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Fishes provide a unique and intriguing model system for studying the genomic origin and evolutionary mechanisms underlying sex determination and high sex-chromosome turnover. In this study, the mode of sex determination was investigated in Siamese fighting fish, a species of commercial importance. Genome-wide SNP analyses were performed on 75 individuals (40 males and 35 females) across commercial populations to determine candidate sex-specific/sex-linked loci. In total, 73 male-specific loci were identified and mapped to a 5.6 kb region on chromosome 9, suggesting a putative male-determining region (pMDR) containing localized dmrt1 and znrf3 functional sex developmental genes. Repeat annotations of the pMDR revealed an abundance of transposable elements, particularly Ty3/Gypsy and novel repeats. Remarkably, two out of the 73 male-specific loci were located on chromosomes 7 and 19, implying the existence of polygenic sex determination. Besides male-specific loci, five female-specific loci on chromosome 9 were also observed in certain populations, indicating the possibility of a female-determining region and the polygenic nature of sex determination. An alternative explanation is that male-specific loci derived from other chromosomes or female-specific loci in Siamese fighting fish recently emerged as new sex-determining loci during domestication and repeated hybridization. [ABSTRACT FROM AUTHOR]

The African catfish (Clarias gariepinus) may exhibit the co-existence of XX/XY and ZZ/ZW sex-determination systems (SDSs). However, the SDS of African catfish might be influenced by a polygenic sex-determination (PSD) system, comprising multiple independently segregating sex "switch" loci to determine sex within a species. Here, we aimed to detect the existence of PSD using hybrid. The hybrid produced by crossing male African catfish with female bighead catfish (C. macrocephalus , XX/XY) is a good animal model to study SDSs. Determining the SDS of hybrid catfish can help in understanding the interactions between these two complex SDS systems. Using the genotyping-by-sequencing "DART-seq" approach, we detected seven moderately male-linked loci and seventeen female-linked loci across all the examined hybrid specimens. Most of these loci were not sex-linked in the parental species, suggesting that the hybrid exhibits a combination of different alleles. Annotation of the identified sex-linked loci revealed the presence of one female-linked locus homologous with the B4GALNT1 gene, which is involved in the spermatogenesis pathway and hatchability. However, this locus was not sex-linked in the parental species, and the African catfish might also exhibit PSD. [ABSTRACT FROM AUTHOR]

The snakeskin gourami (Trichopodus pectoralis) has a high meat yield and is one of the top five aquaculture freshwater fishes in Thailand. The species is not externally sexually dimorphic, and its sex determination system is unknown. Understanding the sex determination system of this species will contribute to its full-scale commercialization. In this study, a cytogenetic analysis did not reveal any between-sex differences in chromosomal patterns. However, we used genotyping-by-sequencing to identify 4 male-linked loci and 1 female-linked locus, indicating that the snakeskin gourami tends to exhibit an XX/XY sex determination system. However, we did not find any male-specific loci after filtering the loci for a ratio of 100:0 ratio of males:females. This suggests that the putative Y chromosome is young and that the sex determination region is cryptic. This approach provides solid information that can help identify the sex determination mechanism and potential sex determination regions in the snakeskin gourami, allowing further investigation of genetic improvements in the species. [ABSTRACT FROM AUTHOR]

Sex determination systems (SDSs) in anurans are diverse and have undergone independent evolutionary transitions among species. The mode of sexual reproduction of the rice field frog (Hoplobatrachus rugulosus)—an economically viable, edible amphibian species—is not well known. Previous studies have proposed that threshold temperature conditions may determine sex in these frogs. To elucidate the SDS in H. rugulosus, we karyotyped 10 male and 12 female frogs, and performed fluorescence in situ hybridization combined with sequencing analyses using DArTseq™. Our results revealed a highly conserved karyotype with no sex chromosome heteromorphism, and the sequencing analyses did not identify any consistent sex-linked loci, supporting the hypothesis of temperature-dependent sex determination. The results of this study, and others, on SDSs in the rice field frog and related species also provide support for the theory that heteromorphic sex chromosomes may lead to an evolutionary trap that prevents variable SDSs. These findings add important information to the body of knowledge on H. rugulosus and are likely to have a significant impact on the productivity and economic success of rice field frog farming. [ABSTRACT FROM AUTHOR]

Jade perch (Scortum barcoo) is a new teleost in the developing aquaculture freshwater finfish grow-out sector in Australia and China. However, key information on the breeding sex determination system (SDS) remains poorly understood, hampering sex control programs and genetic improvement. In this study, the jade perch SDS was examined by investigating genome-wide single-nucleotide polymorphisms (SNPs) using diversity arrays technology and cytogenetics analysis to identify the genomic variants associated with sex-linked regions. Although the cytogenetic results showed no variation in the chromosomal patterns between males and females, one male-specific locus and 13 male-linked loci were observed, suggesting that jade perch exhibits male heterogametic XX/XY SDS. Male-specific loci on the putative Y sex chromosome were also identified as an extremely small proportion of the genome. A homology search of the SNP loci revealed the male-specific loci were homologous to the Gypsy transposable element. This might be a remnant of an initial accumulation of repeats on the Y chromosome at the early stage of sex chromosome differentiation. The results provide a base for sex control breeding biotechnologies and genetic improvements to promote sexual size dimorphism and other new approaches to improve the commercial value of jade perch. • Control of sex and reproduction is a precursor to successful aquaculture. • High-throughput DNA sequencing techniques to investigate sex determination systems. • Initial findings of many sex-linked or sex chromosome-specific molecular markers. • We provide a genetic basis of sexual dimorphism for future manipulation. [ABSTRACT FROM AUTHOR]

Bighead catfish (Clarias macrocephalus , Günther, 1864) is an important aquacultural species that plays a crucial role in the economy of Southeast Asia. Crossbreeding between female bighead catfish and male African catfish (C. gariepinus , Burchell, 1822) is used to produce hybrids with vigorous phenotypes. However, sterility of the hybrid is a major obstacle to their mass production. There is an emerging hypothesis that the complexity of the sex-determination system between two parental species might affect sterility. Previous studies investigated the co-existence of XX/XY and ZZ/ZW sex-determination systems in the African catfish population in Thailand, but in bighead catfish the sex-determination system remains poorly understood. In this study, the sex-determination system of the bighead catfish was examined using Diversity Arrays Technology to identify the genomic variants associated with sex-linked regions. The results support the hypothesis of the previous study that the bighead catfish might exhibit a male heterogametic XX/XY sex-determination system with multiple male-linked loci. One of the male-linked loci showed homology with the GTSF1L gene, which shows a testis-enriched expression pattern. Two of the male-linked loci were partially homologous to transposable element. Male-linked loci on the putative Y sex chromosome were identified as an extremely small proportion of the genome. A PCR-based DNA marker was developed to validate the male-linked loci in the bighead catfish. Our findings provide novel insights into sex-determination mechanisms in clariid catfish and will contribute to genetic improvements in breeding programs. • The bighead catfish might exhibit a XX/XY sex-determination system. • Putative Y chromosome were identified as an extremely small proportion of the genome. • One of the male-linked loci showed homology with the GTSF1L gene. • GTSF1L is associated with sex determination and plays a role in spermatogenesis. • Our findings contribute to genetic improvements in aquaculture breeding programs. [ABSTRACT FROM AUTHOR]