Sex chromosome differentiation in Coleonyx (Gekkota) due to fusion of three ancestral chromosomes and crossover suppression in the Robertsonian sex trivalent

The classical hypothesis of sex chromosome evolution suggests that the arrest of recombination between heterogametic chromosomes evolves due to selection for tight linkage between sex-determining locus and sexually antagonistic loci. Other hypotheses imply that recombination becomes suppressed via neutral mechanisms. Geckos are a good model for studying sex chromosome evolution because they have a variety of independently evolved sex chromosome systems in different clades. We used immunofluorescent synaptonemal complex (SC) analysis to explore the meiotic behavior of complex X1X1X2X2/X1X2Y sex chromosomes in Coleonyx mitratus and sequenced flow-sorted sex chromosome-specific DNA libraries of C. elegans, a related species with a shared sex chromosome system, to identify their genomic content. We found that recombination reduction in the median part of the Robertsonian sex trivalent is shaped by the strong centromere effect and generally lower recombination in the median parts of SCs, implying neutral suppression via centric chromosome fusion. Revealing ancestral chromosomes involved in Coleonyx sex chromosome formation expands our understanding of putative non-random co-option of certain syntenic groups as sex chromosome systems.

性染色体进化的经典假说认为，异型性染色体间的重组停滞源于对性别决定位点与性拮抗位点之间紧密连锁的选择压力。其他假说则提出，重组抑制可通过中性机制实现。壁虎是研究性染色体进化的理想模型类群，因其不同演化支系中存在多种独立起源的性染色体系统。我们采用免疫荧光联会复合体（synaptonemal complex, SC）分析技术，探究了Coleonyx mitratus中复合X1X1X2X2/X1X2Y性染色体的减数分裂行为；同时对拥有相同性染色体系统的近缘物种C. elegans构建流式分选的性染色体特异性DNA文库并进行测序，以解析其基因组构成。研究发现，罗伯逊易位性三价体中部区域的重组率降低，是由强着丝粒效应以及联会复合体中部普遍较低的重组水平共同塑造的，这表明重组抑制可通过着丝粒染色体融合的中性机制实现。阐明参与Coleonyx属性染色体形成的祖先染色体，有助于加深我们对特定同线群以非随机方式被共选为性染色体系统的认知。