The traits of a biotype of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) from South India

Prod. Res. Vol. 21, No. 4, pp. 221-224, 1991 Printed in Great Britain. All rights reserved J. stored

0022-474x/91 $3.00 + 0.00 Copyright 0 1991 Pergamon Press plc

THE TRAITS OF A BIOTYPE OF CALLOSOBRUCHUS MACULATUS (F.) (COLEOPTERA :BRUCHIDAE) FROM SOUTH INDIA RODGER MITCHELL Department of Zoology, Ohio State University, Columbus, OH 43210, U.S.,4. (Received for publicarion 16 May 1991)

Abstract-The South India strain of Callosobruchus macularus (F.), which has been in culture for 10 yr, cannot be distinguished from three cultures established from beetles collected in South India in 1989. These cultures represent a geographic biotype of C. macularus that is widely distributed in the state of Tamii Nadu, India and is stable in culture. Females of the biotype disperse their eggs uniformly, lay eggs on larger beans first, have their oviposition inhibited if beans already carry eggs, and the larvae exhibit contest competition. Fecundity varies greatly within and between cultures. The extremes of fecundity have less effect on the contribution of offspring to the next generation than the elimination of intraspecific competition by selective oviposition.

INTRODUCTION

Laboratory strains of Callosobruchus maculatus (F.) differ greatly in the way their larvae and ovipositing females behave (Dick and Credland, 1986) and these differences are now well characterized (see Fujii et al., 1990, pp. 271-383). The most pressing question at present is: how did the differences between strains evolve? Three processes could be involved either independently or in combination: (1) laboratory strains may be descended from unrepresentative samples; (2) traits may evolve in the laboratory as a result of differences in culturing regimens or genetic drift or both; or (3) the traits of genetically distinct source populations persist in the laboratory. The South India strain (SI) is particularly interesting because it is an extreme deviant in two behavioral traits. Females disperse eggs uniformly (Messina, 1989) and the larvae resolve competition by communicating between burrows (Thanthianga and Mitchell, 1987). SI is a standard used by Toquenaga (1990) in his analysis of contest competition and by Messina (1989) in unravelling the genetics of oviposition behavior. In order to determine how the traits of SI might have originated, new cultures were established from infested beans collected in South India. I report the results of comparing the IO-yr old laboratory strain of SI to new cultures from the source population.

IDENTITY

OF

CULTURES

The new lines are designated cultures to distinguish them from the SI strain. The SI strain was established from a few hundred infested beans (black gram, Vigna mungo L., and mung, I/. radiara L.) obtained in Tirunelveli in July 1979. The beans had larvae in them but no exit holes, hence, the beetles were the first generation to infest the beans. One new culture (TRN) was from infested black gram purchased in January 1989 at the original SI locality. Infested black gram was collected in fields at Madurai and Vambon (150 and 250 km north of Tirunelveli, respectively). Beans harvested in November 1988 at the Tamil Nadu Agricultural College (Madurai) provided the founders of the Madurai (MDU) culture. The Vambon culture (VAM) came from beans harvested in January 1989 at the Agricultural Research Station near Vambon (Pudokottai District, Tamil Nadu). The cultures were maintained on their original host, black gram. Parasitic wasps and, more seriously, pyemotid mites, reduced the founders to 25-100 adults for the first two generations. This increased the risk of obtaining an unrepresentative sample from the source population. 221

