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A quasi-genetic model for plant virus host ranges I. Group reactions within taxonomic boundaries
VIROLOGY
31, 616424
(1967)
A Quasi-genetic
Model
for Plant Virus
I. Group Reactions within Taxonomic
Host Ranges
Boundaries
J. G. BALD Agricultural
Sciences, University
of California,
Los Angeles 90024
AND
T. W. TINSLEY Commonwealth
Forestry
Institute,
University
of Oxford,
England
Accepted December 15, 1966 Reexamination of the experimental host ranges of a number of mechanically inoculated viruses showed in each instance an uneven distribution of susceptible species among groups of related plant, families. Some of the groups, apart from those containing the original field hosts, were significantly different from each other in their aggregate susceptibility to a number of viruses. The most susceptible groups of plants were relatively advanced in the phylogenetic, and therefore probably in the evolutionary, sense but they were generally not the most advanced in the developmental series to which they belonged. The more primitive groups of families were generally less susceptible to the viruses examined, unless like turnip mosaic virus, they had natural field hosts in the more primitive families. On the other hand, the tobacco family and some others were suscept,ible to all the viruses except cucurbit mosaic, which was confined to the cucumber family. Similar uneven distributions of susceptible species appeared on a smaller scale among groups of related genera and species. A simple statistical quasi-genetic model was developed to summarize some of these findings. It assumed a multiplicity of characters for susceptibility distributed unevenly between groups of related plants, and matching characters for infect,ivity unevenly distributed among viruses. The host characters and possibly the matching virus characters were assumed to be less rigidly formulated than processes directly controlled by the cell DNA.
were groups of related families in which nearly every species was susceptible, at least to virus multiplication in the inoculated leaves, and other groups in which few susceptible species were found. A comparative study of TMV and severe etch virus (SEV) (Holmes, 1946) confirmed earlier conclusions for TnIV and showed that the experimental host range of SEV lay within that of T&IV. Price (1940), also by mechanical inoculation, tested the experimental host range of 6 viruses (tobacco necrosis, tobacco ringspot, tomato ringspot, cucumber mosaic, cucurbit mosaic, and alfalfa mosaic) and grouped
INTRODUCTION
Holmes (1935) extended the work of Grant (1934) on t’he experimental host range of tobacco mosaic virus (TMV) and int,roduced a new approach to the study of susceptible species. He inoculated many species of plants, related and unrelated to tobacco, and grouped them with their reactions in families according to the IiGnigsberger Stammbaum of Mez and Ziegenspeck (1926), a taxonomic arrangement based on serological relationships between species. Although species susceptible to inoculation with TMV were widely distributed, there 616
QUASI-GENETIC
MODEL
FOR PLANT
susceptible and insusceptible families according to the Konigsberger Stammbaum. He failed t,o find such regularities as Holmes had found and concluded “t’hat the ability of a plant to support increase of a virus in its tissues is a characteristic of the species itself, and has little or no relationship to taxonomic position of the family.” This implication of order in t’he TMV and SEV host ranges and lack of order in the others posed a contradiction which suggested reexamination of the data. METHODS
The Konigsberger Stammbaum was not used for the arrangement of tested species as it is not generally accepted now as a basis for the classification of plants. Data were arranged according to several classifications; of these Hutchinson’s (1959) system gave the most plausible distribution of susceptible species. In grouping species according to Hutchinson, the division of the flowering plants into Dicotyledons and Monocotyledons was followed by the division of the into the more primitive Dicotyledons Lignosae and the more advanced Herbaceae. The 2 divisions of Dicotyledons and the
I
Compositae
I I
Lobeliaceae
VIRUS
HOST RANGES
617
Monocotyledons allowed a first separation of test species into 3 main groups. These groups were subdivided into the groups of related orders and families, delimited by crosslincs in Hutchinson’s table, “Sequence of orders and families” (pp. 104-121). The data for all host species within one of these subdivisions could be tabulat,ed as a single set of numbers or regrouped with data from other subdivisions according to common origins. In the Herbaceae, from which most of the host species were taken, the basic subdivisions were into 9 groups represented in Fig. 1 by the families of the host species in t,he Herbaceae examined by Holmes and Price. Group 9 could be further divided into 2 sections differing in susceptibility to virus infection: Geraniaceae to Balsaminaceae and Polemnniaceae to Labiatae. Another question that arose was how t,o give quantitative expression to data on host range. Host ranges of plant viruses are usually considered from a qualitat,ive and descriptive point of view. In choosing species for inoculation, the basic problems in quantitat#ive analysis, sampling and freedom from bias, have hardly been considered, The choice of test species has been in-
Gesneracea
Acanthaceae Scrophulariaceae
Boraginaceae
Convolvulaceae SOlanaCiXW
Hydrophyllaceae Polemoniaceae
Balsaminaceae Tropaeoloaceae Oxalidaceae
Portulaccaceae aryophyllaceae
Resedaceae Cruciferae Fumariaceae' Papaveraceae
FIG. 1. Herbaceae (Hutchinson), a phylogenetic diagram of 9 subdivisional as a basis for grouping virus reactions of species. It lists 41 of the 96 families (1959).
(2)
groups of families, used described by Hutchinson
61s
BALD
AND TINSLEY
fluenced mainly by the availabilit,y of seed and the ease wkh which the specks can be grown in an experimental greenhouse. If these characteristics are not linked with suscept’ibility, such a choice may give a relatively unbiased selection of species. A consistent effort was made to ask questions of the data in such a way that answers were not seriously biased by sampling methods. St,aGstically, judgments mere based on differences likely to appear by chance with odds less Ihan 1 in 20; but the numerical data being so heterogeneous, only the simplest forms of statistical analysis were applied. The chi-square test was used on dat’a grouped in contingency t’ables, and the correction for 2 X 2 tables (Fisher and Yates, 1945) was applied. Analysis of variance and correlation were used. The reactions of species were tabulated under the following symbols: + or - indicated infected or not infected, and a lower case “1” indicated infection of inoculated leaves only. If, in t’he original papers the distinction between local and syst,emic infection was made, the categories mere +, 1, nnd - ; but 1 was assumed to be included in + whenever local infections were recorded but not distinguished from systemic infections (e.g., Price, 1940). Opportunity was left, by addition of other symbols to increase the number of catcgorics if that should be necessary. Of t,he virus isolates used by Holmes in his 1946 host’ range study of TJIV and SW, the TMV was characteristic of the form dominant in tobacco (Bald, 1960), and the etch virus, although symptomatically severe, was also chnracterist,ic of the virus as it occurs in tobacco. Careful characterization of the virus isolates adds to the relinbilit’y of t,he conclusions that may be drawn from Holmes’ data. RESULTS
Host Range of TMV ancl SEV Hutchinson’s division into woody and herbaceous Dicotyledons and i\lonocotyledons is shown in Table 1 applied to Holmes’ (1946) data for T!\IV and SW. The majority of infections occurred among the Herbaceae. The distinction between t,he reactions of IJgnosne plus Monocotyledons
and Herbaceae was similar to that, between the 2 sides of the IkIez and Ziegenspcck family tree (Holmes 193S, 1946). The less susceptible groups of families in Herbaceae were in an evolutionary sense mostly the more primitive. They included TABLE
1
NUMBERS OF TESTED ANGIOSPERM SPECIES SUSCEPTIBLE OR INSUSCEPTIBLE To INFECTION WITH TMV OR SEVn
Group
Some orders or Subdivision families 1 represented -I- -’
Lignosae
2 Rosales Leguminales 5, 6 Nyctaginaceae Violaceae 7 Cucurbit,aceae Begoniaceae 8 Malvaceae Euphorbiaceae 10, 12 Hypericaceae Apocynales 14 Bignoniales Verbenales
T&W
SEV
~t l-f1 ------
0
-
120 0 0 21
11200
4
0
1800’9 ! ’
0
015 0 0 15
0,040o
4
13210
5
0
6
Total Herbaceae
1, Ranales 2 Papaveraceae Cruciferae 3 Caryophyllaceae / Chenopodiaceae 4 Primulaceae ) Plantaginaceae 5’ Umbelliferae 6 Dipsacaceae 7 Lobeliaceae 1 Compositae 8 Solsnaceae Scrophldnriaceae 9ai Geraniales
4>200
0 0200 2 6 27 17 9 4 37 i2 23 5 42(10 28 i
0
3500;
8
QUASI-GENETIC TABLE
-~
SEV -
9b Polemoniales Labiatae
Herbaceaecont.
