Pollinating fig wasps’ simple solutions to complex sex ratio problems: a review

Local mate competition (LMC) favours female biased clutch sex ratios because it reduces competition between brothers and provides extra mating opportunities for sons. Fig wasps seem to fit LMC model assumptions and lay female-biased sex ratios as predicted. These female biased sex ratios increase fitness greatly. In line with predictions, their sex ratios become less female-biased as the number of mothers laying in the same fig increases. However, this variation results in comparatively small fitness benefits compared to just biased ratios and data suggest substantial mismatches with LMC theory. The mismatches are due to several factors. (1) Multiple foundresses typically lay too many daughters. (2) Single foundress sex ratios are explained by sequential oviposition and ladies-last models. (3) Mortality that typically exceeds 10% may decouple the link between primary sex ratios, the focus of model predictions, and secondary sex ratios of adult wasps that are counted by researchers. (4) Model assumptions are frequently violated: (a) clutch sizes are unequal, (b) oviposition may not be simultaneous (c) cryptic/multiple wasp species inhabit the same host, (d) foundress numbers are systematically undercounted, (e) inbreeding coefficient calculations are inaccurate, and (f) male wasps sometimes disperse. These data and calculations suggest that alternative explanations must be considered seriously. Substantial data show that wasps typically lay most of their male eggs first followed by mostly female eggs require a new approach. These “slope” strategies result in more accurate sex ratios that are automatically adjusted to foundress number, own and relative clutch sizes and to sequential clutches. This effect will alter sex ratios in all species once the egg capacity of a fig is crossed or when interference reduces clutch sizes. In addition to this passive response, the females of about half the studied species have a conditional response that reduces female bias under higher foundress numbers by laying more sons. Therefore, wasps seem to use a very simple strategy that increases their fitness. Natural selection could have optimized parameters of the slope strategy and possibly the existence of the slope strategy itself. Variation in the slope strategy that is the result of natural selection is adaptive. Research should therefore focus on quantifying variables of this slope strategy. Currently, it is unclear how much of the variation is adaptive as opposed to being coincidental by-products. Graphical Abstract

Offspring sex ratio shifts of the solitary parasitoid wasp, Trichopria drosophilae (Hymenoptera: Diapriidae), under local mate competition

Localmate competition (LMC) models predict a female-biased offspring sex ratio when a single foundress oviposits alone in a patch and an increasing proportion of sons with increasing foundress number. We tested whether the solitary pupal parasitoid, Trichopria drosophilae (Hymenoptera: Diapriidae), adjusted offspring sex ratio with foundress number when parasitizing Drosophila melanogaster pupae. Mean number of female offspring was higher than that of males, with a male proportion of 26 ± 16% when only one foundress oviposited. However, male proportion reached 58 ± 26%, 48 ± 22%, and 51 ± 19% in three-, five and seven-foundress cohorts. That the male proportion of offspring increased with foundress number is consistent with LMC models.

Multiple-foundress associations in a temperate population of Halictus ligatus (Hymenoptera; Halictidae)

Thirteen pleometrotic (multiple-foundress) nests of the primitively social sweat bee Halictus ligatus were excavated in the summer of 1984 at Victoria, near Toronto, southern Ontario. Subordinate foundresses were significantly smaller than both dominant females in pleometrotic nests and females that nested solitarily. Most subordinates were smaller than the workers that they helped to raise. These small females could have been surviving workers from the previous summer or the offspring of workers. It seems unlikely that they were malnourished reproductive brood individuals produced as a result of parental manipulation. In successful nests, the number of workers produced was positively correlated with the number of founding females such that productivity per foundress remained fairly constant. Pleometrotic nests also produced more reproductives than haplometrotic (single-foundress) ones. Subordinates may occasionally lay reproductive brood eggs. The increased productivity of multiple-foundress nests was not quite sufficient, by itself, to select for subordinate behaviour. The small subordinates had lower potential productivities in comparison to the larger females. This decreased reproductive potential, when combined with the increased productivity of pleometrotic nests, was sufficient to make subordinate behaviour selectively advantageous. When both factors are taken together, subordinate behaviour is selected for as long as the coefficient of relatedness between dominant and subordinate individuals is greater than 1/4. This indicates that high coefficients of relatedness are not necessary for pleometrosis to be selectively advantageous under the conditions found in this study. Dominant females may suffer increased reproductive competition from their numerous workers. This, plus the difficulty of ensuring association with siblings in spring, may be the reason why multiple-foundress associations were uncommon at this locality. The data presented here are compared with those from other studies of this species. The factors promoting pleometrosis in halictines are compared with those that result in multiple-foundress associations in temperate polistine wasps.

A LABORATORY STUDY ON THE SELECTIVE ADVANTAGE OF FOUNDRESS ASSOCIATIONS IN POLISTES FUSCATUS (HYMENOPTERA: VESPIDAE)

Overwintering adults of the social wasp Polistes fuscatus (Fabricius) were collected in the early spring and placed in nest boxes under controlled conditions. Colonies were initiated, and one-, two-, and three-foundress associations were formed. Successful single-foundress colonies were similar to successful multiple-foundress colonies in developmental time for immatures, nest size, and total production of offspring. Multiple-foundress colonies were more likely to produce offspring than were single-foundress colonies. Half of the foundress wasps failed to reproduce, either because the colony failed or because they were non-reproductive subordinates. Apparently, wasps that were likely to fail at reproduction tended to join colonies rather than to initiate colonies. The relevance of this data to hypotheses on the selective advantage of foundress associations is discussed.