Protein variation and systematics of three subgenera of Malayan rats (Rodentia: Muridae, Genus Rattus Fischer)

Comp. Biodwm. Physiol.. Vol. 04B. pp. 329 to 337

0305-04ql 79 11014)329502.00,0

Pergamon Press Lid 1979. Printed in Great Britain

PROTEIN VARIATION AND SYSTEMATICS OF THREE SUBGENERA OF MALAYAN RATS (RODENTIA: MURIDAE, GENUS R A T T U S FISCHER) K. L. CHAN,S. S. DHALIWALand H. S. YONG Department of Genetics and Cellular Biology, University of Malaya, Kuala Lumpur, Malaysia (Received 12 March 1979) Abstract--l. Nine erythrocyte proteins coded by a separate locus each were analysed in and among seven Malayan species of Rattus belonging to three subgenera. 2. Electrophoretic data obtained confirm the specific status of the seven taxa and divide the seven species into three groups which correspond with Ellerman's (1949) subgenera Stenomys, Maxomys and Leopoldamys. 3. A comparative study together with 11 other species of Malayan Rattus previously analysed show that, with few exceptions, the overall relationships among the 18 species based on electrophoretic data correspond well with conclusions based on morphological evidence. 4. Malayan species of Rattus are relatively very diverse genetically (S = 0.27, range 0.01-0.94).

INTRODUCTION cameroni in his huang-fulvescens group and R. cremorEighteen species of Rattus are found in the Malay iventer in the cremoriventer group. Yong (1969) Peninsula (Chasen, 1940; Ellerman & Morrison-Scott, showed that the karyotype and diploid chromosome 1955; Harrison, 1961; Medway, 1978). According to number of these three species are similar (2n = 46). Ellerman (1949), Malayan species of Rattus belong to The three species were considered to be closely five of the seven subgenera recognized, namely, related and were assigned to a single species group. Misonne (1969) raised Ellerman's subgenus Maxomys Rattus, Stenomys, Maxomys, Lenothrix and Leopoldamys. Biochemical studies on the component species to generic status and regarded the taxa cameroni and of the subgenera Rattus and Lenothrix have been bukit as species of the genus Maxomys (cremoriventer was not considered). reported (Chan, 1977; Chan et al., 1978). R. sabanus and R. edwardsi represent the only two The subgenus Stenomys comprise the Malayan species R. bowersii and R. muelleri; the subgenus species in the subgenus Leopoldamys (Ellerman, 1949). Maxomys the species R. cremoriventer, R. cameroni While Bonhote (1903) ascribed separate subgroup (formely R. fulvescens) and R. bukit (formerly R. nivi- status to the two species, Ellerman (1949) and Tate renter) (see Medway & Yong, 1976; Medway, 1978) (1947) regarded the two species to be closely related and the subgenus Leopoldamys the species R. sabanus and placed them together in the edwardsi-sabanus group. Yong (1969) also regarded the two species to and R. edwardsi (Ellerman, 1949). Ellerman (1949) divided Stenomys into four species be closely related on the basis of cytological evidence groups on the basis of skull characters. R. bowersii (both 2n = 42) and assigned them to a single species was assigned to the bowersii group while R. muelleri group. Misonne (1969) retained Ellerman's subgenus was placed in the callitrichus group. Both Bonhote Leopoldamys. However, according to Misonne (1969), (1903) and Tate (1947) also regarded bowersii and this subgenus should also include R. whiteheadi and muelleri as members of separate species groups on the R. rajah of Ellerman's subgenus Lenothrix. From the above summary of the systematics of the basis of morphological characters. Employing cytological characters, Yong (1969) showed that the two component species of the three subgenera of Malayan species are different in their karyotype and diploid Rattus, it appears that there are some disagreement chromosome number (R. bowersii, 2n = 40; R. muel- with regard to the relationships among component leri, 2n = 42) and suggested that the two species be species within and between subgenera. Previous bioassigned to separate species groups. Misonne (1969) chemical studies on two subgenera of Malayan rats, re-evaluated the systematics of African and Indo--Aus- Rattus (Chan, 1977) and Lenothrix (Chan et al., 1978) tralian Muriade employing mainly dental characters. have shown that electrophoretic data have been useful The subgenus Stenomys was retained by Misonne in the elucidation of relationships among the species (1969), however, R. bowersii and R. muelleri were of Rattus studied. With the data obtained, it had been regarded as members of the subgenus Bullimus in- possible to solve some of the discrepancies between stead. conclusions based on morphological and cytological The subgenus Maxomys was divided into five evidence with respect to the relative affinities among species groups by Ellerman (1949) and Malayan the species of Rattus considered. The present investispecies of this subgenus were all included in the nivi- gation aims at a comparative study of Malayan renter group. Bonhote (1903) placed the three species species of Rattus belonging to the subgenera in his derdoni group but assigned each of the three Stenomys, Maxomys and Leopoldamys employing the species to different subgroups. Tate (1947) placed R. same electrophoretic markers used previously in the c s.P t,4,4s , 329

