Influence of body size on fecundity and sperm management in the parasitoid wasp Anisopteromalus calandrae

Abstract

A large body size is considered to be advantageous to the reproductive success of females as a result of several factors, such as the allocation of more resources to reproduction and the efficient management of sperm transferred by males. In the present study, the effects of female body size, female mating status and additional food availability on fecundity and the offspring sex ratio are investigated in the parasitoid wasp Anisopteromalus calandrae Howard (Hymenoptera: Pteromalidae). Because of haplodiploid sex determination, females must fertilize eggs to produce female offspring but not to produce male offspring. As predicted, female fecundity and the number of female offspring are positively correlated with body size. However, although the volume of the spermatheca increases with female body size, the amount of sperm stored in the spermatheca is relatively constant, irrespective of body size. Consequently, larger females produce a greater proportion of male offspring, especially at the end of the oviposition sequence, suggesting that larger females that possess more resources for reproduction and produce a larger number of offspring are more likely to suffer sperm depletion. The results of the present study also show that mated females have an increased fecundity compared with virgin females, although the opportunity to feed on honey along with host feeding has no impact upon fecundity or the sex ratio.

Sperm-limited males continue to mate, but females cannot detect the male state in a parasitoid wasp

“…In the experiments below, the amount of sperm transferred to the females was estimated based on the proportion of daughters (fertilized eggs) produced by the females. The rationale for this approach is that studies of A. calandrae that have directly counted the number of sperm in female spermathecae have revealed that females efficiently use the stored sperm when they fertilize eggs in oviposition (Khanh et al 2005;Bressac et al 2009;Kasamatsu and Abe 2015). In our previous study, we confirmed that after laying clutches with mixed sexes (demonstrating that they have been inseminated), females produce only sons (unfertilized eggs), suggesting that they have run out of sperm.…”

“…In our previous study, we confirmed that after laying clutches with mixed sexes (demonstrating that they have been inseminated), females produce only sons (unfertilized eggs), suggesting that they have run out of sperm. This finding was confirmed by dissecting such females, which had no or very few sperm in their spermathecae (Kasamatsu and Abe 2015;unpublished data).…”

“…To obtain the individuals used in the following experiments, subcultures were created, in which five males and five females were moved from the stock culture into a new petri dish with 120 beans, with each bean containing a few hosts. To standardize the size of the experimental individuals, hosts that had developed for more than 14 days were used (Kasamatsu and Abe 2015). Prior to the emergence of the offspring generation, the 120 beans in each subculture were divided into units of three or four beans each that were kept in plastic cases (40 × 30 × 20 mm 3 ).…”

“…Therefore, individuals were ready for the experiments within 24 h after emergence. Adults are already sexually mature on the day of emergence: females are ready to accept copulation, and males possess a sufficient amount of sperm to inseminate females (Bressac et al 2009;Kasamatsu and Abe 2015). All the experiments were conducted under a constant temperature of 28°C and a photoperiod of L16:D8 h. To minimize observer bias, blind methods were used when recording all behavioral data in the following experiments.…”

“…Offspring production was examined for the first, third, fifth, and seventh females among those that mated successfully in the first eight successive mating and for females that mated 6 h and 24 h later. The detailed procedures for measuring offspring production are described elsewhere (Kasamatsu and Abe 2015). Briefly, a lone inseminated female was allowed to produce a clutch just after the 24 h later mating trial by laying eggs on a batch of cowpea seeds containing at least 100 bean beetle hosts for 4 days.…”