# Multiple Origins of Sex Chromosomes in Nothobranchius Killifishes

## TL;DR

This paper explores how sex chromosomes evolved multiple times in killifish species, revealing insights into the genetic mechanisms behind sex determination.

## Contribution

The study identifies multiple independent origins of sex chromosomes in Nothobranchius killifishes and links these to recombination patterns and chromosomal rearrangements.

## Key findings

- Sex chromosomes evolved independently at least four times in Nothobranchius killifishes.
- Sex-determining regions are often located near centromeres or chromosome rearrangement breakpoints.
- Two distinct evolutionary strata of sex chromosomes were found in N. guentheri, with a younger stratum containing a potential SD gene.

## Abstract

Sex chromosomes have evolved repeatedly across eukaryotes. The emergence of a sex‐determining (SD) locus is expected to progressively restrict recombination, driving convergent molecular differentiation. However, evidence from taxa like teleost fishes, representing over half of vertebrate species with unmatched diversity in SD systems, challenges this model. Teleost sex chromosomes are often difficult to detect as they experience frequent turnovers, resetting the differentiation process. Nothobranchius killifishes, which include the XY system shared by
N. furzeri
and N. kadleci and X1X2Y systems in six other species, offer a valuable model to study sex chromosome turnovers. We characterised X1X2Y systems in five killifish species and found that sex chromosomes evolved at least four times independently. Sex‐determining regions resided near centromeres or predicted chromosome rearrangement breakpoints in
N. brieni
and
N. guentheri
, suggesting recombination cold spots may facilitate sex chromosome evolution. Chromosomes representing the XY system in
N. furzeri/N. kadleci were sex‐linked also in the outgroup
Fundulosoma thierryi
, with several genes, including gdf6, residing in the region of differentiation. Although the X1X2Y systems of
N. guentheri
,
N. lourensi
(both Coastal clade), and
N. brieni
(Kalahari clade) involved different chromosomes, they shared a potential SD region. We uncovered two sex‐linked evolutionary strata of distinct age in
N. guentheri
. However, its potential SD gene amhr2 was located in the younger stratum and is hence unlikely to be the ancestral SD gene in this lineage. Our findings suggest recombination landscapes shape sex chromosome turnover and that certain synteny blocks are repeatedly co‐opted as sex chromosomes in killifishes.