K. L. CHAN,S. S. DHALIWALand H. S. YONG

330

comparative study of component species of the subgenera Rattus and Lenothrix such that the interrelationships among all 18 species of Malayan Rattus may be compared to obtain a better insight into the systematics of Malayan Rattus.

MATERIALS

AND METHODS

Species of Rattus belonging to the three subgenera were all trapped from the wild from various localities: R. bowersii

R. muelleri R. cremoriventer R. cameroni R. bukit R. sabanus

R. edwardsi

13 specimens from Ulu Gombak Forest Reserve, S¢langor; 1 from Subang Forest Reserve, Selangor, and 1 from Cameron Highlands, Pahang. 26 specimens from Subang Forest Reserve, S¢langor and 2 from Ulu Gombak Forest Reserve, Selangor. 9 specimens from Ulu Gombak Forest Reserve, S¢langor. 9 specimens from Cameron Highlands, Pahang. 5 specimens from Kedah Peak, Kedah. 11 specimens from Ulu Gombak Forest Reserve, Selangor; 6 from Sungai Buloh, Selangor and 4 from Subang Forest Reserve, Selangor. 3 specimens from Genting Highlands, Pahang and 2 from Cameron Highlands, Pahang.

The experimental methods employed and the electrophoretic markers analysed were identical to those previously described in the study of rat species of the subgenus Rattus (Chan, 1977). Hemolysates of each of the species studied here were firstly analysed for intraspecific variation at each of the nine markers. Hemolysates of the seven species belonging to the three subgenera investigated were then analysed under identical conditions of electrophoresis for interspecific variation in electrophoretic mobility and pattern at each of the markers. In a similar manner, they were then compared in various combinations with hemolysates of species belonging to the subgenera Rattus and Lenothrix. RESULTS

The electrophoretic phenotypes of the nine erythrocyte markers observed in and among the seven specie., of Rattus investigated are illustrated in Fig. 1. For each of the species, only phenotypes determined by alleles with frequencies greater than 0.05 are illustrated. With respect to malate dehydrogenase (MDH), all the species studied here with the exception of R. bukit are monomorphic for this marker. The two forms of MDH observed in R, bukit are assumed to be determined by two alleles at the locus controlling MDH. The mobility of the MDH-I phenotype of R. bukit is identical to the MDH phenotype observed in R. bowersii, R. muelleri, R. cremoriventer and R. cameroni and also to the common MDH phenotype observed in all component species of the subgenera Rattus and Lenothrix. R. sabanus and R. edwardsi exhibit a similar MDH phenotype which is specific to members of these two species only. Interspecific variation of acid phosphatase (AP) was not observed in any of the seven species. Two different AP phenotypes were observed among the seven

