Caste development and sex ratio of the Ryukyu drywood termite Neotermes sugioi and its potential mechanisms

Abstract

Sex allocation is one of the most studied traits in evolutionary biology because its theoretical predictions match the empirical data. Here, using the Ryukyu dry-wood termite Neotermes sugioi, we investigated several factors that could bias the sex allocation in three populations (Okinawa, Ishigaki/Iriomote, and Yonaguni). Our survey showed that there were more queen-only colonies than king-only colonies in these populations, suggesting a longer lifespan of the queens than that of the kings. In this condition, sex-asymmetric reproductive value (SRV) theory predicts female bias, because even after the short-lived kings die, the long-lived queens can continue reproduction with their sons. However, sex allocation in this species seemed to be biased toward males. Furthermore, we examined the possibility of intrasexual competition among siblings (ICS). If ICS is the cause of the bias, the allocation is expected to change depending on the total investment in sexual offspring. However, the biomass of both male and female alates increased linearly with the increase in the total biomass of the alates in these populations. Thus, neither the SRV nor the ICS theory could explain the male-biased sex ratio of N. sugioi. On the basis of these results, we discuss the remaining possibilities in this species.

Figure 1

Relationship between caste developmental pathway and numerical sex ratios in the three populations. (a) The caste developmental pathway of Neotermes sugioi. E egg, L larva, Pse pseudergates, PS pre-soldier, S soldier, N nymph, A alate. (b) Comparisons of numerical sex ratios between castes. In each box, the cross symbol indicates the mean, bar in the box indicates the median, and the box top and bottom indicate the first and third quartiles, respectively. The asterisk next to each bar between boxes indicates p < 0.05 (glmm analysis with sequential Bonferroni), and "n.s." indicates “not significant”, i.e., p ≥ 0.05. The gray boxes indicate deviations from equal sex ratios at p < 0.05 (Wald tests in glm analysis with sequential Bonferroni), and the white boxes indicate p ≥ 0.05.

Figure 2

Characteristics of sex allocation and results of the test for intrasexual competition among siblings (ICS) and sex-asymmetric longevity corresponding to sex-asymmetric reproductive value (SRV) in the three populations. (a) Histogram of the proportion of biomass of male alates in the colonies of the three populations. (b) Plot analysis of the model of ICS. (c) Pattern-frequency of reproductives in the three populations. Solid lines (males) and dashed lines (females) in (b) show the correlations between the total biomass of alates in a colony and the biomass of alates of each sex in a colony, and numerals on the graph (c) indicate the number of colonies. CI confidence interval.