Vucic, T. | Search bioRxiv

bioRxiv2024

Identification of Y-chromosome turnover in newts fails to support a sex chromosome origin for the Triturus balanced lethal system

Non-recombining regions of the genome often have profound effects on evolution, resulting in phenomena such as sex chromosomes and supergenes. Amongst the strangest examples are balanced lethal systems, such as that found in newts of the genus Triturus. These systems halve reproductive output, and the evolution of such a deleterious trait is difficult to explain. For Triturus an intriguing model proposes that the balanced lethal system evolved from an ancestral Y-chromosome. To test this hypothesis, we identify the Y-chromosome of Triturus and verify whether it, or the balanced lethal system, is homologous to the Y-chromosome of its sister genus Lissotriton, which does not possess the balanced lethal system. We identify a set of candidate Y-linked markers in T. ivanbureschi and place them on a high-density linkage map that we construct with 7,233 RADseq markers. We validate male specificity of the markers across the genus, and then place both the Triturus and Lissotriton Y-linked regions within previously constructed target capture linkage maps that include genes linked to the balanced lethal system. We observe that neither the Triturus balanced lethal system, nor the Triturus Y-chromosome are homologous to the Lissotriton Y-chromosome. This is the first molecular evidence of a transition between Y-chromosome systems within salamanders. However, unless additional sex chromosome turnover events are involved, our data does not support a sex chromosome origin of the balanced lethal system.

evolutionary biology↗

bioRxiv2024

PAV-spotter: using signal cross-correlations to identify Presence/Absence Variation in target capture data

High throughput sequencing technologies have become essential in the fields of evolutionary biology and genomics. When dealing with non-model organisms or genomic gigantism, sequencing whole genomes is still relatively costly and therefore reduced-genome representations are frequently obtained, for instance by target capture approaches. While computational tools exist that can handle target capture data and identify small-scale variants such as single nucleotide polymorphisms and micro-indels, options to identify large scale structural variants are limited. To meet this need, we introduce PAV-spotter: a tool that can identify presence/absence variation (PAV) in target capture data. PAV-spotter conducts a signal cross-correlation calculation, in which the distribution of read counts per target between samples of different a priori defined classes - e.g. male versus female, or diseased versus healthy - are compared. We apply and test our methodology by studying Triturus newts: salamanders with gigantic genomes that currently lack an annotated reference genome. Triturus newts suffer from a hereditary disease that kills half their offspring during embryogenesis. We compare the target capture data of two different types of diseased embryos, characterized by unique deletions, with those of healthy embryos. Our findings show that PAV-spotter helps to expose such structural variants, even in the face of medium to low sequencing coverage levels, low sample sizes, and background noise due to mis-mapped reads. PAV-spotter can be used to study the structural variation underlying supergene systems in the absence of whole genome assemblies. The code, including further explanation, is available through the PAV-spotter GitHub repository: https://github.com/Wielstra-Lab/PAVspotter.

bioengineering↗