species. R. cremoriventer, R. cameroni, R. bukit, R. sabanus and R. edwardsi exhibit a similar AP phenotype which is identical to that observed in R. canus. R. bowersii and R. muelleri are similar for the other AP phenotype which is identical to that observed in all component species of the subgenus Rattus. No intraspecific variation of superoxide dismutase (SOD) (previously designated tetrazolium oxidase) was detected in any of the seven species. R. cremorirenter possesses a SOD phenotype which is species specific. R. cameroni and R. bukit are similar for a SOD phenotype which is identical in mobility to that observed in R. rajah. The four remaining species, R. bowersii, R. muelleri, R. sabanus and R. edwardsi are similar for a SOD phenotype identical to that observed in all members of the subgenus Rattus and in R. surifer, R. whiteheadi and R. canus. A number of adenylate kinase (AK) isozymes were observed in each of the species studied. There is a set of electrophoretically fast isozymcs with strong AK activity and a slower set with relatively weak activity which does not consistently stain up clearly. For practical purposes, only the cathodal set of isozymes are considered for comparative purposes and this set of isozymes is assumed to be determined by a single gene locus. Polymorphism of AK was observed in R. bowersii, the two common AK phenotypes detected being presumably homozygous for two different AK alleles. The more common AK-1 phenotype of R. bowersii is identical to that observed in R. muelleri and also in R. annandalei. A different AK phenotype common to R. cremoriventer, R. cameroni, R. bukit, R. sabanus and R. edwardsi is also seen in R. surifer. Only products of the A locus controlling lactate dehydrogenase (LDH) of erythrocytes are consistently present in all the Rattus species so far studied (Chan, 1977; Chan et al., 1978). This is also true for the seven species presently investigated. Polymorphism of LDH was not detected in any of the seven species. Interspecific comparison show that R. bowersii and R. muelleri possess a similar LDH phenotype which is identical to that observed in R. annandalei. A different LDH phenotype common to R. cremoriventer, R. cameroni, R. bukit and R. sabanus is also seen in R. canus. R. edwardsi exhibits a unique LDH phenotype not seen in other species of Rattus. Polymorphism of 6-phosphogluconate dehydrogenase (6PGD) was detected in R. cremoriventer and R. sabanus. The more common 6PGD-1 phenotype of R. cremoriventer is identical to the 6PGD phenotype observed in R. cameroni and R. bukit only. The 6PGD phenotype of R. edwardsi has an electrophoretic mobility similar to the phenotype determined by the less frequent 6PGD allele in R. sabanus. The mobility of the 6PGD phenotype of R. muelleri is different from that of R. bowersii but identical to that observed in R. annandalei. Interspecific variation of nucleoside phosphorylase (NP) was not detected in any of the seven species. The NP phenotypes observed in R. cremoriventer, R. cameroni and R. bukit have identical mobilities and a r e also similar to that observed in R. canus. R. muelleri exhibits a NP phenotype of identical mobility to that observed in R. annandalei only. The NP phenotypes of R. bowersii, R. sabanus and R. edwardsi are species specific.

Protein variation and systematics in Malayan rats MDH

331

LDH

6P6D CZ) C ~ (~

C~

C2~ C:~

c~CZ~

C:Z~ C:Z~ CZ~ CZ~ c~

B C D

E

F G

~ A

~ B

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C

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F

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NP ~::~ c:~, ~

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~ O ~ C D O 0 0 0

~

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A

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~

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B

C

D

E

F

G

A

B

C

D

E

F

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A

e

C

O

E

F

G

Fig. 1. Diagrams illustrating the observed electrophoretic variation of the nine markers analysed in and among component species of the subgenera Stenomys, Maxomys and Leopoldamys. A-R. bowersii, B-R. rtmelleri, C-R. cremoriventer, D-R. cameroni, E-R. bukit, F-R. sabanus, G-R. edwardsi.

All the seven species are monomorphic for phosphohexose isomerase (PHI). Interspecific comparisons show that R. cameroni and R. bukit exhibit identical P H I phenotypes, the other five species exhibit species specific P H I phenotypes. For practical purposes, variation of hemoglobin (Hb) in and among the species of Rattus studied is considered to be determined by a single gene locus (Chan, 1977; Chan et al., 1978). Polymorphism of H b was not observed in any of the seven species and each of them exhibits a species specific H b phenotype. Table 1 summarizes the allele frequencies at loci which are considered to be polymorphic in the combined sample population of five of the seven species

studied according to the 5% criterion (Ayala et al., 1970). R. muelleri and R. edwardsi are monomorphic for all the nine erythrocyte markers which are considered to be determined by a separate gene locus each. Based on the 5% criterion, the average proportion of loci which are polymorphic in the seven species studied is 0.078. The average proportion of loci heterozygous per individual for these species is 0.013. All the seven species may be differentiated from one another by electrophoretic phenotypes of one or more of the markers analysed. Table 2 lists the markers for which species specific phenotypes were observed in the seven species.

