Sex ratio in two species of Pegoscapus wasps (Hymenoptera: Agaonidae) that develop in figs: can wasps do mathematics, or play sex ratio games?

Abstract

The fig pollinating wasps (Hymenoptera: Agaonidae) have obligate arrhenotoky and a breeding structure that fits local mate competition (LMC). it has been traditionally assumed that LMC organisms adjust the sex ratio by laying a greater proportion of male eggs when there is superparasitism (several foundresses in a host). We tested the assumption with two wasp species, Pegoscapus silvestrii, pollinator of Ficus pertusa and Pegoscapus tonduzi, pollinator of Ficus eximia (= F. citrifolia), in the Central Valley of Costa Rica. Total number of wasps and seeds were recorded in individual isolated naturally colonized syconia. There was a constant additive effect between the number of foundresses and the number of males produced in the brood of a syconium, while the number of females decreased. Both wasp species seem to have precise sex ratios and probably lay the male eggs first in the sequence, independently of superparasitism and clutch size: consequently, they have a non-random sex allocation. Each syconium of Ficus pertusa and of F. eximia colonized by one foundress had similar mean numbers of females, males, and seeds. The two species of wasps studied do not seem to adjust the sex ratio when there is superparasitism. Pollinating fig wasp behavior is better explained by those models not assuming that females do mathematical calculations according to other females' sex ratios, size, number of foundresses, genetic constitution, clutch size or environmental conditions inside the syconium. Our results are in agreement with the constant male number hypothesis, not with sex ratio games. Rev. Biol. Trop. 57 (3): 605-621. Epub 2009 September 30.

The efficacy of natural selection in producing optimal sex ratio adjustments in a fig wasp species

“…Assuming that foundresses sharing a fig have equal clutch sizes, settle randomly on figs and that all figs produce the same number of females, irrespective of the number of foundresses, Herre [14] calculated the evolutionary stable strategy (ESS) sex ratio as r n à ¼ (n À 1)(2n h À 1)=n(4n h À 1), where n h is the harmonic mean number of foundresses since s = 1/n h (see electronic supplementary material for alternatives). Data from many fig wasp species are in line with these predictions, showing that mothers have a smaller fraction of sons when there are fewer foundresses and species with lower n h have fewer sons for a specific number of foundresses [14][15][16][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. However, several authors have questioned if LMC models are appropriate for fig wasps [15,22,24,27,[30][31][32][34][35][36][37][38][39].…”

“…Data from many fig wasp species are in line with these predictions, showing that mothers have a smaller fraction of sons when there are fewer foundresses and species with lower n h have fewer sons for a specific number of foundresses [14][15][16][20][21][22][23][24][25][26][27][28][29][30][31][32][33]. However, several authors have questioned if LMC models are appropriate for fig wasps [15,22,24,27,[30][31][32][34][35][36][37][38][39].…”

Evolutionary stable sex ratios with non‐facultative male‐eggs first sex allocation in fig wasps

“…Son production in these species would not be limited by the number of available oviposition sites (Kinoshita et al , Raja et al , Wang et al ) and any decrease in the total number of eggs laid would be at the expense of potential female eggs laid. Some fig wasp species’ foundresses have even been shown to oviposit a constant number of males (Ramírez‐Benavides et al ), and primary offspring sex ratios (the proportion of sons laid initially) are therefore simply a function of initial male number and how many oviposition sites are left for females. In these species, natural selection should act to optimize the number of initial male eggs for the most frequently encountered patch conditions.…”

Antennae and the role of olfaction and contact stimulation in mate recognition by males of the pollinating fig wasp Ceratosolen gravelyi (Hymenoptera: Agaonidae)

“…Another important finding of this study is that individual male pollinators may mate several times with the same female. This condition could result in a more female-biased sex ratios if sperm is limiting (Hamilton 1979;Ramírez-Benavides et al 2009;Peng et al 2014), since more gametes might be transferred over the successive matings. Occurrence of a male mating successively several times with the same female has also been documented to occur in the two pollinators of Ficus ottoniifolia in Gabon.…”