Why Do Some Sex Chromosomes Degenerate More Slowly Than Others? The Odd Case of Ratite Sex Chromosomes

Abstract

The hallmark of sex chromosome evolution is the progressive suppression of recombination which leads to subsequent degeneration of the non-recombining chromosome. In birds, species belonging to the two major clades, Palaeognathae (including tinamous and flightless ratites) and Neognathae (all remaining birds), show distinctive patterns of sex chromosome degeneration. Birds are female heterogametic, in which females have a Z and a W chromosome. In Neognathae, the highly-degenerated W chromosome seems to have followed the expected trajectory of sex chromosome evolution. In contrast, among Palaeognathae, sex chromosomes of ratite birds are largely recombining. The underlying reason for maintenance of recombination between sex chromosomes in ratites is not clear. Degeneration of the W chromosome might have halted or slowed down due to a multitude of reasons ranging from selective processes, such as a less pronounced effect of sexually antagonistic selection, to neutral processes, such as a slower rate of molecular evolution in ratites. The production of genome assemblies and gene expression data for species of Palaeognathae has made it possible, during recent years, to have a closer look at their sex chromosome evolution. Here, we critically evaluate the understanding of the maintenance of recombination in ratites in light of the current data. We conclude by highlighting certain aspects of sex chromosome evolution in ratites that require further research and can potentially increase power for the inference of the unique history of sex chromosome evolution in this lineage of birds.

Repeat Sequence Mapping Shows Different W Chromosome Evolutionary Pathways in Two Caprimulgiformes Families

“…The large blocks of microsatellites and the heterochromatin amount in the W chromosome of the Common Potoo may be the result of the response mechanisms to different evolutionary pressures, leading this genome to accumulate repetitive sequences and resulting in the morphological differentiation and enlargement of this sex chromosome, which is uncommon for most bird species [20,24,60]. We can speculate that such a difference in the degeneration progression of homologous sex chromosomes might result from the lineage-specific mechanisms that influenced the rate of W-chromosome differentiation [8,21]. These characteristics highlight the importance of the Common Potoo as a model species for studies concerning the differentiation of the ZW sex system.…”

“…The avian W chromosome is far less conserved and has run into shifting differentiation degrees, becoming highly differentiated from the homologous Z chromosome and broadly heterochromatic [8]. In contrast, the Z chromosome length is strongly conserved through most bird lineages [9,10].…”

“…Data concerning the evolution of sex chromosomes in birds indicate they have originated through the same mechanism-a gradual suppression of recombination around the sex-determining region and across the chromosome and subsequent loss of genetic material; however, different lineages went through different paths, as reviewed in Yazdi et al [8]. Heteromorphic ZW chromosomes are observed in the karyotype of most bird species, from different lineages.…”

The Diversity and Evolution of Sex Chromosomes in Frogs

“…In the past decade, with the advancement and reduced price of genomic sequencing, studies of many non-model organisms have revealed a remarkable diversity in the rate of sex chromosome differentiation, as well as in the dynamics of birth and death of sex chromosomes (i.e., sex chromosome turnovers) in many fishes, amphibians and reptiles (reviewed by [ 19 ]). In many cases, there is very limited differentiation between the sex chromosomes in the heterogametic sex despite their old age [ 20 , 21 , 22 , 23 , 24 , 25 ]. In other words, they do not follow the degeneration path predicted by the canonical model of sex chromosome evolution.…”

A brief review of vertebrate sex evolution with a pledge for integrative research: towards ‘sexomics’

“…Genomics of avian sex chromosomes is well studied and revealed great interspecies diversity of pseudoautosomal regions (PAR) and Z/W differentiation, from relatively modest degradation in some palaeognath species to extreme degradation in most modern birds [ 285 , 286 ]. The PAR is short in many neognaths, and even without genes in chicken [ 287 ].…”