TY - JOUR

T1 - X chromosomes show relaxed selection and complete somatic dosage compensation across Timema stick insect species

AU - Parker, Darren

AU - Jaron, Kamil S

AU - Dumas, Zoe

AU - Robinson-Rechavi, Marc

AU - Schwander, Tanja

PY - 2022/8/7

Y1 - 2022/8/7

N2 - Sex chromosomes have evolved repeatedly across the tree of life. As they are pre-sent in different copy numbers in males and females, they are expected to experi-ence different selection pressures than the autosomes, with consequences including a faster rate of evolution, increased accumulation of sexually antagonistic alleles and the evolution of dosage compensation. Whether these consequences are general or linked to idiosyncrasies of specific taxa is not clear as relatively few taxa have been studied thus far. Here, we use whole-genome sequencing to identify and characterize the evolution of the X chromosome in five species of Timema stick insects with XX:X0 sex determination. The X chromosome had a similar size (approximately 12% of the genome) and gene content across all five species, suggesting that the X chromosome originated prior to the diversification of the genus. Genes on the X showed evidence of relaxed selection (elevated dN/dS) and a slower evolutionary rate (dN + dS) than genes on the autosomes, likely due to sex- biased mutation rates. Genes on the X also showed almost complete dosage compensation in somatic tissues (heads and legs), but dosage compensation was absent in the reproductive tracts. Contrary to predic-tion, sex- biased genes showed little enrichment on the X, suggesting that the advan-tage X- linkage provides to the accumulation of sexually antagonistic alleles is weak. Overall, we found the consequences of X- linkage on gene sequences and expression to be similar across Timema species, showing the characteristics of the X chromosome are surprisingly consistent over 30 million years of evolution.KEYWORDSinsects, population genetics, sex chromosomes, sexual selection & conflicts

AB - Sex chromosomes have evolved repeatedly across the tree of life. As they are pre-sent in different copy numbers in males and females, they are expected to experi-ence different selection pressures than the autosomes, with consequences including a faster rate of evolution, increased accumulation of sexually antagonistic alleles and the evolution of dosage compensation. Whether these consequences are general or linked to idiosyncrasies of specific taxa is not clear as relatively few taxa have been studied thus far. Here, we use whole-genome sequencing to identify and characterize the evolution of the X chromosome in five species of Timema stick insects with XX:X0 sex determination. The X chromosome had a similar size (approximately 12% of the genome) and gene content across all five species, suggesting that the X chromosome originated prior to the diversification of the genus. Genes on the X showed evidence of relaxed selection (elevated dN/dS) and a slower evolutionary rate (dN + dS) than genes on the autosomes, likely due to sex- biased mutation rates. Genes on the X also showed almost complete dosage compensation in somatic tissues (heads and legs), but dosage compensation was absent in the reproductive tracts. Contrary to predic-tion, sex- biased genes showed little enrichment on the X, suggesting that the advan-tage X- linkage provides to the accumulation of sexually antagonistic alleles is weak. Overall, we found the consequences of X- linkage on gene sequences and expression to be similar across Timema species, showing the characteristics of the X chromosome are surprisingly consistent over 30 million years of evolution.KEYWORDSinsects, population genetics, sex chromosomes, sexual selection & conflicts

KW - insects

KW - population genetics

KW - sex chromosomes

KW - sexual selection & conflicts

U2 - https://doi.org/10.1111/jeb.14075

DO - https://doi.org/10.1111/jeb.14075

M3 - Article

JO - Journal of Evolutionary Biology

JF - Journal of Evolutionary Biology

SN - 1010-061X

ER -