RODGER MITCHELL

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METHODS All lineages have been cultured in the same way. Every generation >200 adults are sieved onto > 500 fresh beans and left for l-2 days. Egg densities are generally < 2 eggs/bean. The new cultures were maintained on their original host (black gram) and SI was cultured on mung. Black gram was purchased in Columbus, Ohio, U.S.A. and the mung beans were the Berkin variety from the Johnson Seed Co., Enid, Okla., U.S.A. The culturing and experiments were done at ambient conditions (22-26°C). A standard protocol (Mitchell, 1990) was used to measure fecundity, responses to numbers of beans, egg dispersion, and discrimination of bean weight at generation 4-5. Virgin females that developed alone in a bean were paired and given either 4, 20, or 100 beans for life. The oviposition data indicated how females dispersed their eggs and responded to numbers of beans. The uniformity of egg placement was measured by determining how many eggs had to be moved to convert the observed distribution to a uniform distribution of eggs. That number is the “errors” made by a female. The number of errors expected with random egg placement (the Poisson) was determined and an index of uniformity “U” calculated as: (Poisson errors-observed errors)/Poisson errors (Messina and Mitchell, 1989). Beans were sorted into sets with 0, 1, or 2 eggs and each set weighed. A second set of fecundity tests were run at generation 12 of the new cultures and generation 121 of SI. The Berkin variety of mung was used because it is one of the hosts being used in a current reference study of laboratory strains.

RESULTS Fecundity

At generations 45 (Table 1) the new cultures showed fecundities insignificantly higher (5-10 eggs) than SI. The standard errors of the mean exceeded 10% of the mean in two cultures and these two cultures diverged in fecundity by generation 12. VAM fecundity declined from 60 to 40 eggs and TRN increased from 60 to 84 eggs (Tables 1 and 2). SI fecundity remained at 55-58. The standard errors of the means (Tables 1 and 2) are similar indicating that the SI strain was as variable after 121 generations as the new cultures were after 4 generations. Responses to resource abundance

Females laid fewer eggs when given only a few beans. All lineages laid 15-24 eggs on 4 beans but VAM showed only a small reduction at intermediate resource densities of 20 beans. Egg dispersion

The pattern of egg dispersion was consistent among cultures (Table 2) with 75-85% of the females achieving uniformity when given 100 beans. Females given 20 beans maintained a uniform distribution up to 1.1-1.2 eggs/bean.

Table 1. Mean f SE number of eggs laid by virgin pairs of C. macularus given 4,20, and 100 black gram for generations 4-5 of the new cultures and generation 110of SI. The number of replicates for each number of hosts is given and a Single Classification ANOVA tested for similarities of fecundity for each number of hosts No. of eggs laid on Culture (replicates) SI (5, 5, 5) TRN (6, 7, 6) MDU (11, 5, 4) VAM (5, 5, 4) F

..

100 beans

20 beans

4 beans

55.6 f 5.13 60.3 f 7.19 65.4 f 3.22 60.4 + 7.03 0.553,,,*,, NS

31.0+ 1.64 40.8 f 5.30 42.0 f 5.62 53.6 + 2.56 3.767,~~ P i 0.05

14.8 f 2.06*** 20.3 + 7.09,’ 16.4 f 3.27**’ 24.2 f 5.67.’ 0.5030, 10 NS

Regressions for reduction of fecundity lP < 0.01; **P< 0.001

Table 2. The fecundity, errors (U), and weight discrimination of the new cultures at generation 12 and generation 121 of SI. Virgin pairs of C. maethus given 100 mung beans for life. Letters indicate means that are similar (P> 0.05). The arcsin-square root transformed U-values were tested for differences with a I-test. Letters indicate values that are similar under a r-test (P< 0.01) Culture (replicates) SI (IO) TRN (20) MDU (20) VAM (20j

Fecundity 58.0 f 84.1 * 62.0 f 40.6k

5.35, 3.77; 5.51h 3.18;*

Errors per female (U) 0.50 (0.97), 0.60 (0.97). 0.20 (0.98). 0.20(0.98);

Deviation (mg) of I-egg beans from av. wt +6.95 + i4.54 f +6.25 + +2.89k

0.49, 4.27, 0.99. 0.53,

*I-tests show fecundity differs from generations 45 P c 0.01

The traits of a biotype of Callosobruchus maculatus (F.)

