Multiple evidences suggest sox2 as the main driver of a young and complex sex determining ZW/ZZ system in turbot (Scophthalmus maximus)

Abstract

A major challenge in evolutionary biology is to find an explanation for the variation in sex-determining (SD) systems across taxa and to understand the mechanisms driving sex chromosome differentiation. We studied the turbot, holding a ZW/ZZ SD system and no sex chromosome heteromorphism, by combining classical genetics and genomics approaches to disentangle the genetic architecture of this trait. RAD-Seq was used to genotype 18,214 SNPs on 1,135 fish from 36 families and a genome wide association study (GWAS) identified a ~ 6 Mb region on LG5 associated with sex (P < 0.05). The most significant associated markers were located close to sox2, dnajc19 and fxr1 genes. A segregation analysis enabled narrowing down the associated region and evidenced recombination suppression in a region overlapping the candidate genes. A Nanopore/Illumina assembly of the SD region using ZZ and WW individuals identified a single SNP fully associated with Z and W chromosomes. RNA-seq from 5-90 day-old fish detected the expression along the gonad differentiation period of a short non-coding splicing variant (ncRNA) included in a vertebrate-conserved long non-coding RNA overlapping sox2. qPCR showed that sox2 was the only differentially expressed gene between males and females at 50-55 days post fertilization, just prior the beginning of gonad differentiation. More refined information on the involvement of secondary genetic and environmental factors and their interactions on SD was gathered after the analysis of a broad sample of families. Our results confirm the complex nature of SD in turbot and support sox2 as its main driver.

Comparative gonad transcriptome analysis in cobia (Rachycentron canadum)

“…sox3 is the male-determining factor on the Y chromosome in the fish Oryzias dancena ( Takehana et al, 2014 ), while it was found mainly expressed in the ovaries of yellowfin seabream ( Acanthopagrus latus ) ( Li et al, 2020 ). In addition, sox2 is the master sex determination gene in turbot ( Martínez et al, 2019 ). Notably, the sox9 gene, a major transcription factor in testicular development, has been widely studied in various fish species, for example, rainbow trout and Japanese flounder ( Paralichthys olivaceus ) ( Baron et al, 2005 ; Raghuveer and Senthilkumaran, 2010 ; Li et al, 2018 ).…”

Network architecture and sex chromosome turnovers

“…Some genes are repeatedly implicated in vertebrate sex determination. The first vertebrate sex‐determining gene identified was SRY in mammals, [ 53 ] and closely related Sox genes have been recruited as sex determiners at least twice during the evolution of fishes. Doublesex/MAB‐3 (DM) domain genes play an important role in regulating gonadal cell fates in a wide variety of metazoans, [ 66 ] and DM genes have been identified as the sex determiner in birds, [ 44 ] frogs, [ 45 ] and medaka.…”