Data from: Multiple origins of sex chromosome fusions correlated with chiasma localization in Habronattus jumping spiders (Araneae: Salticidae)

Entelegyne spiders rarely show fusions yielding neo-Y chromosomes, which M. J. D. White attributed to a constraint in spiders, namely their proximal chiasma localization acting to upset meiotic segregation in males with fusions. Of the 75 taxa of Habronattus and outgroups studied, 47 have X1X20 sex chromosomes in males, 10 have X1X2Y, 15 have X1X2X3Y, 2 have X0, and one has both X1X20 and X1X2X3Y. Chromosome numbers and behavior suggest neo-Ys formed by an autosome-X fusion to make X1X2Y, with a second fusion to an autosome to make X1X2X3Y. Phylogeny shows at least 8-15 gains (or possibly some losses) of neo-Y (i.e. X-autosome fusions), a remarkable number for such a small clade. In contrast to the many X-autosome fusions, at most one autosome-autosome fusion is indicated. Origins of neo-Y are correlated significantly with distal localization of chiasmata, supporting White’s hypothesis that evolution of neo-Y systems is facilitated by looser pairing (distal chiasmata) at meiosis. However, an alternative (or contributing) explanation for the correlation is that X-autosome fusions were selected to permit isolation of male-favored alleles to the neo-Y chromosome, aided by distal chiasmata limiting recombination. This intralocus sexual conflict hypothesis could explain both the many X-autosome fusions, and the stunning complexity of male Habronattus courtship displays.

全突蛛类（Entelegyne spiders）极少出现可产生新Y染色体（neo-Y chromosome）的染色体融合事件，M. J. D. 怀特（M. J. D. White）将此归因于蜘蛛类群存在的一种演化限制：即其近端交叉定位（proximal chiasma localization）会干扰发生融合的雄性个体的减数分裂分离过程。在本次研究涉及的75个雅蛛属（Habronattus）类群及外类群（outgroups）中，47个类群的雄性拥有X₁X₂O型性染色体，10个为X₁X₂Y型，15个为X₁X₂X₃Y型，2个为X₀型，另有1个类群同时存在X₁X₂O与X₁X₂X₃Y两种性染色体系统。染色体数目与行为特征表明，新Y染色体（neo-Ys）的形成首先通过常染色体（autosome）-X染色体融合产生X₁X₂Y型系统，随后经第二次与常染色体的融合形成X₁X₂X₃Y型系统。系统发育（phylogeny）分析显示，新Y染色体（即X染色体-常染色体融合事件）至少经历了8-15次独立起源（或存在部分丢失事件），对于如此小型的演化支（clade）而言，这一数量极为可观。与大量X染色体-常染色体融合事件形成鲜明对比的是，本次研究仅检出至多1次常染色体-常染色体融合事件。新Y染色体的起源与交叉远端定位（distal localization of chiasmata）存在显著相关性，这一结果支持怀特的假说：减数分裂过程中更松散的染色体配对（即远端交叉）可促进新Y染色体系统的演化。不过，针对这一相关性也存在另一种（或作为补充）解释：X染色体-常染色体融合事件受到选择青睐，使得雄性偏好的等位基因（alleles）能够被隔离至新Y染色体上，而远端交叉可通过限制重组过程辅助这一过程。这一基因座内性冲突（intralocus sexual conflict）假说，既可解释X染色体-常染色体融合事件的普遍性，也可阐释雅蛛属雄性极具特色的复杂求偶展示（courtship displays）行为。