Table 1. Allele frequencies at loci polymorphic in the species studied Species R. R. R. R.

bowersii cremoriventer bukit sabanus

Locus

Number

AK 6PGD MDH 6PGD

15 9 5 20

Allele frequencies AK 1 = 6PGD 1 = MDH 1 = 6PGD l =

0.94 0.94 0.90 0.75

AK 2 = 6PGD 2 = MDH 2 = 6PGD 2 =

0.06 0.06 0.10 0.25

K. L. CHAS, S. S. DHALIWALand H. S. YONG

332

:--

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>

--

0

z3

~

C~

t'T"

Table 2. Electrophoretic markers at which species specific phenotypes were observed in the species studied

~ E

E

n

R. R. R. R. R. R. R.

6I~

Species

Markers

bowersii muelleri cremoriventer cameroni bukit sabanus edwardsi

Hb, NP, PHI, 6PGD Hb, PHI Hb, NP HB Hb Hb, NP, PHI, 6PGD Hb, NP, LDH

For the elucidation of relationships among the species studied, the electrophoretic data obtained have been employed in two ways (Chan, 1977; Chan et aL, 1978). In the first approach each electrophoretic marker is treated as a classical taxonomic character and two species are considered to be similar for a particular marker only if the most frequent allele at the locus controlling the marker in the combined sample population of the two species is identical to one another. Referring to Fig. 1, it can be seen that the marker MDH separates R. sabanus and R. edwardsi as a phyletic group from the other five species. Similarly, the markers AP, AK and LDH segregate R. bowersii and R. muelleri as a group from the other five species. On the other hand, the marker NP separates R. cremoriventer, R. cameroni and R. bukit as a group from the other species. The relationships among the seven species based on such an approach may be represented as shown in Fig. 2A. In. the, phenetic approach, coefficients of genetic similarit)~ (S) for all paired combinations of species studied were computed according to Roger's formula (cited in Johnson & Selander, 1971) and the matrix of similarity coefficients obtained are shown in Table 3. Cluster analysis was performed using the unweighted pair-group method (Sokal & Sneath, 1963) and the derived dendrogram showing the relationships among the seven species of Rattus studied is illustrated in Fig. 2B. Rattus

AP AK

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DISCUSSION

[

Fig. 2. Dendrograms showing the relationships among the component species of the three subgenera derived by the classical approach (A) and by the phenetic approach (B) based on biochemical data.

The degree of genic variability in the seven species of Malayan rats belonging to the three subgenera of Rattus studied is relatively low (H = 0.013) compared with the mean H value of 0.04 obtained for six species and one subspecies of rats of the subgenus Rattus