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Bean weight Eggs were laid on beans that were 4-7 mg above the average weight, except for VAM. VAM selected beans were 2.9 mg above average weight. The average was not a significant deviation, but most females (18/20 x2 = 12.8, P c 0.01, Table 2) selected larger beans. Mung beans with one TRN egg (Table 2) were separated by size to determine survival. The frequencies of emergences were similar; 72% emerged from 46 large (65-99 mg) mung and 78% from 45 small (3 l-50 mg) mung. Bean size had no apparent effect on larval survival. Competition All but one larvae was excluded when larvae were in competition

(n = 56).

DISCUSSION After 120 generations of laboratory culture, the behavioral traits of the SI strain match three cultures newly established from the source population. Fecundity is variable within and between the new cultures and in the established laboratory stock. Over the last 10 yr SI fecundity ranged from 92.0 + 2.21 (n = 9, generation 36) to 47.8 &-2.4 (n = 5, generation 102) and is now 55-58. Fecundity changed significantly from generation 4 to 12 in the new cultures. These changes may be the result ol genetic bottlenecks or genetic drift. If there is more than one egg on a bean, only one larva survives (Thanthianga and Mitchell, 1987). Females given 100 beans very nearly eliminated larval mortality by dispersing their eggs uniformly. The larval mortality was determined for all the females given 100 beans in these experiments. Of the 6934 eggs laid by 114 females, only 56 eggs were added to beans carrying an egg. One larva died when two larvae enter a bean. If a female laid her eggs at random (a Poisson was calculated for each female) larvae from 1871 eggs would die in competition. Hyperdispersion eliminated all but 3% of the deaths expected with random oviposition. When few beans were available, all females lay fewer eggs and the eggs they hold back could be laid at a later time (Credland and Wright, 1988; Thanthianga and Mitchell, 1990). Oviposition was inhibited when offspring were almost certainly likely to die in competition with established larvae. Contest competition may be a key trait that maintains the suite of oviposition traits. If larvae from eggs added to an occupied bean usually die in competition, uniform egg dispersion will be advantageous. Larval competition was reduced from that expected under random oviposition by 97%. Hyperdispersion appears to be a dominant genetic trait, with evidence of polygenic, epistatic non-additive components (Messina, 1989). Females in free ranging populations will have a potential advantage if they hold back eggs and disperse from localities in which their offspring are likely to die in competition with older larvae. That potential advantage can be realized if a female finds unexploited beans on which to lay the eggs she withheld. In my stock cultures females are moved to fresh beans after a large number of individuals have emerged. Their oviposition would be inhibited prior to transfer and the inhibition reversed when females are given fresh beans. Inhibition of oviposition under crowded conditions appears to be an advantageous trait under my culturing regimen as well as in nature. All the lineages selected larger beans for oviposition but the size of mung beans had no effect on larval survival. Size discrimination is found in all tested strains and is expressed most strongly in the African strains (Mitchell, 1990). Those strains show a density dependent increase in survival with bean size (Toquenaga, 1990). Fecundity varies and both increased (TRN) and decreased (VAM) from generation 4 to generation 12 in the laboratory. The number of eggs laid by a female may not be as important as where the eggs are placed. If the females in these experiments placed their eggs at random only 73% of the larvae would survive intraspecific competition. The observed survival is 99% because the eggs are placed nearly uniformly over the beans. Obviously, the fecundity of hyperdispersing females would have to fall by 26% before they would leave fewer offspring than random ovipositors. Fitness is determined by the interactions between fecundity and behavior. We may be able to explain the observed variation in fecundity when those interactions are understood.

