CE02 - Terre vivante

Characterization of a sex chromosome born from a feminizing endosymbiont genome – SymchroSex

Submission summary

Although sex may be the most impactful trait of an individual, the mechanisms by which sex chromosomes emerge, and the genes that orient development into one sex or the other, are not well characterized. A sex chromosome is assumed to derive from an autosome that acquired a sex-determining gene. A mounting number of studies have shown that these events occur quite frequently in several clades, with chromosome pairs replacing others for the task of sex determination. However, these replacements have seldom been tracked down while they are underway. Hence, despite the commonness of sex chromosomes in animal species, the conditions that favor the emergence and turnover of sex chromosomes lack empirical validation. Likewise, the genes controlling sex are identified in very few animal taxa. This project aims to shed light on these two uncertainties by studying a sex chromosome that has recently appeared in a crustacean species, the terrestrial isopod Armadillidium vulgare.
Armadillidium vulgare is characterized by WZ sex chromosomes (female heterogamety). However, a new sex chromosome has emerged from the insertion of the genome of a feminizing Wolbachia endosymbiont in its host’s genome. This horizontal DNA transfer event has effectively created a feminizing sex chromosome. Despite its very young age (the f element shows 99.7% sequence identify with Wolbachia) the f element is already quite frequent in natural populations and may currently be replacing the WZ sex-determining system. The f element is characterized as eight genomic scaffolds that have not been located in the A. vulgare genome. While the f element most certainly locates on a chromosome, its non-mendelian transmission rates suggest that it may be linked to a segregation distorter.
Study of the f element of A. vulgare offers the rare opportunity to reach our objectives: (1) to better understand how new sex chromosomes arise, and (2) to identify sex-determining gene(s) which are known in only a handful of taxa. As the f element is of bacterial origin, objective 2 may even allow pinpointing the genes used by endosymbionts to manipulate the sex of their hosts. To reach these goals, we devised a project comprising three work packages (WPs).
In WP1 we will fully reconstruct the sequence of the f element, characterize its content and genomic context, and locate it on a chromosome. To do so, we will use the latest sequencing and optical mapping technologies to produce a reference-grade assembly of the A. vulgare genome, with chromosome-size scaffolds. This assembly will serve as a basis for the other WPs.
In WP2, we will sequence and analyze full genomes of individuals from diverse countries to clarify the recent history of the f element. In particular, we will reconstruct phased haplotypes to reveal traces of positive selection, the effect of which may explain the apparent evolutionary success of the f element. This analysis will be complemented by crossing experiments and genotyping of progenies to test for the presence of a segregation distorter.
WP3 will set to pinpoint feminization genes in/near the f element by two complementary approaches. As we suspect that inactive, non-feminizing f variants exist (some males carry the f element) we will identify and fully sequence these variants, and search for gene-disrupting mutations. We will also link individual genotypes at SNPs to the feminizing ability of f variants through an association study designed to locate feminizing genes. In parallel, we will the investigate transcription patterns of f-linked genes in reared individuals during their sexual differentiation, through transcriptomics and RT-qPCR, with the assumption that a feminizing gene must be expressed during this phase. These convergent approaches should yield a set of feminizing gene candidates for future analyses.

Project coordination

Jean Peccoud (Ecologie et biologie des interactions)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

EBI Ecologie et biologie des interactions

Help of the ANR 295,920 euros
Beginning and duration of the scientific project: January 2021 - 48 Months

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