FOR
l-continued
TMV some orders or families represented +1-+1-
Group
-__
MODEL
9 22 2 6 3 24
Total
ledons
1
1
.,
,.,,
Q In 3 main groups of Angiosperms and in subdivisional groups of families, arranged according to Hutchinson’s (1959) classification of flowering plants (pp. 104-121). The 3 main groups of Angiosperms are the 2 divisions of Dicotyledones, Lignosae, and Herbaceae, and the Monocotyledones. + = systemic infection; 1 = infection of inoculated leaves; - = no infection.
families in subdivisions 1, 2, 5, 6, and the more primitive section of subdivisions 9, 9a. In these subdivisions, SEV failed to infect any species, and all but one of the T&TV infect,ions were confined to the inoculated leaves. Subdivisions 3, 4, 7, 8, and 9b, which included the more advanced families, were on the whole more susceptible (Tables 1 and 2, Fig. 1). For each virus the difference between the 2 sets of subdivisions was highly significant. Of the more advanced groups, subdivision 7, Campanulaceae to Compositae, was the least susceptible (Table 1, Fig. 1). The infection figures for TMV alone were not significantly different from those of the less susceptible groups, but the proportion of species susceptible to SEV was higher than in most groups. Species in subdivisions 3, 4 were highly susceptible to TMV, but systemic infection seldom occurred. Local infect,ions often gave necrotic lesions. These reactions were similar to those most common in the more primitive groups descended directly from Ranunculaceae, but they occurred in a greater proportion of species. Subdivision 9b contained a center of susceptibility to both viruses in the Hydrophyllaceae (Holmes, 1946). In the most advanced families of the same group,
PLANT
VIRUS
HOST
619
RANGES
Boraginaceae to Labiatae, local infections recurred and became dominant. Again in group S, which contained Solanaceae, the major center of susceptibility, the most advanced families showed the same tendency to localize infection. It was combined in the Scrophulariaceae with a suppression of symptoms where systemic infection occurred. In general the major centers of infect,ion were in advanced but not in climax families. Host Range of 5 Unrelated Viruses (Price, 1s/to) Price’s experiment,s on comparative host range included 6 plant viruses. Data for 5 were analyzed because one, cucurbit mosaic, infected only species of Cucurbitaceae. The 5 were tobacco necrosis, TNV; tobacco ringspot, TRV; tomato (Lycopersicon) ringspot, LRV; cucumber mosaic, CMV; and alfalfa mosaic, AMV. In Price’s Table 1, containing his own results, the positive but not the negative results of other workers were also listed. Such incomplete data were not included in the analysis as they would have introduced bias in the absence of records for insusceptible species. All data were given as infection or no infection of a tested plant species. For comparison Holmes’ results were bulked in the same form. The difference bet,ween the 2 viruses examined by Holmes and the viruses examined by Price, which gave rise to t,he basic difference in their views on host range, was centered TABLE
2
SUBDIVISION OF TMVAND SEV-TESTED SPECIES WVI'IHIN THE IIERRACEAE INTO 2 LOTS OF FAMILIES~
Infection Subdivisional groups
+ TMV
-
1 SEV’ TMV / SEV --,
3, 1, 4, 2, 5, 7, 8, 6, 9b 9a
LXoinfection
--I-
681 1 580 i~ 102 18
TMV j SEV -I-
22 0 / 39 22 , 1::
a One group is descended more or less directly from the Ranales; the other on the whole is more advanced phylogenetically. Data condensed from Table 1.
620
BALD
AND
in t,he reactions of Hutchinson’s mxonomic division, Lignosae (fundamentally woody plants). TRIV and SEV infected a smaller proportion of t,ested species in this division than in Herbaceae (fundamentally herbaceous plants), but the viruses test,ed by Price did not. This difference will be discussed in another paper. Within the Herbaceae, which included a majority of host species t’ested, Price’s data were grouped in subdivisions. Subdivisions 1 and 2 were combined because only 2 species of Ranunculaceae were inoculated by Price (10 tests). Subdivisions 5 and 6 were also combined, and subdivision 8 was divided into Solannceae and other families. Caryophyllaceae were detached from subdivision 3 and combined with 9a for reasons given elsewhere (Bald and Tinsley, 1967). For comparison with Holmes’ data numbers of infections with t’he 5 viruses were added and converted to percentages, e.g., numbers of species in Solanaceae (subdivision S) infect’ed and not infected by t,he 5 viruses tested by Price were 10-2, 11-0, 5-0, 19-0, 22-O = 67169 = 97 $5 infect,ed in 69 tests. Holmes’ data were put in the same form (Table 3). Calculation of the correlation between the sets of percentage values in Table 3 for TRIV SEV and Price’s 5 viruses gave Y = ~0.9Oy,~ n = 7, p < 0.001, which was clearly significant. Price’s data for 5 viruses revealed much the same pattern of susceptible and insusceptible subdivisions in the Herbacene as Holmes’ data for TAIL and SET’. CompaGm Strains
xith
l’wnip
Mosaic
Virus
The experimental host’ range of various strains of turnip mosaic virus (TulUV) provided another example of group susceptibility. The strains were derived from turnip, cabbage, and anemone, i.e., from species of the families Cruciferae and Ranunculaceae which are in a region of the Herbaceae relatively insusceptible to t,he viruses examined above. By contrast the viruses tested by Holmes and Price were taken originally from Solanaceae (Herb., subd. S), Cucurbitaceae (Lign., subd. 7), or Leguminales (l,ign., subd. 2), and strains of
TINSLEY TABLE PER
CENT
INFECTION
SUBDIVISIONAL
TESTED
GROUPS
Holmes TMV, SEV Subdivision Herbaceae
3
AMONG
OF
SPECIES
IN
FAMILIES~
Price 5 viruses
TuMV 5 strains
of I ’
1, 2 3Caryoph. 4 5, 6 7 8 (Solsn.) 8 (Rem.) ga + Caryoph. 9b Borag. Lab
rests
Per cent
rests*
50 40 20 16 100 110 50 30
24 58 45 25 40 90 56 33
39 42 28 40 52 6gb 63 42
Per cent* -
rests
33 93 Gl 28 65 97* 81 40
130 37 6 G 49 72 18 12
Per cent
I-
75 59 17 17 31 61 50 8
55 64 60 15 13 30 50 53 - - - 0 Averages are for 2 groups of unrelated viruses and 5 strains of turnip mosaic virus (TuMV). Values for TMV and SEV were uniformly twice the numbers of species tested; some of the tests made by Price and many with turnip mosaic virus were made with 1 to 4 viruses or strains per species. * Calculation of tests and per cent given in text, using subdivision 8 (Solanaceae), Price’s 5 viruses as an example. 66 42
-I
-I
bot,h cucumber and alfalfa mosaic were known to cause diseases in Solanaceae. The turnip mosaic host lists combined for analysis were : Cabbage
mosaic
Cabbage ring
black
Turnip
mosaic
Cabba.ge black ringspot Anemone mosaic
Larsen and Walker (1939)) Wisconsin Tompkins, Gardner, and Thomas (1938)) California Tompkins (1938), California Hollings (1957), England Hollings
(1957)) England
For comparison with T&IV and SEV (Holmes, 1946) and Price’s 5 viruses, infections were listed as positive or negative for each species. Thereafter they were bulked by families and subdivisions, and numbers infected were reduced to percentages as for other data in Table 3. Comparison between groups of viruses in subdivision 9b was confined t,o the 2 families Boraginaceae-Labiatac
QUASI-GENETIC
MODEL
FOR PLANT
VIRUS
621
HOST RANGES TABLE
4
because only a single species in the pair of families Polemoniaceae, Hydrophyllaceae was inoculated with a TuMV strain. In the families related to Cruciferae and Ranunculaceae t’he TuRiV st,rains were more infective than TMV, SEV (Holmes, 1946) and the 5 viruses inoculated by Price (1940). Elsewhere in the HerbaCeae the TuMV st,rains were less infective than most of the viruses, but the same pattern of relatively susceptible and insuspectible subdivisional groups of families appeared (Table 3). The correlation with Price’s data, apart from the subdivisions 1 and 2, was T = 0.922, 12 = 6, p = cu. 0.001, and wit,h Holmes’ data, r = 0.789, p < 0.02. This indicated considerable agreement, in group susceptibility except in the subdivisions where the original field hosts of turnip mosaic mere located.