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Females that withhold eggs when they encounter beans carrying eggs will contribute more offspring to the next generation if they find fresh beans. All the major oviposition traits, except fecundity, can be explained as the result of a selective advantage associated with the conditions in the field and laboratory environments. Credland’s (1990) proposal to recognize biotypes of C. maculatus was difficult to apply as long as the source of the differences between cultures was unknown. Those problems are resolved for the SI biotype. It is a true breeding geographic variant of C. maculatus. The laboratory strains from Africa are more variable in oviposition behavior (Mitchell, 1990) and have larvae that tolerate competition. There is less advantage for hyperdispersion if the larvae tolerate competition. Perhaps oviposition traits vary among these strains because a variety of oviposition tactics can evolve when the larvae show scramble competition. The large, over 200 mg, African cowpea cultivars support several larvae, but no more than 2-3 larvae can mature in the 40-60 mg cowpeas grown in South India. The most common hosts, black gram and mung, generally weigh less. Could the competitive traits of SI larvae be adaptations for dealing with the small-seeded Indian cultivars? The answer to this question may be lost in the history of the evolution of the pulses in India. CONCLUSIONS

The South Indian population of C. maculatus is a distinct geographical biotype that remains stable in culture. Broadly distributed in the State of Tamil Nadu, India, the SI biotype is distinguished by five major traits. (1) Larvae show a strict contest competition: as only one adult survives per bean; (2) females disperse their eggs almost uniformly over the available beans; (3) oviposition is strongly inhibited if the available beans carry eggs; (4) larger beans are selected for oviposition and (5) fecundity is variable, ranging from 40 to 90 eggs. Acknowledgements-I am indebted to Vice Chancellor S. Jayaraj of the Tamil Nadu Agricultural University for his enthusiastic support and to Professor P. C. Sundara Babu for his personal attention to this project. It was a special pleasure to work at the Madurai Campus thanks to the kind generosity of Professors M. S. Venugopal and K. Gunatiligaraj.

REFERENCES Credland P. F. (1990) Biotype variation and host change in bruchids: causes and effects in the evolution of bruchid pests. In Bruchids and Legumes: Economics, Ecology and &evolution (Edited by Fujii K., Gatehouse A. M. R., Johnson C. D., Mitchel R. and Yoshida T.), pp. 271-287. Kluwer Academic, Dordrecht, The Netherlands. C&land P. F. and Wright A. W. (1988) The effect of artificial substrates and host extracts on oviposition by Callosobruchus maculatus (F.) (Coleoptera: Bruchidae). J. stored Prod. Res. 24, 157-164. Dick K. M. and Credland P. F. (1986) Egg production and development of three strains of Callosobruchus macularus (F.) (Coleoptera: Bruchidae). J. stored Prod. Res. 23, 43-48. Fujii K., Gatehouse A. M. R., Johnson C. D., Mitchel R. and Yoshida T., Eds (1990) Bruchids and Legumes: Economics, Ecology and Coeoolution, 407 pp. Kluwer Academic, Dordrecht, The Netherlands. Messina F. J. (1989) Genetic basis for variable oviposition behavior in Callosobruchus maculatus (Coleoptera: Bruchidae). Ann. Em. sot. Am. 82, 792-796. Messina F. J. and Mitchell R. (1989) Intraspecific variation in the egg-spacing behavior of the seed beetle Callosobruchus maculatus. J. Insect Behav. 2, 727-742. Mitchell R. (1990) Behavioral ecology of Callosobruchus maculates. In Bruchids and Legumes: Economics, Ecology and Coeuolution (Edited by Fujii K., Gatehouse A. M. R., Johnson C. D., Mitchel R. and Yoshida T.), pp. 317-330. Kluwer Academic, Dordrecht, The Netherlands. Thanthianga C. and Mitchell R. (1987) Vibrations mediate the prudent resource exploitation by competing larvae of the bruchid bean weevil Callosobruchus maculatus. Ent. exp. Appl. 44, 15-21. Thanthianga C. and Mitchell R. (1990) The fecundity and oviposition behavior of a South Indian strain of Callosobruchus maculatus. Ent. exp. appl. 57, 133-142. Toquenaga Y. (1990) The mechanisms of contest and scramble competition in bruchid species. In Bruchids and Legumes: Economics, Ecology and Coeuolution (Edited by K. Fujii, Gatehouse A. M. R., Johnson C. D., Mitchel R. and Yoshida T.), pp. 341-349. Kluwer Academic, Dordrecht, The Netherlands.