Klein Kora, Kokko Hanna
School of Biological Sciences, Institute of Evolutionary Biology, The University of Edinburgh, Edinburgh, United Kingdom.
Institute of Organismal and Molecular Evolution, Johannes Gutenberg University of Mainz, Mainz, Germany.
Evolution. 2025 Sep 12;79(8):1444-1455. doi: 10.1093/evolut/qpaf066.
Sex ratio theory usually assumes obligate sex; rare exceptions with facultative sex typically consider idiosyncratic cases of cyclic parthenogens. Here, we construct a general theoretical framework for facultative parthenogens. We show that facultative parthenogenesis selects for female-biased sex ratios by elevating the class reproductive value of females. The degree of this bias depends on the future rate of parthenogenesis. This complicates calculations for cyclic parthenogens, but in stable environments (with stable rates of parthenogenesis), the optimal sex ratio can result automatically from constraints caused by preexisting sex chromosomes: if sexually produced offspring retain unbiased sex ratios while parthenogenetically produced offspring are female (example: stick insects), optimality is achieved for any rate of parthenogenesis. Conversely, in birds and haplodiploids, parthenogenesis produces males, resulting in suboptimal sex ratios. Nevertheless, male-producing parthenogenesis can invade and reach an equilibrium frequency, if the reproductive value of parthenogenetically produced brood is compromised by less than 50%. We argue that this condition is not met in birds due to inviable WW and homozygous ZZ offspring. For haplodiploids, on the other hand, our work resurrects a somewhat forgotten idea by Bull (1981) that haplodiploidy in Hymenoptera evolved from a diplodiploid ancestor with complementary sex determination.
性别比例理论通常假定为专性有性生殖；兼性有性生殖的罕见例外情况通常考虑周期性孤雌生殖的特殊案例。在此，我们构建了一个适用于兼性孤雌生殖的通用理论框架。我们表明，兼性孤雌生殖通过提高雌性的类生殖价值来选择偏雌性的性别比例。这种偏差的程度取决于未来孤雌生殖的速率。这使得周期性孤雌生殖的计算变得复杂，但在稳定环境中（孤雌生殖速率稳定），最优性别比例可自动由现有性染色体造成的限制产生：如果有性生殖产生的后代保持无偏差的性别比例，而孤雌生殖产生的后代为雌性（例如：竹节虫），那么对于任何孤雌生殖速率都能实现最优性。相反，在鸟类和单双倍体生物中，孤雌生殖产生雄性，导致次优的性别比例。然而，如果孤雌生殖产生的一窝后代的生殖价值受损小于50%，产生雄性的孤雌生殖就能入侵并达到平衡频率。我们认为鸟类不满足这一条件，因为会出现WW和纯合ZZ不可存活的后代。另一方面，对于单双倍体生物，我们的研究重新提出了布尔（1981年）一个有些被遗忘的观点，即膜翅目昆虫的单双倍体是从具有互补性别决定的双倍体祖先进化而来的。