QHolmes (1946). b + = Systemic infection with symptoms; s = systemic infect,ion without symptoms; I = local multiplication; - = no infection. Values for a random distribution of infections are shown in parentheses.
Group Reactions within Solanaceae
Solanae Cestreae
Differences
in group
susceptibility
may
also be found among groups of genera and species within a family. Although the Solanaceae were highly susceptible to TMV and SEV (Holmes, 1946), there were vnriations in the reactions of species t,hat. could be summarized in contingency t’ables and submitted IO the chi-square test. The turnip mosaic strains allowed- a second test of the same kind. Solanaceac (Engler and Prantl) includes 2 tribes which were represented in these tests by 10 species or more, Solanae and Cestreae (subtribe Sicotianinae only; Cestrinae and Goetzeinae were not represented). The results of inoculations with TIJIV and SEV are in Table 1, and of the turnip mosaic strains in Table 5. In t’he Solanae the proportion of infected
plants
with no symptoms
or
local multiplication of virus only was significantly less t’han in Nicotiana and some closely related genera. Turnip mosaic infected only 2 tested species in Solanse, each with one strain, and in t’he Cestreae 24 lvere infected
of 31 tested.
In spite of these
resuhs Cestreae are not uniformly susceptible. In a study of reactions of TXIV to the genera Cestrunz and Petunia which are both in Cestreae (Bald, 1958), all 5 tested species of Cestrum were insusceptible to the tobacco
REACTIONS OF SPECIES IN 2 TI~IBES OF SOLANA~EAE TO TMV AND SEVa, b
Tribe Solanae Cestreae
+ 20 (27.5) 51 (43.5)
TABLE
-
s, 1 12 (6.2) 4 (9.8)
6 (4.3) 5 (6.7)
5
REACTIONS OF SPECIES IN 2 TRIBES OF SOLANACEAE TO 5 STRAINS OF TURNIP
Mosarc VIRUSa Tribe
+
1
2 (5.5) 12 (8.5)
0 (4.7) 12 (7.3)
18 (9.8) 7 (15.2)
(1Symbols as in Table 4. Values for a random distribution of infections are shown in paren-
theses. form of TMV, and t’ested Petunia spp. were uniformly susceptible. TMV discriminated between related genera as well as between higher taxa. DISCUSSION
In his study first, of TMV and then of TRlV and SEV: Holmes (1938, 1946) laid the foundation for theoretical as opposed to pragmatic host range studies, but his choice of a taxonomic system increased the difficult,ies of coordinating Price’s (1940) results with his, and of finding generalizations applicable to a number of viruses. In this sense Hutchinson’s system of classification was superior, although in some of its aspects, notably the division of Dicotyledons into Lignosae and Herbaceae, it is contested by a number of taxonomist8s. However, the greatest difficulty in generalizing from the work of Holmes and Price lay in the lack of statistical treatment. The effectiveness of statistical methods in revealing characteristic group reactions leads to the suggestion that such reactions are most easily explained on a stat)istical basis.
622
BALD AND TINSLEY
A group of related families or any other convenient taxonomic grouping may be considered a genetic pool containing factors for susceptibility. A single factor might not need to be unitary and invariable, i.e., rigidly formulated, so long as it’ was functionally uniform. For example, the presence of a particular factor might imply availability at the right time of one kind of enzyme at a particular site in the cell. In different plant species the enzyme might exist in slightly different forms; it might be made through slightly different metabolic pathways or through similar genetic information differently situated in the plant genomes. In the virus genomes also equivalent variability in an infectivity fact,or might exist without impairing its capacity to mat,ch the complementary factor for susceptibility in the host. Such a system would imply considerable flexibility in the matching of virus and plant genomes. It might arise in part from the virus having its genetic information carried by RNA. If plant DNA is not directly involved in virus synthesis (and perhaps even if it were involved), t,he virus would have to match the products of the DNA code, not the code itself. The coding sequence of the plant DNA should have far less importance than in the multiplication of a DNA virus. In a taxonomic group of plants the probability of a single species having a susceptible reaction would be related to the number and frequencies of such factors for susceptibility in the group as a whole. If the number and frequencies were high, many representatives of the taxonomic group might be found susceptible to a number of viruses, if low, they might be susceptible only to a few viruses specially adapted to the genetic constitution of that taxonomic group. A simplified numerica’ model can be made to illustrate the general nature of such a system. Suppose 4 viruses, p, q, r, and z can each infect species in a taxonomic group of plants if t#hey possess 3 infectivit’y factors matching complementary factors for SUSceptibility in the t,ested plant species. Table 6 shows viruses p and q with 2 fact,ors in common and one unshared; r, 2 factors unshared by p and one unshared by q.
Viruses p and z have no infectivity factors in common. Three examples are given in Table 6. In t’he taxonomic group of plants represented by (1) in the lower section of the table, there is a high probability of occurrence of each of the 6 complementary factors for susceptibility; in (2) the probabilities are uniformly lower; and in (3) some are high and some low. Assuming a random distribution of susceptibility factors among individual tested species, the expected numbers of susceptible species would in example (I), upper sect,ion, Table 6, range from 45 to 60%>, in example (2) from 3 to 7%, and in example (3) from G to 24%. In example (3) if one virus carried the infectivit’y factors ace and another the factors bdf there would be a contrast, not a general agreement in the degree of susceptibility of the taxonomic group to the 2 viruses. The expected percentages of susceptible species TABLE
6
ARITHMETIC MODEL OF THE HOST RANGE OF 4 VIRUSES IN 3 TAXONOMIC GROUPS OF PLANTS~
Virus Infectivity factors P cl r e
Taxo-
nomic group
abc
-
abd ade def
Per cent of susceptiblespeciesin taxonomicgroup (1) 56.P 45.3 60.3 54.0
(4
(3)
7.0 3.0 6.0 7.2
16.1 5.1 24.2 14.4
Probability of occurrenceof susceptibility factorsa’ - f’ in singlespecieswithin 3 taxonomicgroups -
(1) (2) (3)
a Each virus has 3 infectivity factors that must match 3 susceptibility factors in a host species for infection to occur. Above, percent of susceptible species; below, probability of occurrence of each factor for susceptibi1it.y in a single plant, species. bPercentage (P) of species with susceptibility factors a’b’c’ (lower section of table) corresponding to infectivity factors abc. P = 0.85 X 0.7 X 0.95 x 100 = X.6.
