Sex chromosome and sex locus characterization in goldfish, Carassius auratus (Linnaeus, 1758)

Abstract

Background: Goldfish is an important model for various areas of research, including neural development and behavior and a species of significant importance in aquaculture, especially as an ornamental species. It has a male heterogametic (XX/XY) sex determination system that relies on both genetic and environmental factors, with high temperatures being able to produce female-to-male sex reversal. Little, however, is currently known on the molecular basis of genetic sex determination in this important cyprinid model. Here we used sequencing approaches to better characterize sex determination and sex-chromosomes in an experimental strain of goldfish. Results: Our results confirmed that sex determination in goldfish is a mix of environmental and genetic factors and that its sex determination system is male heterogametic (XX/XY). Using reduced representation (RAD-seq) and whole genome (pool-seq) approaches, we characterized sex-linked polymorphisms and developed male specific genetic markers. These male specific markers were used to distinguish sex-reversed XX neomales from XY males and to demonstrate that XX female-to-male sex reversal could even occur at a relatively low rearing temperature (18°C), for which sex reversal has been previously shown to be close to zero. We also characterized a relatively large non-recombining region (~11.7 Mb) on goldfish linkage group 22 (LG22) that contained a high-density of male-biased genetic polymorphisms. This large LG22 region harbors 373 genes, including a single candidate as a potential master sex gene, i.e., the anti-Mullerian hormone gene (amh). However, no sex-linked polymorphisms were detected in the coding DNA sequence of the goldfish amh gene. Conclusions: These results show that our goldfish strain has a relatively large sex locus on LG22, which is likely the Y chromosome of this experimental population. The presence of a few XX males even at low temperature also suggests that other environmental factors in addition to temperature could trigger female-to-male sex reversal. Finally, we also developed sex-linked genetic markers, which will be important tools for future research on sex determination in our experimental goldfish population. However, additional work would be needed to explore whether this sex locus is conserved in other populations of goldfish.

Identification of Sex-Specific Markers Through 2b-RAD Sequencing in the Sea Urchin (Mesocentrotus nudus)

“…Our results may help elucidate the origin of sex-determination systems. Although it is difficult to observe heteromorphic sex chromosomes, sex-determining regions have been reported in several aquaculture animals (Wen et al, 2020;Xue et al, 2020). Liu et al (2018) obtained a male-specific sequence 8,661 bp in length from both bighead carp (Hypophthalmichthys nobilis) and silver carp (Hypophthalmichthys molitrix) via 2b-RAD-seq, and suggested that it was located on the Y chromosome or in a nascent Y-linked region.…”

Sex determination mechanisms and sex control approaches in aquaculture animals

“…Meanwhile, hypoxia can turn genotypic females into phenotypic males and also upregulate the sex-determining gene dmy, which leads to a male-dominant population (Cheung et al, 2014). Besides, environmental sex determination effect also has been observed in many other fish species with GSD, such as Nile tilapia (O. niloticus) (Wang et al, 2017), goldfish (Carassius auratus) (Wen et al, 2020), and common carp (C. carpio) (Wu et al, 2003).…”

Research advances and future perspectives of genomics and genetic improvement in allotetraploid common carp

“…Although some progress has been made in common carp, it is difficult to identify a clear signal for sex determination in this tetraploid species, in which sex determination is influenced by both genetic and environmental factors, and by epigenetic modifications 149–151 . An earlier study of goldfish characterized an 11.7‐Mb non‐recombining region likely located on the Y chromosome in male individuals 152 . Moreover, sex differentiation is commonly believed to be controlled by germ cells, 153,154 whereas in tetraploid goldfish, it appears to be controlled by somatic cells 155 .…”