Abstract
Males and females of Artemia franciscana, a crustacean commonly used in the aquarium trade, are highly dimorphic. Sex is determined by a pair of ZW chromosomes, but the nature and extent of differentiation of these chromosomes is unknown. Here, we characterize the Z chromosome by detecting genomic regions that show lower genomic coverage in female than in male samples, and regions that harbor an excess of female-specific SNPs. We detect many Z-specific genes, which no longer have homologs on the W, but also Z-linked genes that appear to have diverged very recently from their existing W-linked homolog. We assess patterns of male and female expression in two tissues with extensive morphological dimorphism, gonads, and heads. In agreement with their morphology, sex-biased expression is common in both tissues. Interestingly, the Z chromosome is not enriched for sex-biased genes, and seems to in fact have a mechanism of dosage compensation that leads to equal expression in males and in females. Both of these patterns are contrary to most ZW systems studied so far, making A. franciscana an excellent model for investigating the interplay between the evolution of sexual dimorphism and dosage compensation, as well as Z chromosome evolution in general.
Highlights
In many species, sex is determined by a pair of sex chromosomes: X and Y chromosomes in male-heterogametic species, or Z and W in female-heterogametic species
Both of these patterns are contrary to most ZW systems studied so far, making A. franciscana an excellent model for investigating the interplay between the evolution of sexual dimorphism and dosage compensation, as well as Z chromosome evolution in general
While ZW sex determination had been hypothesized in Artemia for a long time (Bowen 1963), it was not until the recent production of a linkage map that it was fully supported (De Vos et al 2013)
Summary
Sex is determined by a pair of sex chromosomes: X and Y chromosomes in male-heterogametic species, or Z and W in female-heterogametic species. While “dosage compensation” mechanisms that regulate gene expression of the X/Z to make up for loss of Y/W-linked genes are widespread in male-heterogametic species, they appear to be limited to a few dosage-sensitive genes in female-heterogametic species (with the exception of Lepidoptera, reviewed in Gu and Walters 2017 and Mank 2013). Why this should be the case, and what evolutionary forces are at play, is still unclear (Mank et al 2011; Gu and Walters 2017)
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