QUASI-GENETIC
MODEL
FOR
would he 64.6 and 3.6. These examples suggest, that several t’axonomic groups might react in a very similar way to 2 viruses sharing common infectivity factors (p, q) or to t,wo viruses with no common infectivity factors (p, z). Also that if some susceptibility factors were frequently present in a taxonomic group, and some were seldom found, susceptibility to one virus and insusceptibility to others might occur. In a crude way, therefore, this model illustrates correlations and contrasts of host range that’ commonly exist. They will be discussed further in another paper. Further points for comment are (a) the nature of the viruses chosen for this study, (b) the plant species inoculated, and (c) the requirements for host range studies from the statistical point of view. (a) The viruses chosen for this study the two main were diverse, including morphological types, rods and so-called spheres. TMV is generally transmitted AMV, and mechanically : SEV, CNV, TuJIV by aphids; TRV and LRV by nematodes; and TNV by one of the lower fungi. All are easily inoculated mechanically and multiply in parcnchymatous tissues. Many conclusions applicable to these viruses are very likely applicable to similar viruses, but t’here is no evidence here to show whether they apply t,o viruses transmitted by other kinds of vectors or viruses wit,h different tissue relations. (b) The plant species inoculated by the various authors whose results were examined were sometimes the same, but sometimes clifferent. Price (1940) inoculated about 140 species, not all with all viruses; Holmes (1946), 310. Fifty-two species were found that both had inoculated: Price with 4 viruses, TNV, TRV, CRIV, and Ai\IV; Holmes wit)h TMV and SEV. About, 50 more species inoculated by Price wit,h 1 to 4 viruses were also inoculat’ed by Holmes. The 5 turnip mosaic virus strains studied were inoculated to a number of species that neither l’rice nor Holmes used. There was most coincidence among Solanaceae, where many species were tested, and generally least in families from which few species were snmplcd. Fortunately, the smaller samples
PLANT
VIRUS
HOST
RANGES
623
provided data more likely to be representat,ive of the family than of the individual species. Hopefully, this increased the reliabilit,y of the general conclusions about group susceptibility. (c) There are several requirements for a statistical investigation t’hat are only partly satisfied in the data at present available. These include (i) recording negative as well as positive results. Some of the classic host range studies are unfit for quantitative treatment because only positive results were published. (ii) Often hhe reason given for not’ publishing results was that they were of uncertain validity. A degree of uncertainty does not invalidate such data for statistical treatment. The modern view is that in any case bhere is a t’hreshold for detection of viruses in plant tissues. Below the threshold t’here may be undetectable infections in single cells or small groups of cells, and these will have to be classed witsh examples of complete immunity. Confounding two such classes is unimportant so long as one maintains a consistent criterion for judging infection. Holmes (1946) recorded as positive only those t’ested species giving more than 10 lesions in back inoculations to Nicotiana glutinosa. (iii) Holmes also set a standard in recording the type of response for each host species. The types of reaction may be indispensable for decisions on relative susceptibility. The 4 basic reactions to inoculation appear to be (a) complete immunity, (b) subliminal infections, (c) local multiplication in inoculated leaves only, (d) systemic invasion. Classes a and b are confounded. Other classes representing degrees of susceptibilit,y may be added to these 4 (Holmes, 1946). It is legitimat,e for statistical purposes t,o make a division at any level so long as the classes separated represent real differences in susceptibility, or in host reactions. (iv) The complexit,y of host relations is such that it is advisable t,o use well defined strains of viruses in host range studies. The example of TRW and cucurbit mosaic shows that strains of one virus can vary as widely in host range as unrelated viruses; and TMV and SET’ show how closely the host ranges of unrelated viruses may correspond. A mixture of st’rains, unless it is a
624
BALD
AND
feature of the experimental design, might lead to confusion. (v) The sampling of test species for experimental host range st,udies is both a taxonomic and statistical problem. The numbers of species in single families range from less than 10 to more than 20,000. A comprehensive study of host range would demand first a crit’ical selection of families among more than 400, t,hen of representative tribes, genera, and species, a final selection of species, and possibly of genotypes within species. Obtaining seed and growing test plants would remain a major problem. Seed of crop plants, ornnmentals, and botanical collections are most readily obtainable, but t’hey represent a small fraction of all species. Here a systematist could help by reducing the ideal t’o the possible and a statistician by prescribing a sampling technique and other elements of experimental design. Sampling will become simpler as problems become more confined, but the same kind of advice will be essential even in more restricted host range studies. REFERENCES BALD, J. G. (1958). Host reactions to two viruses in Nicotiana and the related genera Cestr?Lm and Plant Physiob. 33, Suppl. xli. Petunia. (Abstract). BALD, J. G. (1969). Forms of tobacco mosaic virus. Nature 188, 645-647. BALD, J. G., and TINSLEY, T. W. (1967). Virus reactions and taxon0m.y. Xature in press. BESSEY, C. E. (1915).‘ The phylogeny and tax-
TINSLEY onomy of Angiosperms. Ann. Missouri Botan. Gardens 2, 109-164. FISHER, R. A., and YATES, E. (1945). “Statistical Tables for Biological, Agricultural, and Medical Research.” Oliver and Boyd, Edinburgh. GRANT, T. J. (1934). The host range and behavior of the ordinary tobacco mosaic virus. Phytopathology 24, 311-336. HOLLINGS, M. (1957). Anemone mosaic-a virus disease. flnn. ilppl. Biol. 45, 44-61. HOLMES, F. 0. (1938). Taxonomic relationships of plants susceptible to infection by tobaccomosaic virus. Phytopathology 28, 5%6G. HOLMES, F. 0. (1940). A comparison of the experimental host ranges of tobacco-etch and tobacco-mosaic viruses. Phytopathology 36. 643-659. HUTCHINSON, J. (1955). “British Flowering Plants.” P. R. Gawthorn Ltd., London. HUTCHINSOK, J. (1959). “The Families of Flowering Plants,” Vol 1, Dicotyledons; Vol. 2, Monocotyledons. Oxford Univ. Press (Clarendon), London and New York. LARSEN, R. H., and WALKER, J. C. (1939). A mosaic disease of cabbage. J. ;Igr. Res. 59, 367-392. MEZ, C., and ZIEGENSPECK, H. (1926). Der Kiinigsberger serodiagnostische Stammbaum. Botan. Arch. 13, 483-485. PRICE, W. C. (1940). Comparative host range of six plant viruses. Am. J. Botany 27, 530-541. TOMPKINS, C. 111. (1938). A mosaic disease of turnip. J. Agr. Res. 57, 589-602. TOMPKINS, C. M., GARDNER, M. W., and THOMAS, H. R. (1938). Black ring, a virus disease of cabbage and ot,her crucifers. J. Agr. Res. 57, 929943.
