Sex chromosomes are predicted to harbour elevated levels of sexually antagonistic variation due to asymmetries in the heritability of recessive traits in the homogametic versus heterogametic sex. This evolutionary dynamic may manifest as high recessive load specifically affecting the homogametic sex, and the retention of haplotype diversity in small populations. We tested the hypothesis that the Z chromosome in the butterfly Bicyclus anynana carries a high inbred load for male fertility and viability. Homozygosity of Z chromosome blocks was produced by daughter–father backcrosses, and inferred from marker loci positioned via a linkage map. Male sterility was, in general, unrelated to homozygosity in any region of the Z, but there was an extreme deficit of homozygous males within a 2 cM interval in all families. In contrast, no corresponding skew in Z genotype was detected in their (hemizygous) sisters. The same pattern was observed in historically inbred lines, indicating a high frequency of recessive lethals in the ancestral population. Allele-frequency changes between 1993 and 2006 (70 generations at N e ~ 160) show that, despite the loss of many haplotypes, diversity was retained significantly above the neutral expectation. Effective overdominance in the lethal region can account for this effect locally but not in other parts of the chromosome, that are also associated with persistent linkage disequilibrium. These unexpected patterns suggest the operation of other factors, such as epistatic selection, recombination suppression, assortative mating and meiotic drive. Our results highlight the role of balancing selection in maintaining the inbred load and linked genetic diversity.

中文翻译：

---

圈养蝴蝶种群中隐性 Z 连锁致死率和单倍型多样性的保留

由于同配性别与异配性别中隐性性状的遗传性不对称，性染色体被预测为具有较高水平的性拮抗变异。这种进化动态可能表现为高隐性负载，特别影响同配性别，以及在小种群中保留单倍型多样性。我们检验了蝴蝶 Bicyclus anynana 中的 Z 染色体对雄性生育力和生存力具有高近交负荷的假设。Z 染色体块的纯合性由父代回交产生，并从通过连锁图定位的标记基因座推断。一般而言，雄性不育与 Z 的任何区域的纯合性无关，但在所有家庭中，2 cM 间隔内的纯合雄性极度缺乏。相比之下，在他们的（半合子）姐妹中没有检测到 Z 基因型的相应偏斜。在历史上的近交系中观察到相同的模式，表明祖先种群中隐性致死的频率很高。1993 年至 2006 年（N e ~ 160 的 70 代）之间的等位基因频率变化表明，尽管丢失了许多单倍型，但多样性仍显着高于中性预期。致死区域的有效过度支配可以在局部解释这种影响，但不能解释染色体的其他部分，这些部分也与持久的连锁不平衡有关。这些意想不到的模式表明其他因素的作用，如上位选择、重组抑制、分类交配和减数分裂驱动。