Miniscule differences between sex chromosomes in the giant genome of a salamander

Melissa C Keinath,Nataliya Timoshevskaya,S Randal Voss,Jeramiah J Smith,Vladimir A Timoshevskiy

doi:10.1038/s41598-018-36209-2

Melissa C Keinath, Nataliya Timoshevskaya + Show 3 more

Open Access

https://doi.org/10.1038/s41598-018-36209-2

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Abstract

In the Mexican axolotl (Ambystoma mexicanum), sex is determined by a single Mendelian factor, yet its sex chromosomes do not exhibit morphological differentiation typical of many vertebrate taxa that possess a single sex-determining locus. As sex chromosomes are theorized to differentiate rapidly, species with undifferentiated sex chromosomes provide the opportunity to reconstruct early events in sex chromosome evolution. Whole genome sequencing of 48 salamanders, targeted chromosome sequencing and in situ hybridization were used to identify the homomorphic sex chromosome that carries an A. mexicanum sex-determining factor and sequences that are present only on the W chromosome. Altogether, these sequences cover ~300 kb of validated female-specific (W chromosome) sequence, representing ~1/100,000th of the 32 Gb genome. Notably, a recent duplication of ATRX, a gene associated with mammalian sex-determining pathways, is one of few functional (non-repetitive) genes identified among these W-specific sequences. This duplicated gene (ATRW) was used to develop highly predictive markers for diagnosing sex and represents a strong candidate for a recently-acquired sex determining locus (or sexually antagonistic gene) in A. mexicanum.

Highlights

In many species, sex is determined by the inheritance of highly differentiated sex chromosomes, which have evolved independently many times throughout the tree of life[1,2,3]
It has been suggested that a majority of salamanders have homomorphic sex chromosomes[18,20], evidence for genetic sex determination in most species is largely based on the observation of 1:1 sex ratios from clutches without thorough demonstration of Mendelian inheritance
A second bacterial artificial chromosome (BAC) corresponding to a marker from the opposite end of LG9 (E12A6) localized to the opposite tip of the same chromosome, indicating that this chromosome corresponds precisely to LG9 (Fig. 1)

Summary

Introduction

Sex is determined by the inheritance of highly differentiated (heteromorphic) sex chromosomes, which have evolved independently many times throughout the tree of life[1,2,3]. Subsequent studies showed normal sex ratios from matings with the F1 males and most of the F1 females, but several of the F1 females produced spawns of all females, suggesting they carried the unusual WW genotype[24] Following these foundational studies, early genetic mapping studies used cold shock to inhibit meiosis II and assessed triploid phenotypes to estimate the frequencies of equatorial separation and map distances between recessive mutations and their linked centromeres[25]. Early genetic mapping studies used cold shock to inhibit meiosis II and assessed triploid phenotypes to estimate the frequencies of equatorial separation and map distances between recessive mutations and their linked centromeres[25] Based on these analyses, the sex determining locus was predicted to occur near the end of an undefined chromosome[25] and later estimated to be 59.1 cM distal to the centromere (essentially, freely recombining)[23]

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