31, 616424
(1967)
A Quasi-genetic
Model
for Plant Virus
I. Group Reactions within Taxonomic
Host Ranges
Boundaries
J. G. BALD Agricultural
Sciences, University
of California,
Los Angeles 90024
AND
T. W. TINSLEY Commonwealth
Forestry
Institute,
University
of Oxford,
England
Accepted December 15, 1966 Reexamination of the experimental host ranges of a number of mechanically inoculated viruses showed in each instance an uneven distribution of susceptible species among groups of related plant, families. Some of the groups, apart from those containing the original field hosts, were significantly different from each other in their aggregate susceptibility to a number of viruses. The most susceptible groups of plants were relatively advanced in the phylogenetic, and therefore probably in the evolutionary, sense but they were generally not the most advanced in the developmental series to which they belonged. The more primitive groups of families were generally less susceptible to the viruses examined, unless like turnip mosaic virus, they had natural field hosts in the more primitive families. On the other hand, the tobacco family and some others were suscept,ible to all the viruses except cucurbit mosaic, which was confined to the cucumber family. Similar uneven distributions of susceptible species appeared on a smaller scale among groups of related genera and species. A simple statistical quasi-genetic model was developed to summarize some of these findings. It assumed a multiplicity of characters for susceptibility distributed unevenly between groups of related plants, and matching characters for infect,ivity unevenly distributed among viruses. The host characters and possibly the matching virus characters were assumed to be less rigidly formulated than processes directly controlled by the cell DNA.
were groups of related families in which nearly every species was susceptible, at least to virus multiplication in the inoculated leaves, and other groups in which few susceptible species were found. A comparative study of TMV and severe etch virus (SEV) (Holmes, 1946) confirmed earlier conclusions for TnIV and showed that the experimental host range of SEV lay within that of T&IV. Price (1940), also by mechanical inoculation, tested the experimental host range of 6 viruses (tobacco necrosis, tobacco ringspot, tomato ringspot, cucumber mosaic, cucurbit mosaic, and alfalfa mosaic) and grouped
INTRODUCTION
Holmes (1935) extended the work of Grant (1934) on t’he experimental host range of tobacco mosaic virus (TMV) and int,roduced a new approach to the study of susceptible species. He inoculated many species of plants, related and unrelated to tobacco, and grouped them with their reactions in families according to the IiGnigsberger Stammbaum of Mez and Ziegenspeck (1926), a taxonomic arrangement based on serological relationships between species. Although species susceptible to inoculation with TMV were widely distributed, there 616
QUASI-GENETIC
MODEL
FOR PLANT
susceptible and insusceptible families according to the Konigsberger Stammbaum. He failed t,o find such regularities as Holmes had found and concluded “t’hat the ability of a plant to support increase of a virus in its tissues is a characteristic of the species itself, and has little or no relationship to taxonomic position of the family.” This implication of order in t’he TMV and SEV host ranges and lack of order in the others posed a contradiction which suggested reexamination of the data. METHODS
The Konigsberger Stammbaum was not used for the arrangement of tested species as it is not generally accepted now as a basis for the classification of plants. Data were arranged according to several classifications; of these Hutchinson’s (1959) system gave the most plausible distribution of susceptible species. In grouping species according to Hutchinson, the division of the flowering plants into Dicotyledons and Monocotyledons was followed by the division of the into the more primitive Dicotyledons Lignosae and the more advanced Herbaceae. The 2 divisions of Dicotyledons and the
I
Compositae
I I
Lobeliaceae
VIRUS
HOST RANGES
617
Monocotyledons allowed a first separation of test species into 3 main groups. These groups were subdivided into the groups of related orders and families, delimited by crosslincs in Hutchinson’s table, “Sequence of orders and families” (pp. 104-121). The data for all host species within one of these subdivisions could be tabulat,ed as a single set of numbers or regrouped with data from other subdivisions according to common origins. In the Herbaceae, from which most of the host species were taken, the basic subdivisions were into 9 groups represented in Fig. 1 by the families of the host species in t,he Herbaceae examined by Holmes and Price. Group 9 could be further divided into 2 sections differing in susceptibility to virus infection: Geraniaceae to Balsaminaceae and Polemnniaceae to Labiatae. Another question that arose was how t,o give quantitative expression to data on host range. Host ranges of plant viruses are usually considered from a qualitat,ive and descriptive point of view. In choosing species for inoculation, the basic problems in quantitat#ive analysis, sampling and freedom from bias, have hardly been considered, The choice of test species has been in-
Gesneracea
Acanthaceae Scrophulariaceae
Boraginaceae
Convolvulaceae SOlanaCiXW
Hydrophyllaceae Polemoniaceae
Balsaminaceae Tropaeoloaceae Oxalidaceae
Portulaccaceae aryophyllaceae
Resedaceae Cruciferae Fumariaceae' Papaveraceae
FIG. 1. Herbaceae (Hutchinson), a phylogenetic diagram of 9 subdivisional as a basis for grouping virus reactions of species. It lists 41 of the 96 families (1959).
(2)
groups of families, used described by Hutchinson
61s
BALD
AND TINSLEY
fluenced mainly by the availabilit,y of seed and the ease wkh which the specks can be grown in an experimental greenhouse. If these characteristics are not linked with suscept’ibility, such a choice may give a relatively unbiased selection of species. A consistent effort was made to ask questions of the data in such a way that answers were not seriously biased by sampling methods. St,aGstically, judgments mere based on differences likely to appear by chance with odds less Ihan 1 in 20; but the numerical data being so heterogeneous, only the simplest forms of statistical analysis were applied. The chi-square test was used on dat’a grouped in contingency t’ables, and the correction for 2 X 2 tables (Fisher and Yates, 1945) was applied. Analysis of variance and correlation were used. The reactions of species were tabulated under the following symbols: + or - indicated infected or not infected, and a lower case “1” indicated infection of inoculated leaves only. If, in t’he original papers the distinction between local and syst,emic infection was made, the categories mere +, 1, nnd - ; but 1 was assumed to be included in + whenever local infections were recorded but not distinguished from systemic infections (e.g., Price, 1940). Opportunity was left, by addition of other symbols to increase the number of catcgorics if that should be necessary. Of t,he virus isolates used by Holmes in his 1946 host’ range study of TJIV and SW, the TMV was characteristic of the form dominant in tobacco (Bald, 1960), and the etch virus, although symptomatically severe, was also chnracterist,ic of the virus as it occurs in tobacco. Careful characterization of the virus isolates adds to the relinbilit’y of t,he conclusions that may be drawn from Holmes’ data. RESULTS
Host Range of TMV ancl SEV Hutchinson’s division into woody and herbaceous Dicotyledons and i\lonocotyledons is shown in Table 1 applied to Holmes’ (1946) data for T!\IV and SW. The majority of infections occurred among the Herbaceae. The distinction between t,he reactions of IJgnosne plus Monocotyledons
and Herbaceae was similar to that, between the 2 sides of the IkIez and Ziegenspcck family tree (Holmes 193S, 1946). The less susceptible groups of families in Herbaceae were in an evolutionary sense mostly the more primitive. They included TABLE
1
NUMBERS OF TESTED ANGIOSPERM SPECIES SUSCEPTIBLE OR INSUSCEPTIBLE To INFECTION WITH TMV OR SEVn
Group
Some orders or Subdivision families 1 represented -I- -’
Lignosae
2 Rosales Leguminales 5, 6 Nyctaginaceae Violaceae 7 Cucurbit,aceae Begoniaceae 8 Malvaceae Euphorbiaceae 10, 12 Hypericaceae Apocynales 14 Bignoniales Verbenales
T&W
SEV
~t l-f1 ------
0
-
120 0 0 21
11200
4
0
1800’9 ! ’
0
015 0 0 15
0,040o
4
13210
5
0
6
Total Herbaceae
1, Ranales 2 Papaveraceae Cruciferae 3 Caryophyllaceae / Chenopodiaceae 4 Primulaceae ) Plantaginaceae 5’ Umbelliferae 6 Dipsacaceae 7 Lobeliaceae 1 Compositae 8 Solsnaceae Scrophldnriaceae 9ai Geraniales
4>200
0 0200 2 6 27 17 9 4 37 i2 23 5 42(10 28 i
0
3500;
8
QUASI-GENETIC TABLE
-~
SEV -
9b Polemoniales Labiatae
Herbaceaecont.
