Cynoglossus semilaevis

Cynoglossus semilaevis has a female heterogametic sex-determination system, with females being ZW and males being ZZ.^[4] Female tongue soles grow up to 2-4 times larger and faster than males, making them more desirable for aquaculture. To maximize yields, efforts have been made to produce all-female stocks through artificial gynogenesis.^[5] However, the sex ratio of C. semilaevis populations in both the wild and aquaculture environments is male-skewed due to the occurrence of pseudomales^[6] or individuals with female ZW chromosomes that become physiologically male.^[7] Offspring of pseudomales tend to grow more slowly than those of genetic males and are more likely to become pseudomales themselves, further skewing the sex ratio.^[4][6] This phenomenon is influenced by a combination of genotypic sex determination and temperature-dependent sex determination.^[8] Higher temperatures during the gonadal sex differentiation stage (approximately 56–62 days post-hatching) significantly increase the likelihood of sex reversal. For example, one study found that the likelihood of females becoming pseudomales increased to 73% when reared at a higher temperature (28 °C) as opposed to a 14% sex-reversal rate when reared at an ambient temperature (22 °C).^[4][9] Furthermore, the offspring of the pseudomales reared at 28 °C had a sex-reversal rate of 94% despite being reared at 22 °C.^[4] This high rate of females becoming pseudomales in the F2 generation was attributed to the offspring inheriting their Z chromosome from their pseudomale father and retaining paternal methylation patterns.^[10]

A single SNP, Cyn_Z_6676874, allows certain females to become pseudomales.^[11] Females with a thymine at this site can undergo sex reversal, but females with an adenine cannot.^[11] There are also three known female-specific molecular markers that can be used to distinguish between the various tongue sole sexes.^[6] Two SNPs, SNP_chr_8935925_C_T and SNP_chr_8936186_C_G, and one indel were found to enable accurate differentiation between males, females, and pseudomales.^[6]

Gene expression in the gonads of males and females are different before sex determination, after sex determination, and after exposure to higher temperatures.^[12] Prior to sex determination, males and females have differentially expressed genes (DEGs) related to muscle development.^[12] Following sex determination, females have upregulated female-specific genes, figla and foxl2, and downregulation of the male-specific genes, dmrt1 and amh, under both ambient and high-temperature conditions.^[12] When exposed to higher temperatures, males, females, and pseudomales have hundreds of DEGs in common that are mainly involved in biological processes and molecular functions.^[12]

Pseudomale gonads have more similar gene expression patterns to males than females.^[8] DEGs between males and pseudomales played a role in spermatogenesis and energy metabolism.^[12] Whereas, DEGS between females and pseudomales were related to steroid hormones, helicase activity, sexual differentiation, and development.^[12]