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.

在单倍二倍体动物（haplodiploid animals）中，雌性个体的产生需要受精作用，而雄性个体的产生则无需此过程。因此，单倍二倍体物种可呈现出非同寻常的性比。限制性性别分配（constrained sex allocation）指雌性无法产生雌性后代的情况，该现象既可因雌虫处于处女态引发，也可在交配后因授精失败、自私遗传元件或生理限制而出现。本研究针对凯利棒蓟马（Pezothrips kellyanus）的限制性性别分配机制展开探究。该物种中，部分已交配雌虫的后代种群呈现显著的雌性偏向，然而另有部分雌虫的后代却为极端雄性偏向，其背后原因至今不明。通过控制母本遗传内共生体（maternally inherited endosymbionts）的杂交实验，我们确认存在限制性性别分配的雌虫已成功完成授精。此外，雄性偏向的后代产生并非由父本遗传，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的可能性。随后，我们进一步排除了交配时机以及父本适合度效应（雄虫体型）对性别分配的影响。但研究发现，限制性性别分配仅出现于小型雌虫中：这类雌虫所产的卵小于大型雌虫所产的卵，而后者的后代种群多呈雌性偏向。综上，双峰性别分配模式与母本状态密切相关，其起源或为适应性演化或为非适应性过程，可能与近期在部分单倍二倍体物种中发现的卵大小介导的受精机制有关——该机制表明卵大小对受精成功率具有正向影响。