FOR
l-continued
TMV some orders or families represented +1-+1-
Group
-__
MODEL
9 22 2 6 3 24
Total
ledons
1
1
.,
,.,,
Q In 3 main groups of Angiosperms and in subdivisional groups of families, arranged according to Hutchinson’s (1959) classification of flowering plants (pp. 104-121). The 3 main groups of Angiosperms are the 2 divisions of Dicotyledones, Lignosae, and Herbaceae, and the Monocotyledones. + = systemic infection; 1 = infection of inoculated leaves; - = no infection.
families in subdivisions 1, 2, 5, 6, and the more primitive section of subdivisions 9, 9a. In these subdivisions, SEV failed to infect any species, and all but one of the T&TV infect,ions were confined to the inoculated leaves. Subdivisions 3, 4, 7, 8, and 9b, which included the more advanced families, were on the whole more susceptible (Tables 1 and 2, Fig. 1). For each virus the difference between the 2 sets of subdivisions was highly significant. Of the more advanced groups, subdivision 7, Campanulaceae to Compositae, was the least susceptible (Table 1, Fig. 1). The infection figures for TMV alone were not significantly different from those of the less susceptible groups, but the proportion of species susceptible to SEV was higher than in most groups. Species in subdivisions 3, 4 were highly susceptible to TMV, but systemic infection seldom occurred. Local infect,ions often gave necrotic lesions. These reactions were similar to those most common in the more primitive groups descended directly from Ranunculaceae, but they occurred in a greater proportion of species. Subdivision 9b contained a center of susceptibility to both viruses in the Hydrophyllaceae (Holmes, 1946). In the most advanced families of the same group,
PLANT
VIRUS
HOST
619
RANGES
Boraginaceae to Labiatae, local infections recurred and became dominant. Again in group S, which contained Solanaceae, the major center of susceptibility, the most advanced families showed the same tendency to localize infection. It was combined in the Scrophulariaceae with a suppression of symptoms where systemic infection occurred. In general the major centers of infect,ion were in advanced but not in climax families. Host Range of 5 Unrelated Viruses (Price, 1s/to) Price’s experiment,s on comparative host range included 6 plant viruses. Data for 5 were analyzed because one, cucurbit mosaic, infected only species of Cucurbitaceae. The 5 were tobacco necrosis, TNV; tobacco ringspot, TRV; tomato (Lycopersicon) ringspot, LRV; cucumber mosaic, CMV; and alfalfa mosaic, AMV. In Price’s Table 1, containing his own results, the positive but not the negative results of other workers were also listed. Such incomplete data were not included in the analysis as they would have introduced bias in the absence of records for insusceptible species. All data were given as infection or no infection of a tested plant species. For comparison Holmes’ results were bulked in the same form. The difference bet,ween the 2 viruses examined by Holmes and the viruses examined by Price, which gave rise to t,he basic difference in their views on host range, was centered TABLE
2
SUBDIVISION OF TMVAND SEV-TESTED SPECIES WVI'IHIN THE IIERRACEAE INTO 2 LOTS OF FAMILIES~
Infection Subdivisional groups
+ TMV
-
1 SEV’ TMV / SEV --,
3, 1, 4, 2, 5, 7, 8, 6, 9b 9a
LXoinfection
--I-
681 1 580 i~ 102 18
TMV j SEV -I-
22 0 / 39 22 , 1::
a One group is descended more or less directly from the Ranales; the other on the whole is more advanced phylogenetically. Data condensed from Table 1.
620
BALD
AND
in t,he reactions of Hutchinson’s mxonomic division, Lignosae (fundamentally woody plants). TRIV and SEV infected a smaller proportion of t,ested species in this division than in Herbaceae (fundamentally herbaceous plants), but the viruses test,ed by Price did not. This difference will be discussed in another paper. Within the Herbaceae, which included a majority of host species t’ested, Price’s data were grouped in subdivisions. Subdivisions 1 and 2 were combined because only 2 species of Ranunculaceae were inoculated by Price (10 tests). Subdivisions 5 and 6 were also combined, and subdivision 8 was divided into Solannceae and other families. Caryophyllaceae were detached from subdivision 3 and combined with 9a for reasons given elsewhere (Bald and Tinsley, 1967). For comparison with Holmes’ data numbers of infections with t’he 5 viruses were added and converted to percentages, e.g., numbers of species in Solanaceae (subdivision S) infect’ed and not infected by t,he 5 viruses tested by Price were 10-2, 11-0, 5-0, 19-0, 22-O = 67169 = 97 $5 infect,ed in 69 tests. Holmes’ data were put in the same form (Table 3). Calculation of the correlation between the sets of percentage values in Table 3 for TRIV SEV and Price’s 5 viruses gave Y = ~0.9Oy,~ n = 7, p < 0.001, which was clearly significant. Price’s data for 5 viruses revealed much the same pattern of susceptible and insusceptible subdivisions in the Herbacene as Holmes’ data for TAIL and SET’. CompaGm Strains
xith
l’wnip
Mosaic
Virus
The experimental host’ range of various strains of turnip mosaic virus (TulUV) provided another example of group susceptibility. The strains were derived from turnip, cabbage, and anemone, i.e., from species of the families Cruciferae and Ranunculaceae which are in a region of the Herbaceae relatively insusceptible to t,he viruses examined above. By contrast the viruses tested by Holmes and Price were taken originally from Solanaceae (Herb., subd. S), Cucurbitaceae (Lign., subd. 7), or Leguminales (l,ign., subd. 2), and strains of
TINSLEY TABLE PER
CENT
INFECTION
SUBDIVISIONAL
TESTED
GROUPS
Holmes TMV, SEV Subdivision Herbaceae
3
AMONG
OF
SPECIES
IN
FAMILIES~
Price 5 viruses
TuMV 5 strains
of I ’
1, 2 3Caryoph. 4 5, 6 7 8 (Solsn.) 8 (Rem.) ga + Caryoph. 9b Borag. Lab
rests
Per cent
rests*
50 40 20 16 100 110 50 30
24 58 45 25 40 90 56 33
39 42 28 40 52 6gb 63 42
Per cent* -
rests
33 93 Gl 28 65 97* 81 40
130 37 6 G 49 72 18 12
Per cent
I-
75 59 17 17 31 61 50 8
55 64 60 15 13 30 50 53 - - - 0 Averages are for 2 groups of unrelated viruses and 5 strains of turnip mosaic virus (TuMV). Values for TMV and SEV were uniformly twice the numbers of species tested; some of the tests made by Price and many with turnip mosaic virus were made with 1 to 4 viruses or strains per species. * Calculation of tests and per cent given in text, using subdivision 8 (Solanaceae), Price’s 5 viruses as an example. 66 42
-I
-I
bot,h cucumber and alfalfa mosaic were known to cause diseases in Solanaceae. The turnip mosaic host lists combined for analysis were : Cabbage
mosaic
Cabbage ring
black
Turnip
mosaic
Cabba.ge black ringspot Anemone mosaic
Larsen and Walker (1939)) Wisconsin Tompkins, Gardner, and Thomas (1938)) California Tompkins (1938), California Hollings (1957), England Hollings
(1957)) England
For comparison with T&IV and SEV (Holmes, 1946) and Price’s 5 viruses, infections were listed as positive or negative for each species. Thereafter they were bulked by families and subdivisions, and numbers infected were reduced to percentages as for other data in Table 3. Comparison between groups of viruses in subdivision 9b was confined t,o the 2 families Boraginaceae-Labiatac
QUASI-GENETIC
MODEL
FOR PLANT
VIRUS
621
HOST RANGES TABLE
4
because only a single species in the pair of families Polemoniaceae, Hydrophyllaceae was inoculated with a TuMV strain. In the families related to Cruciferae and Ranunculaceae t’he TuRiV st,rains were more infective than TMV, SEV (Holmes, 1946) and the 5 viruses inoculated by Price (1940). Elsewhere in the HerbaCeae the TuMV st,rains were less infective than most of the viruses, but the same pattern of relatively susceptible and insuspectible subdivisional groups of families appeared (Table 3). The correlation with Price’s data, apart from the subdivisions 1 and 2, was T = 0.922, 12 = 6, p = cu. 0.001, and wit,h Holmes’ data, r = 0.789, p < 0.02. This indicated considerable agreement, in group susceptibility except in the subdivisions where the original field hosts of turnip mosaic mere located.
