Pirro, S. | bioRxiv Science

bioRxiv2020

Gene-rich X chromosomes implicate intragenomic conflict in the evolution of bizarre genetic systems

Haplodiploidy and paternal genome elimination (HD/PGE) are common in invertebrates, having evolved at least two dozen times, all from male heterogamety (i.e., systems with X chromosomes). However, why X chromosomes are important for the evolution of HD/PGE remains debated. The Haploid Viability Hypothesis posits that X-linked genes promote the evolution of male haploidy by facilitating purging recessive deleterious mutations. The Intragenomic Conflict Hypothesis holds that conflict between genes drives genetic system turnover; under this model, X-linked genes could promote the evolution of male haploidy due to conflicts with autosomes over sex ratios and genetic transmission. We studied lineages where we can distinguish these hypotheses: species with germline PGE that retain an XX/X0 sex determination system (gPGE+X). Because evolving PGE in these cases involves changes in transmission without increases in male hemizygosity, a high degree of X linkage in these systems is predicted by the Intragenomic Conflict Hypothesis but not the Haploid Viability Hypothesis. To quantify the degree of X linkage, we sequenced and compared 7 gPGE+X species genomes with 11 related species with typical XX/XY or XX/X0 genetic systems, representing three transitions to gPGE. We find highly increased X linkage in both modern and ancestral genomes of gPGE+X species compared to non-gPGE relatives, and recover a significant positive correlation between percent X linkage and the evolution of gPGE. These are among the first empirical results suggesting a role for intragenomic conflict in the evolution of novel genetic systems like HD/PGE. Significance StatementSex determination systems such as haplodiploidy, in which males gene transmission is haploid, are surprisingly common, however, the evolutionary paths to these systems are poorly understood. X chromosomes may play a particularly important role, either by increasing survival of males with only maternal genomes, or due to conflicts between X-chromosomal and autosomal genes. We studied X-chromosome gene richness in three arthropod lineages in which males are diploid as adults but only transmit their maternally-inherited haploid genome. We find that species with such atypical systems have far more X chromosomal genes than related diploid species. These results suggest that conflict between genetic elements within the genome drives the evolution of unusual sex determination systems.

evolutionary biology↗

bioRxiv2020

Draft Genome of the Sea Cucumber Holothuria glaberrima, a model for the study of regeneration

Regeneration is one of the most fascinating and yet least understood processes of animals. Echinoderms, one of the closest related invertebrate groups to humans, can contribute to our understanding of the genetic basis of regenerative processes. Amongst echinoderms, sea cucumbers have the ability to grow back most of their body parts following injury, including the intestine and nervous tissue. The cellular and molecular events underlying these abilities in sea cucumbers have been most extensively studied in the species Holothuria glaberrima. However, research into the regenerative abilities of this species have been impeded due to the lack of adequate genomic resources. Here, we report the first draft genome assembly of H. glaberrima and demonstrate its value for future genetic studies. Using only short sequencing reads, we assembled the genome into 2,960,762 scaffolds totaling 1.5 gigabases with an N50 of 15 kilobases. Our BUSCO assessment of the genome resulted in 882 (90.2%) complete and partial genes from 978 genes queried. We incorporated transcriptomic data from several different life history stages to annotate 41,076 genes in our final assembly. To demonstrate the usefulness of the genome, we fully annotated the melanotransferrin (Mtf) gene family, which have a potential role in regeneration of the sea cucumber intestine. Using these same data, we extracted the mitochondrial genome, which showed high conservation to that of other holothuroids. Thus, these data will be a critical resource for ongoing studies of regeneration and other studies in sea cucumbers.

genomics↗