Table 3. Matrix of similarity coefficients

R. R. R. R. R. R.

~owersii muelleri cremoriventer cameroni bukit sabanus

0.55

0.12 0.12

0.12 0.11 0.66

0.10 0.10 0.65 0.88

0.12 0.12 0.35 0.35 0.35

0.12 0.11 0.23 0.22 0.22 0.47

Protein variation and systematics in Malayan rats

333

(Chan, 1977) and a value of 0.037 for five species Bonhote (1903) that the three species be regarded as belonging to the subgenus Lenothrix (Chan et al., members of separate subgroups. 1978). Nevertheless, the mean value of H for all 19 With respect to R. sabanus and R. edwardsi, biotaxa of Malayan Rattus works out to be 0.029 chemical evidence suggest that they be regarded as (0.000-0.085) which is in general comparable to values members of separate species groups within the subobtained for various species of rodents, Dipodomys genus Leopoldamys as the S value between the two (Johnson & Selander, 1971); Peromyscus(Avise et al., species is 0.47 which is lower than that obtained 1974a, Avise et al., 1974b, Kilpatrick & Zimm.erman, between R. bowersii and R. muelleri which are con1975); Geomys (Selander et al., 1974); Siomodon,(John- sidered as members of different species groups. This is son et al., 1972) and Rattus rattus (Patton et al., 1975). in line with the opinion of Bonhote (1903). However, Genic variability was not detected in 4 of the 19 taxa Tate (1947), Ellerman (1949) and Yong (1969) assigned of Rattus studied, namely, R. t. tiomanicus, R. aroenti- the two species to a single species group. ranter, R. cameroni and R. edwardsi. In the remaining For the purpose of assessing the inter-relationships taxa, genic variation was detected in one or two of the between component species of the five subgenera of nine markers analysed and there appears to be no Malayan rats which have been studied, similarity correlation between the extent of genic variability coefficients were calculated for all paired combinaobserved in the 19 taxa of Rattus and the locality or "tions of the 19 taxa of Malayan Rattus and the matrix natural habitats from which they were trapped. The of similarity coefficients is shown in Table 4. Figure markers AP and LDH was found to be monomorphic 3A illustrates the dendrogram showing the relationin all 19 taxa of rats studied while electrophoretic ships among the 19 taxa of Rattus derived from data variation of the other seven markers are observed in in Table 4 by cluster analysis employing the unone or more of the Rattus species studied. In poly- weighted pair-group method of Sokal & Sneath morphism of most of the markers, two common (1962). This dendrogram obtained by the phenetic alleles were detected in populations of the various approach is very similar to that derived by the classispecies but for 6PGD, three alleles with frequencies of cal approach and it will be utilized for the purpose of 0.05 were detected in two taxa, R. t. jalorensis and R. comparison of relationships among Malayan Rattus norvegicus. based on other types of evidence. Figure 3B shows the There has been in general no dispute among overall relationships among Malayan Rattus based on workers with respect to the specific status of the seven cranial and external morphological characters (Ellertaxa belonging to the three subgenera of rats investi- man, 1949). gated. The electrophoretic data obtained confirm A comparison of dendrogram A with dendrogram their specific status and each of them may be differen- B in Fig. 3 shows a number of differences with respect tiated from component species of the subgenera to the inter-relationships between component species Rattus and Lenothrix at one or more of the electro- of the five subgenera of Malayan rats. Thus biochemiphoretic markers employed. cal evidence suggests that R. annandalei of Ellerman's In the elucidation of relationships among the seven subgenus Rattus is more closely related to the comspecies studied, the application of electrophoretic data ponent species of the subgenus Stenomys (S = 0.675) by either the classical or phenetic approach clearly than to component species of Rattus (S = 0.345). In divides the seven species into three groups: addition, R. annandalei is seen to be more closely related biochemically to R. muelleri (S = 0.74) than 1. R. bowersii and R. muelleri the latter is to R. bowersii (S = 0.55). Yong (1969) 2. R. cremoriventer, R. cameroni and R. bukit showed that the chromosome number of R. muelleri 3. R. sabanus and R. edwardsi and R. annandalei is 2n = 42. Thus biochemical eviwhich correspond with Eilerman's subgenus dence and cytological evidence based on chromosome Stenomys, Maxomys and Leopoldamys respectively numbers are not in conflict in this respect. Misonne delineated on the basis of skull characters (Ellerman, (1969) did not study R. annandalei. In t.he subgenus Lenothrix, it has been shown that 1949). In the subgenus Stenomys, the present results sup- with the exception of R. whiteheadi and R. inas which port morphological and cytological evidence that R. are relatively closely related to one another, the other bowersii and R. muelleri may be regarded as members species in the subgenus, R. rajah, R. surifer and R. of separate species groups. The S value between the canus are relatively distantly related (Chan et al., 1978). From Fig. 3A, it can be seen that R. canus is two species is 0.55. In the subgenus Maxomys, biochemical evidence more closely related biochemically to the component suggests that the component species be divided into species of Ellerman's subgenus Maxomys than to any two species groups; R. cremoriventer belonging to a other species of Malayan Rattus. This finding is no species group separate from the other comprising R. doubt in conflict with conclusions based on other cameroni and R. bukit. Tate (1947) regarded R. cre- types of evidence. Tate (1947) placed R. canus in a moriventer and R. cameroni as members of different separate group and Misonne (1969) regarded canus as species groups while Ellerman (1949) and Yong (1969) a species of the genus Lenothrix. Yong (1969) also regarded the three species as members of the same regarded R. canus to be unrelated cytologically to the species group. Although the biochemical data component species of Ellerman's Lenothrix and other obtained show that R. cameroni and R. b~kit are more Rattus species. Nevertheless Yong (1969) showed that closely related to one another (S = 0.85) than .either is R. canus has a chromosome number of 2n = 46 which to R. cremoriventer (S = 0.65), the averag~ S value • is identical to the chromosome number seen in the among the 3 species is relatively high (S = 0.73). The three component species of Ellerman's subgenus present data does not however support the opinion of Maxomys. Although similarity in chromosome

