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.

Highlights

  • Sex chromosomes are the main players in the genetic sex determination of mammals and birds

  • Dmrt1 is located on the Z chromosome and its higher expression in males relative to females drives the formation of male gonads [2]

  • Molecular data shows that the ancestor of all extant birds lived >100 MYA [40] and avian sex chromosomes are ancient, making the maintenance of recombination and low degeneration level of ratite sex chromosomes even more striking

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Summary

Introduction

Sex chromosomes are the main players in the genetic sex determination of mammals and birds. The most prevalent hypothesis concerns the action of mutations with opposing fitness effects in the two sexes [12,13,14] According to this theory, the first step involves the establishment of a sex-determining region on a pair of autosomes by restricting recombination between initial sex-determining genes [15]. A recombination-suppressing mutation, commonly assumed an inversion, tightly links the sex-determining region and SA allele, preventing the damage to the sex in which the SA allele is deleterious [13] This process is hypothesized to occur in a step-wise manner until recombination is suppressed across most of the chromosome. Molecular data shows that the ancestor of all extant birds lived >100 MYA [40] and avian sex chromosomes are ancient, making the maintenance of recombination and low degeneration level of ratite sex chromosomes even more striking. Takagi et al 1972 [27]; 2 Ansari et al 1988 [28]; 3 Pigozzi 2011 [31]; 4 Tsuda, et al 2007 [30]; 5 Nanda, et al 2008 [29]; Zhou, et al 2014 [36]; 7 Yazdi and Ellegren 2014 [37]; 8 Xu et al 2019 [23]

Selective Processes
Sexually Antagonistic Selection
Balancing Selection
Purifying Selection
Neutral Processes
Rate of Molecular Evolution
Chromatin State of Sex Chromosomes
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