What Fig Wasp Sex Ratios May or May Not Tell Us about Sex Allocation Strategies

Fig wasps (Hymenoptera, Agaonidae) have been widely utilized in studies of sex ratio evolution, especially local mate competition (LMC), and good fits have been obtained between empirical data and model predictions incorporating LMC and inbreeding effects. These models assume that foundress females within a patch (a fig) oviposit synchronously, producing roughly equal numbers of offspring with the same progeny sex ratios. Working with the fig wasp Eupristina belagaumensis, which pollinates the fig tree Ficus drupacea in India, we investigated whether these assumptions are valid, and then produced an alternative model which incorporates revised biological assumptions. Egg loads at adult emergence were compared with those remaining in females after they had completed their egg-laying and had died. As foundress numbers increased, so did variation in the numbers of eggs which the females managed to lay. In multi-foundress figs certain females (most likely the first ones to enter) often contributed almost complete egg loads while others (which may have entered when most of the oviposition sites had been utilised) contributed relatively few eggs. Assumptions of previous models were therefore violated. Our model assumes totally sequential oviposition by foundresses and differential contributions to broods. The predicted overall sex ratios of the fig wasp progeny are qualitatively similar to models based on LMC and inbreeding effects, despite being based on different biological assumptions, suggesting that previous models may have given the right answers for the wrong reasons. Explicit tests between the models are possible, as the sequential oviposition model predicts that the progeny sex ratios of individual foundresses should vary depending on where in the sequence they enter a fig.