QHolmes (1946). b + = Systemic infection with symptoms; s = systemic infect,ion without symptoms; I = local multiplication; - = no infection. Values for a random distribution of infections are shown in parentheses.
Group Reactions within Solanaceae
Solanae Cestreae
Differences
in group
susceptibility
may
also be found among groups of genera and species within a family. Although the Solanaceae were highly susceptible to TMV and SEV (Holmes, 1946), there were vnriations in the reactions of species t,hat. could be summarized in contingency t’ables and submitted IO the chi-square test. The turnip mosaic strains allowed- a second test of the same kind. Solanaceac (Engler and Prantl) includes 2 tribes which were represented in these tests by 10 species or more, Solanae and Cestreae (subtribe Sicotianinae only; Cestrinae and Goetzeinae were not represented). The results of inoculations with TIJIV and SEV are in Table 1, and of the turnip mosaic strains in Table 5. In t’he Solanae the proportion of infected
plants
with no symptoms
or
local multiplication of virus only was significantly less t’han in Nicotiana and some closely related genera. Turnip mosaic infected only 2 tested species in Solanse, each with one strain, and in t’he Cestreae 24 lvere infected
of 31 tested.
In spite of these
resuhs Cestreae are not uniformly susceptible. In a study of reactions of TXIV to the genera Cestrunz and Petunia which are both in Cestreae (Bald, 1958), all 5 tested species of Cestrum were insusceptible to the tobacco
REACTIONS OF SPECIES IN 2 TI~IBES OF SOLANA~EAE TO TMV AND SEVa, b
Tribe Solanae Cestreae
+ 20 (27.5) 51 (43.5)
TABLE
-
s, 1 12 (6.2) 4 (9.8)
6 (4.3) 5 (6.7)
5
REACTIONS OF SPECIES IN 2 TRIBES OF SOLANACEAE TO 5 STRAINS OF TURNIP
Mosarc VIRUSa Tribe
+
1
2 (5.5) 12 (8.5)
0 (4.7) 12 (7.3)
18 (9.8) 7 (15.2)
(1Symbols as in Table 4. Values for a random distribution of infections are shown in paren-
theses. form of TMV, and t’ested Petunia spp. were uniformly susceptible. TMV discriminated between related genera as well as between higher taxa. DISCUSSION
In his study first, of TMV and then of TRlV and SEV: Holmes (1938, 1946) laid the foundation for theoretical as opposed to pragmatic host range studies, but his choice of a taxonomic system increased the difficult,ies of coordinating Price’s (1940) results with his, and of finding generalizations applicable to a number of viruses. In this sense Hutchinson’s system of classification was superior, although in some of its aspects, notably the division of Dicotyledons into Lignosae and Herbaceae, it is contested by a number of taxonomist8s. However, the greatest difficulty in generalizing from the work of Holmes and Price lay in the lack of statistical treatment. The effectiveness of statistical methods in revealing characteristic group reactions leads to the suggestion that such reactions are most easily explained on a stat)istical basis.
622
BALD AND TINSLEY
A group of related families or any other convenient taxonomic grouping may be considered a genetic pool containing factors for susceptibility. A single factor might not need to be unitary and invariable, i.e., rigidly formulated, so long as it’ was functionally uniform. For example, the presence of a particular factor might imply availability at the right time of one kind of enzyme at a particular site in the cell. In different plant species the enzyme might exist in slightly different forms; it might be made through slightly different metabolic pathways or through similar genetic information differently situated in the plant genomes. In the virus genomes also equivalent variability in an infectivity fact,or might exist without impairing its capacity to mat,ch the complementary factor for susceptibility in the host. Such a system would imply considerable flexibility in the matching of virus and plant genomes. It might arise in part from the virus having its genetic information carried by RNA. If plant DNA is not directly involved in virus synthesis (and perhaps even if it were involved), t,he virus would have to match the products of the DNA code, not the code itself. The coding sequence of the plant DNA should have far less importance than in the multiplication of a DNA virus. In a taxonomic group of plants the probability of a single species having a susceptible reaction would be related to the number and frequencies of such factors for susceptibility in the group as a whole. If the number and frequencies were high, many representatives of the taxonomic group might be found susceptible to a number of viruses, if low, they might be susceptible only to a few viruses specially adapted to the genetic constitution of that taxonomic group. A simplified numerica’ model can be made to illustrate the general nature of such a system. Suppose 4 viruses, p, q, r, and z can each infect species in a taxonomic group of plants if t#hey possess 3 infectivit’y factors matching complementary factors for SUSceptibility in the t,ested plant species. Table 6 shows viruses p and q with 2 fact,ors in common and one unshared; r, 2 factors unshared by p and one unshared by q.
Viruses p and z have no infectivity factors in common. Three examples are given in Table 6. In t’he taxonomic group of plants represented by (1) in the lower section of the table, there is a high probability of occurrence of each of the 6 complementary factors for susceptibility; in (2) the probabilities are uniformly lower; and in (3) some are high and some low. Assuming a random distribution of susceptibility factors among individual tested species, the expected numbers of susceptible species would in example (I), upper sect,ion, Table 6, range from 45 to 60%>, in example (2) from 3 to 7%, and in example (3) from G to 24%. In example (3) if one virus carried the infectivit’y factors ace and another the factors bdf there would be a contrast, not a general agreement in the degree of susceptibility of the taxonomic group to the 2 viruses. The expected percentages of susceptible species TABLE
6
ARITHMETIC MODEL OF THE HOST RANGE OF 4 VIRUSES IN 3 TAXONOMIC GROUPS OF PLANTS~
Virus Infectivity factors P cl r e
Taxo-
nomic group
abc
-
abd ade def
Per cent of susceptiblespeciesin taxonomicgroup (1) 56.P 45.3 60.3 54.0
(4
(3)
7.0 3.0 6.0 7.2
16.1 5.1 24.2 14.4
Probability of occurrenceof susceptibility factorsa’ - f’ in singlespecieswithin 3 taxonomicgroups -
(1) (2) (3)
a Each virus has 3 infectivity factors that must match 3 susceptibility factors in a host species for infection to occur. Above, percent of susceptible species; below, probability of occurrence of each factor for susceptibi1it.y in a single plant, species. bPercentage (P) of species with susceptibility factors a’b’c’ (lower section of table) corresponding to infectivity factors abc. P = 0.85 X 0.7 X 0.95 x 100 = X.6.
