Chromosomal fusions and evolutionary forces: Exploring the neo-sex chromosome system of Anolis distichus

The evolutionary dynamics of sex chromosomes differ from those of autosomes due to their unique patterns of inheritance and regions of hemizygosity in non-recombining areas. However, the study of sex chromosomes and sex-linked gene evolution has been limited by the rarity of truly novel sex chromosome complements in model systems. Recent advances in next-generation sequencing have enabled the identification of neo-sex chromosomes, created by the fission or fusion of autosomes with sex chromosomes, providing a new avenue to investigate the dynamics of sex chromosome evolution. Squamate reptiles, particularly Anolis lizards, are an excellent system for studying the consequences of sex-linkage due to their frequent sex chromosome-autosome fusions. The Hispaniolan Bark anole, Anolis distichus, has experienced two sex chromosome and autosome fusions that led to a multiple sex chromosome system (X1X2Y). We present a high-quality whole-genome assembly and annotation of a male A. distichus (X1X2Y), enabling a detailed analysis of all three of its neo-sex chromosomes. We identified the AnoDisX1, AnoDisX2, and AnoDisY chromosomes from assembly scaffolds using an integrative approach, and estimated their degeneration and selection strength. Our results support long-held theories of differential evolutionary pressures on sex chromosomes, such as the Fast X effect and Y degeneration. Additionally, we observed that chromosome 12 has become sex-linked in two different Anolis species, suggesting that some autosomes may be more likely to become sex-linked. Altogether, our genome adds to the diversity of available taxa sequenced and enables novel comparative analyses in a variety of fields, including speciation, chromosomal synteny, and sex chromosome evolution.