The genome of New Zealand trevally (Carangidae: Pseudocaranx georgianus) uncovers a XY sex determination locus

Abstract

BackgroundThe genetic control of sex determinism in teleost species is poorly understood. This is partly because of the diversity of sex determining mechanisms in this large group, including constitutive genes linked to sex chromosomes, polygenic constitutive mechanisms, environmental factors, hermaphroditism, and unisexuality. Here we use a de novo genome assembly of New Zealand silver trevally (Pseudocaranx georgianus) together with whole genome sequencing to detect sexually divergent regions, identify candidate genes and develop molecular makers. ResultsThe de novo assembly of an unsexed trevally (Trevally_v1) resulted in an assembly of 579.4 Mb in length, with a N50 of 25.2 Mb. Of the assembled scaffolds, 24 were of chromosome scale, ranging from 11 to 31 Mb. A total of 28416 genes were annotated after 12.8% of the assembly was masked with repetitive elements. Whole genome re-sequencing of 13 sexed trevally (7 males, 6 females) identified sexually divergent regions located on two scaffolds, including a 6 kb region at the proximal end of chromosome 21. Blast analyses revealed similarity between one region and the aromatase genes cyp19 (a1a/b). Males contained higher numbers of heterozygous variants in both regions, while females showed regions of very low read-depth, indicative of deletions. Molecular markers tested on 96 histologically-sexed fish (42 males, 54 females). Three markers amplified in absolute correspondence with sex. ConclusionsThe higher number of heterozygous variants in males combined with deletions in females support a XY sex-determination model, indicating the trevally_v1 genome assembly was based on a male. This sex system contrasts with the ZW-type sex system documented in closely related species. Our results indicate a likely sex-determining function of the cyp19b-like gene, suggesting the molecular pathway of sex determination is somewhat conserved in this family. Our genomic resources will facilitate future comparative genomics works in teleost species, and enable improved insights into the varied sex determination pathways in this group of vertebrates. The sex marker will be a valuable resource for aquaculture breeding programmes, and for determining sex ratios and sex-specific impacts in wild fisheries stocks of this species.