Single Locus Maintains Large Variation of Sex Reversal in Half-Smooth Tongue Sole (Cynoglossus semilaevis)

Abstract

Sex determination is a fundamental biological process for individual sex development and population sex ratios. However, for some species, the primary sex might be altered during development, and individuals can develop into the opposite sex. Sex reversal may happen in insects, reptiles, amphibians, and fishes. In half-smooth tongue sole (Cynoglossus semilaevis), some genetically female fish irreversibly reverse to pseudomales, resulting in higher costs in aquaculture owing to a lower growth rate of male fish during a 2-yr growth period. Here, we identified a locus with large controlling effect on sex reversal in the half-smooth tongue sole through genome-wide association study with high-density single nucleotide polymorphisms (SNPs). This SNP is located at the third intron of the F-box and leucine rich repeat protein 17 (FBXL17) gene on the Z chromosome, and it has two alleles, A and T. Genetic females with Z^AW genotypes will never reverse into phenotypic males, but those with Z^TW genotypes can sometimes undergo sex reversal. This SNP explains 82.7% of the genetic variation, or 58.4% of the phenotypic variation. Based on our results, a reproductive management program could be developed to improve the phenotypic female ratio in aquaculture, and elucidate the mechanism of sex reversal in half-smooth tongue sole. We expect that these findings will have a substantial impact on the population management in many harvested species where sex reversal occurs.

Keywords: FBXL17; genome-wide association study; half-smooth tongue sole; sex reversal.

Figure 1

Sex differentiation of the half-smooth tongue sole for pseudomales. (A) Ovary and oocytes (×100). (B) Chimeric gonad during the late process of sex reversal from ZW female to pseudomale, mainly composed of spermatangia; the red triangle indicates the oocytes (×100). (C) Testis and sperm cells (×100). (D) Chimeric gonad during the early stage of sex reversal from ZW females to pseudomale; the green triangle indicates the sperm cell (×100).

Figure 2

GWAS of sex reversal in the half-smooth tongue sole. (A) Manhattan plot of SNPs associated with sex reversal, x-axis presents genomic coordinates along chromosomes 1–20 and sex chromosomes W and Z. The y-axis presents a negative logarithm of P-values. The horizontal dashed line is the genome-wide threshold, SNPs above this line are significantly associated with sex reversal. (B) Enlarged plot for the Z chromosome, the two vertical dashed lines indicate the region of the 95% confidence interval for the detected locus. (C) The known genes located in the 95% confidence interval.

Figure 3

GWAS of sex reversal with the half-smooth tongue sole of the Z^TW genotype. No locus was detected associated with sex reversal mapping with individuals of the Z^TW genotype. The green horizontal line is the threshold.

Figure 4

Six possible cross modes of normal males and females. Modes (A) and (B) ensure the genetic female offspring will not reverse to phenotypic males (n); in modes (C–F), the genetic female offspring of (Z^TW) will possibly reverse to phenotypic males (pr). In mode (A), the offspring carries only the A allele, in the next generations, the genetic female will never reverse to the phenotypic male.