Split sex ratio due to maternal condition

In females of haplodiploid animals, female production requires fertilisation, whereas male production does not. Therefore, haplodiploid species can display extraordinary sex ratios. Constrained sex allocation occurs when a female cannot produce daughters. This can be due to virginity but may also occur after mating due to insemination failure, selfish genetic elements or physiological constraints. Here, we investigated the mechanism underlying constrained sex allocation in Pezothrips kellyanus. In this species some mated females produce highly female-biased broods, yet, for unknown reasons, others produce extremely male-biased broods. Using crossing experiments controlled for maternally inherited endosymbionts we confirmed that constrained females were successfully inseminated. Furthermore, male-biased offspring production was not paternally inherited, ruling out paternal sex ratio elements previously identified as sex ratio distorters in some parasitoid wasps. Next, we excluded mating time and paternal fitness effects (male size) on sex allocation. However, we found that constrained sex allocation only occurred in small females producing smaller eggs than large females producing larger eggs and female-biased broods. Consequently, the bimodal sex allocation pattern correlates with maternal condition, and may have arisen (adaptively or non-adaptively) in association with an egg size-mediated fertilization mechanism recently detected in some haplodiploids, with egg size positively affecting fertilization success.

在单倍二倍体（haplodiploid）动物的雌性个体中，雌性后代的产生依赖受精过程，而雄性后代的产生则无需受精。因此，单倍二倍体物种可展现出异常的性比格局。当雌性个体无法产生雌性后代时，便会出现受限性分配（constrained sex allocation）现象，该现象既可因雌性未交配（处女态）引发，也可在交配后因受精失败、自私遗传元件或生理限制而发生。本研究以凯利蓟马（Pezothrips kellyanus）为对象，探究其受限性分配的潜在机制；该物种中部分已交配的雌性可产出雌性占比极高的后代群，然而另有部分雌性会产出雄性占比极高的后代群，其背后原因至今不明。通过开展针对母系遗传内共生菌（endosymbionts）的控制杂交实验，本研究证实出现受限性分配的雌性个体已成功完成受精；此外，雄性偏向型后代的产生并非父本遗传，由此排除了此前在部分寄生蜂中发现的、可作为性比干扰因子的父本性比因子（paternal sex ratio elements）。随后，本研究排除了交配时长以及父本适合度效应（雄性体型）对性分配的影响，但研究发现，受限性分配仅发生于体型较小、所产卵粒尺寸更小的雌性个体中，而体型较大的雌性所产卵粒尺寸更大，其后代群的雌性占比更高。综上，这种双峰性分配格局与母体状态密切相关，其演化起源（适应性或非适应性）或与近期在部分单倍二倍体物种中发现的、由卵粒尺寸介导的受精机制相关——卵粒尺寸对受精成功率存在正向影响。