_-

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R. r. diardii R. t. tiomanicus R. t. jalorensis R. argentiventer R. exulans R. narvegicus R. annandalei R. bowersii R. muelleri R. cremariventer R:cameroni R. bukit R. rajah R. surifer R. whiteheadi R. inas R. canus R. sabanus

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0.31 0.34 0.35 0.34 0.35 0.35 0.56

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Protein variation and systematics in Malayan rats

o =

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Fig. 3. Dendrograms showing the overall relationships among the 18 species of Malayan Rattus based on biochemical evidence (A) and based on cranial and external morphological characters (B).

number need not imply close relationship, it may be noted that in the subgenera Rattus, Maxomys and Leopoldamys, relatively close relationship between component species of each of these three subgenera is correlated with identical chromosome numbers. Biochemical evidence thus suggests that R. canus be grouped together with R. cameroni, R. bukit and R. cremoriventer but belonging to a separate species group. The present consideration of all 18 species of Malayan Rattus clearly does not support the suggestion of Misonne (1969) that R. whiteheadi and R. rajah of Ellerman's subgenus Lenothrix be grouped together with R. sabanus and R. edwardsi in the subgenus Leopoldamys. Biochemical evidence support other morphological and cytological evidence that R. sabanus and R. edwardsi as a group are relatively distantly related to other Rattus species. R. whiteheadi is relatively closely related to R. inas and the two species are relatively distinct from other species of Rattus. Den-

drogram 3A also shows that R. rajah and R. surifer are each relatively distantly related to other species of Rattus as previously suggested by Yong (1969, 1972). On the basis of the electrophoretic data obtained here, Malayan species of Rattus may be divided into seven groups as shown in Table 5 derived from dendrogram 3A. Within the groups the component species may be subdivided into one or more subgroups based on evidence previously discussed. A comparison of the two dendrograms in Fig. 3 shows that the proposed classification based on electrophoretic data essentially retains all the five subgenera proposed by Ellerman (1949) with the exception of a few changes previously discussed. The groups of species proposed may thus be equated to subgcnera of Ellerman and in this respect results of the present study do not support Misonne's (1969) suggestion that canus be regarded as a species of a different genus or that only Ellerman's subgenus Maxomys be raised to generic status.

K. L. CHAN,S. S. DHALIWALand H. S. Yo•o

336

Table 5. Classification of Malayan Rattus based on electrophoretic characters Group I ( = subgenus Rattus) Subgroup (a) R. r. diardii, R. t. tiomanicus, R. t. jalorensis, R. ar,qentiventer, R. exulans (b) R. norvegicus Group II ( = subgenus Stenomys) Subgroup (a) R. annandalei, R. muelleri (b) R. bowersii Group Ill (= subgenus Maxomys) Subgroup (a) R. cameron£ R. bukit (b) R. cremorirenter (c) R. canus Group IV (= subgenus Lenothrix) R. whiteheadi, R. inas

Group V (= subgenus Leopoldamys) Subgroup (a) R. sabanus (b) R. edwardsi Group VI g. surifer

Group VII R. rajah

It has previously been shown that Rattus species belonging to Ellerman's subgenera Rattus and Lenothrix are very diverse genetically. Table 6 shows the mean and range of similarity values between congeneric species of some rodents including the Rattus species studied. A comparison of the data in the table shows that the 18 species of Malayan Rattus studied exhibit the lowest mean coefficient of similarity. In addition, the widest range of similarity values are also observed between Rattus species. This implies that Malayan species of Rattus are comparatively very diverse genetically, comprising species which are very similar as well as those which are relatively very different genetically. Further, it can be seen from Table 6 that the mean and range of similarity values between component species of each of the various groups or subgenera of Rattus proposed are comparable to values observed between species of other genera of rodents. This no doubt suggests that the low mean