QUASI-GENETIC
MODEL
FOR
would he 64.6 and 3.6. These examples suggest, that several t’axonomic groups might react in a very similar way to 2 viruses sharing common infectivity factors (p, q) or to t,wo viruses with no common infectivity factors (p, z). Also that if some susceptibility factors were frequently present in a taxonomic group, and some were seldom found, susceptibility to one virus and insusceptibility to others might occur. In a crude way, therefore, this model illustrates correlations and contrasts of host range that’ commonly exist. They will be discussed further in another paper. Further points for comment are (a) the nature of the viruses chosen for this study, (b) the plant species inoculated, and (c) the requirements for host range studies from the statistical point of view. (a) The viruses chosen for this study the two main were diverse, including morphological types, rods and so-called spheres. TMV is generally transmitted AMV, and mechanically : SEV, CNV, TuJIV by aphids; TRV and LRV by nematodes; and TNV by one of the lower fungi. All are easily inoculated mechanically and multiply in parcnchymatous tissues. Many conclusions applicable to these viruses are very likely applicable to similar viruses, but t’here is no evidence here to show whether they apply t,o viruses transmitted by other kinds of vectors or viruses wit,h different tissue relations. (b) The plant species inoculated by the various authors whose results were examined were sometimes the same, but sometimes clifferent. Price (1940) inoculated about 140 species, not all with all viruses; Holmes (1946), 310. Fifty-two species were found that both had inoculated: Price with 4 viruses, TNV, TRV, CRIV, and Ai\IV; Holmes wit)h TMV and SEV. About, 50 more species inoculated by Price wit,h 1 to 4 viruses were also inoculat’ed by Holmes. The 5 turnip mosaic virus strains studied were inoculated to a number of species that neither l’rice nor Holmes used. There was most coincidence among Solanaceae, where many species were tested, and generally least in families from which few species were snmplcd. Fortunately, the smaller samples
PLANT
VIRUS
HOST
RANGES
623
provided data more likely to be representat,ive of the family than of the individual species. Hopefully, this increased the reliabilit,y of the general conclusions about group susceptibility. (c) There are several requirements for a statistical investigation t’hat are only partly satisfied in the data at present available. These include (i) recording negative as well as positive results. Some of the classic host range studies are unfit for quantitative treatment because only positive results were published. (ii) Often hhe reason given for not’ publishing results was that they were of uncertain validity. A degree of uncertainty does not invalidate such data for statistical treatment. The modern view is that in any case bhere is a t’hreshold for detection of viruses in plant tissues. Below the threshold t’here may be undetectable infections in single cells or small groups of cells, and these will have to be classed witsh examples of complete immunity. Confounding two such classes is unimportant so long as one maintains a consistent criterion for judging infection. Holmes (1946) recorded as positive only those t’ested species giving more than 10 lesions in back inoculations to Nicotiana glutinosa. (iii) Holmes also set a standard in recording the type of response for each host species. The types of reaction may be indispensable for decisions on relative susceptibility. The 4 basic reactions to inoculation appear to be (a) complete immunity, (b) subliminal infections, (c) local multiplication in inoculated leaves only, (d) systemic invasion. Classes a and b are confounded. Other classes representing degrees of susceptibilit,y may be added to these 4 (Holmes, 1946). It is legitimat,e for statistical purposes t,o make a division at any level so long as the classes separated represent real differences in susceptibility, or in host reactions. (iv) The complexit,y of host relations is such that it is advisable t,o use well defined strains of viruses in host range studies. The example of TRW and cucurbit mosaic shows that strains of one virus can vary as widely in host range as unrelated viruses; and TMV and SET’ show how closely the host ranges of unrelated viruses may correspond. A mixture of st’rains, unless it is a
624
BALD
AND
feature of the experimental design, might lead to confusion. (v) The sampling of test species for experimental host range st,udies is both a taxonomic and statistical problem. The numbers of species in single families range from less than 10 to more than 20,000. A comprehensive study of host range would demand first a crit’ical selection of families among more than 400, t,hen of representative tribes, genera, and species, a final selection of species, and possibly of genotypes within species. Obtaining seed and growing test plants would remain a major problem. Seed of crop plants, ornnmentals, and botanical collections are most readily obtainable, but t’hey represent a small fraction of all species. Here a systematist could help by reducing the ideal t’o the possible and a statistician by prescribing a sampling technique and other elements of experimental design. Sampling will become simpler as problems become more confined, but the same kind of advice will be essential even in more restricted host range studies. REFERENCES BALD, J. G. (1958). Host reactions to two viruses in Nicotiana and the related genera Cestr?Lm and Plant Physiob. 33, Suppl. xli. Petunia. (Abstract). BALD, J. G. (1969). Forms of tobacco mosaic virus. Nature 188, 645-647. BALD, J. G., and TINSLEY, T. W. (1967). Virus reactions and taxon0m.y. Xature in press. BESSEY, C. E. (1915).‘ The phylogeny and tax-
TINSLEY onomy of Angiosperms. Ann. Missouri Botan. Gardens 2, 109-164. FISHER, R. A., and YATES, E. (1945). “Statistical Tables for Biological, Agricultural, and Medical Research.” Oliver and Boyd, Edinburgh. GRANT, T. J. (1934). The host range and behavior of the ordinary tobacco mosaic virus. Phytopathology 24, 311-336. HOLLINGS, M. (1957). Anemone mosaic-a virus disease. flnn. ilppl. Biol. 45, 44-61. HOLMES, F. 0. (1938). Taxonomic relationships of plants susceptible to infection by tobaccomosaic virus. Phytopathology 28, 5%6G. HOLMES, F. 0. (1940). A comparison of the experimental host ranges of tobacco-etch and tobacco-mosaic viruses. Phytopathology 36. 643-659. HUTCHINSON, J. (1955). “British Flowering Plants.” P. R. Gawthorn Ltd., London. HUTCHINSOK, J. (1959). “The Families of Flowering Plants,” Vol 1, Dicotyledons; Vol. 2, Monocotyledons. Oxford Univ. Press (Clarendon), London and New York. LARSEN, R. H., and WALKER, J. C. (1939). A mosaic disease of cabbage. J. ;Igr. Res. 59, 367-392. MEZ, C., and ZIEGENSPECK, H. (1926). Der Kiinigsberger serodiagnostische Stammbaum. Botan. Arch. 13, 483-485. PRICE, W. C. (1940). Comparative host range of six plant viruses. Am. J. Botany 27, 530-541. TOMPKINS, C. 111. (1938). A mosaic disease of turnip. J. Agr. Res. 57, 589-602. TOMPKINS, C. M., GARDNER, M. W., and THOMAS, H. R. (1938). Black ring, a virus disease of cabbage and ot,her crucifers. J. Agr. Res. 57, 929943.