Independent Evolution of Sex Chromosomes in Eublepharid Geckos, A Lineage with Environmental and Genotypic Sex Determination.

Eleonora Pensabene,Michail Rovatsos,Lukáš Kratochvíl

doi:10.3390/life10120342

Abstract

Geckos demonstrate a remarkable variability in sex determination systems, but our limited knowledge prohibits accurate conclusions on the evolution of sex determination in this group. Eyelid geckos (Eublepharidae) are of particular interest, as they encompass species with both environmental and genotypic sex determination. We identified for the first time the X-specific gene content in the Yucatán banded gecko, Coleonyx elegans, possessing X1X1X2X2/X1X2Y multiple sex chromosomes by comparative genome coverage analysis between sexes. The X-specific gene content of Coleonyx elegans was revealed to be partially homologous to genomic regions linked to the chicken autosomes 1, 6 and 11. A qPCR-based test was applied to validate a subset of X-specific genes by comparing the difference in gene copy numbers between sexes, and to explore the homology of sex chromosomes across eleven eublepharid, two phyllodactylid and one sphaerodactylid species. Homologous sex chromosomes are shared between Coleonyx elegans and Coleonyx mitratus, two species diverged approximately 34 million years ago, but not with other tested species. As far as we know, the X-specific gene content of Coleonyx elegans / Coleonyx mitratus was never involved in the sex chromosomes of other gecko lineages, indicating that the sex chromosomes in this clade of eublepharid geckos evolved independently.

Highlights

Sex determination is the vital process that determines whether an individual will develop to a male or a female
The clade of vertebrates comprising sauropsids and mammals, demonstrate two major systems of sex determination: the genotypic sex determination (GSD), where the sex is determined by sex-specific genetic factors linked to sex chromosomes, and the environmental sex determination (ESD), where the sex is influenced by environmental factors, most commonly temperature, during a sensitive period of embryonic development and there are no sex-specific differences in genotypes [1,2]
The major support for this hypothesis comes from phylogenetic reconstruction of the sex determination systems in reptiles indicating that transitions from ESD to GSD are common, but transitions in the opposite directions are rare

Summary

Introduction

Sex determination is the vital process that determines whether an individual will develop to a male or a female. The clade of vertebrates comprising sauropsids (reptiles and birds) and mammals, demonstrate two major systems of sex determination: the genotypic sex determination (GSD), where the sex is determined by sex-specific genetic factors linked to sex chromosomes, and the environmental sex determination (ESD), where the sex is influenced by environmental factors, most commonly temperature (temperature-dependent sex determination—TSD), during a sensitive period of embryonic development and there are no sex-specific differences in genotypes [1,2]. The ancestral ESD hypothesis suggests that the common ancestor of amniotes had ESD and that sex chromosomes evolved independently in distant lineages, often from the ancestral ESD [5,6]. The major support for this hypothesis comes from phylogenetic reconstruction of the sex determination systems in reptiles indicating that transitions from ESD to GSD are common, but transitions in the opposite directions are rare. The hypothesis expects that once evolved, GSD seems to be evolutionary

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