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.