Diversity of sex chromosomes in Sulawesian medaka fishes

Abstract

Recent genetic and genomic studies have revealed tremendous diversity in sex chromosomes across diverse taxa. Although theoretical studies predict that sex chromosome evolution can drive the evolution of sexual dimorphism, empirical studies are still limited. A taxonomic group that shows diversity in both sex chromosomes and sexual dimorphism provides excellent opportunity to investigate the link between the evolution of sex chromosomes and sexual dimorphism. The medaka fishes (family Adrianichthyidae) exhibit both rapid sex chromosome turnovers and diversification of sexually dimorphic traits. In the present study, we investigated the sex chromosomes of 13 Oryzias species from Sulawesi, Indonesia, that have diversified in sexual dichromatism. Using pooled sequencing we found sex chromosomes in 9 species that all had XY systems, with a few species being possibly modified by multiple loci. Seven species (O. woworae, O. asinua, O. wolasi, O. matanensis, O. celebensis, O. hadiatyae, and O. dopingdopingensis) share linkage group (LG) 24 as sex chromosomes; however, they differed in the length and magnitude of sequence divergence between the X and Y chromosomes. The sex chromosome of O. eversi was LG4, which has not been reported as a sex chromosome in any other medaka species. In O. sarasinorum, LG16 and LG22 are associated with sex. Although LG16 was found to be sex-linked in another medaka species previously examined, the sex-determining regions did not overlap. Frequent turnovers and the great diversity of the sex chromosomes make Sulawesian medaka species a model system for investigating the roles of sex chromosome evolution in the diversification of sexual dimorphism.

Genetic basis for the evolution of pelvic‐fin brooding, a new mode of reproduction, in a Sulawesian fish

“…It is theoretically predicted that such genes under intralocus sexual conflict are likely to be linked to sex chromosomes to resolve the conflict (Rice, 1987). Our recent study has shown that the pelvic‐fin brooding O. eversi has an XY sex chromosome system, and that the sex chromosome is chr 4 (Ansai et al, 2022). In this study, however, no QTL was detected on chr 4, indicating that the X chromosome contributes little to the evolution of pelvic fin brooding.…”

“…In this study, however, no QTL was detected on chr 4, indicating that the X chromosome contributes little to the evolution of pelvic fin brooding. Although a major locus for pelvic‐fin length was found on chr 24, which is a sex chromosome in O. dopingdopingensis (Ansai et al, 2022), this locus was not located at the region showing substantial X–Y divergence (Ansai et al, 2022). Therefore, other mechanisms such as gonadal sex steroid‐mediated gene expression may play a more important role in female‐specific expression of traits involved in pelvic fin brooding in O. eversi .…”

Sex chromosome evolution: The remarkable diversity in the evolutionary rates and mechanisms

“…For instance, in this special issue: Using a combination of short‐read and long‐read sequencing, Evans et al (2022) and Duhamel et al (2022) perform a chromosome‐scale genome assembly in the Marsabit clawed frog and anther‐smut fungi, respectively. These approaches allow the study of intra‐specific variation of sex chromosome divergence in the frog and reveal independent chromosomal rearrangement in the mating‐type chromosomes of the fungi. With short‐read WGS and the help of existing reference genomes, Sigeman et al (2022) investigate the translocation nature of a novel neo‐sex chromosome in Sylvivoidea songbirds, whilst Rovatsos et al (2022) reveal a XX/XY sex determination system in a blind lizard and further identified a novel combination of homologous genes using the chicken X chromosome as reference. With short‐read whole‐genome pooled sequencing, Ansai et al (2022) and Jeffries et al (2022) investigate the sex‐determining regions and sex chromosome turnover across closely related fish taxa. With RAD‐seq, Keating et al (2022) and Meuser et al (2022) perform analysis in multiple individuals per sex to investigate possible sex‐determining regions and infer sex chromosome turnovers. …”

“…The authors further demonstrated a sex chromosome turnover from an XY to another XY system in the banded geckos of the genus Coleonyx . Using pooled sequencing, Ansai et al (2022) find that nine Oryzias fish species have XX/XY sex chromosome systems with several linkage groups used to determine sex in different species, whilst no sex chromosomes are found in four other investigated species. Their results suggest frequent turnovers in this lineage as well.…”