and wide range of similarity values between the 18 species of Rattus studied must be attributed to the large genetic differences between species of the different groups of Rattus. The average coefficient of similarity between pairs of the seven subgenera of Rattus listed in Table 5 isO.18 (0.08-0.34). This value is comparable to the value of 0.16 (0.07-0.19) obtained by Johnson & Selander (1971) between species of Dipodomys and Peroganthus hispidus. The present consideration of all 18 species of Malayan Rattus thus confirm the earlier suggestion (Chan et al., 1978) that species of the genus Rattus are very diverse genetically. It also shows that the various groups or subgenera of Malayan Rattus suggested are more equivalent to genera of other rodents with respect to the degree of genetic divergence. There is however no intention here to propose that these groups of Malayan Rattus be accorded generic status on the basis of the present results. This study has

Table 6. Roger's coefficientof genetic similarity (S) between various congeneric species of rodents Taxa

Mean and range of S values

References

Dipodomys 11 species

0.61 (0.31-0.89)

Johnson & Selander (1971)

0.66 (0.34-0.99)

Avise et al. (1974b)

0.76 (0.76-0.77)

Johnson et al. (1972)

0.77 (0.764).77)

Selander et al. (1969)

0.84 (0.83-0.86)

Patton et al. (1972)

0.27 (0.01-0.94)

Present investigation

0.71 (0.50-0.94)

Present investigation

0.63 (0.55--0.79)

Present investigation

0.63 (0.52-0.88)

Present investigation

0.69

Present investigation

0.47

Present investigation

Peromyseus

16 species Sigmodon

2 species Mus

2 semispecies Thomomys

2 semispecies Rattus

18 species Group I (Rattus) 5 species Group II (Stenomys) 3 species Group III (Maxomys) 4 species Group IV (Lenothrix) 2 species Group V (Leopoldamys) 2 species

Protein variation and systematics in Malayan rats examined only Malayan species of Rattus belonging to five of the seven subgenera recognized in this very large genus. Further work involving species of other subgenera of Rattus and closely related genera employing additional electrophoretic markers is required to confirm the present finding with respect to the genetic divergence of the genus Rattus. Nevertheless with regard to the relative affinities among the Malayan species of Rattus studied, results based on electrophoretic evidence have been shown to correspond well in general with those based on cranial and external morphological characters and most of the discrepancies between classification of Malayan Rattus based on other types of evidence could be resolved by the biochemical data obtained. Acknowledgements--The authors wish to thank Dr Liro Boo Liat of the Institute for Medical Research, Kuala Lumpur, for the supply of some of the rats utilized in this study. The excellent assistance of Mr Asokan a/l Chooli Krishnan in the field and in the laboratory is gratefully acknowledged. This research was supported by a research grant (Vote F) from the University of Malaya, Kuala Lumpur REFERENCES AVISEJ. C., SMITHM. H., SELANDERR. K., LAWLORT. E. & RAMSEY P. R. (1974a) Biochemical polymorphisms and systematics in the genus Peromyscus. V. Insular and mainland species of the subgenus Haplomylomys. Syst. Zool. 23, 226. AVlSE J. C., SMITH M. H. & SELANDERR. K. (1974b) Biochemical polymorphism and systematics in the genus Peromyscus. VI. Systematic relationships in the boylii species group. J. Mammal. 55, 751. AYALAF. J., MOURAOC. A., PEREZ-SALASS., RICHMONDR. C. & DOBZHANSKYT. (1970) Enzyme variability in the Drosophila willistoni group. I. Genetic differentiation among sibling species. Proc. HatH. Acad. Sci. U.S.A. 67, 225. BONHOTEJ. L. (1903) Report on the mammals. In Fasciculi Malayenses, Zoology Pt. 1. The University Press of Liverpool. CnAN K. L. (1977) Enzyme polymorphisms in Malayan rats of the subgenus Rattus. Biochem. Syst. Ecol. 5, 161. CHAN K. L. (1978) Protein variation and systematics in Malayan rats of the subgenus Lenothrix. Comp. Biochem. Physiol. 59B, 345-351.

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