An investigation of sex-specific loci may provide important insights into fish sex determination strategies. This may be useful for biotechnological purposes, for example, to produce all-male or all-female fish for commercial breeding. The North African catfish species, Clarias gariepinus, has been widely adopted for aquaculture because its superior growth and disease resistance render the species suitable for hybridization with other catfish to improve the productivity and quality of fish meat. This species has either a ZZ/ZW or XX/XY sex determination system. Here, we investigate and characterize these systems using high-throughput genome complexity reduction sequencing as Diversity Arrays Technology. This approach was effective in identifying moderately sex-linked loci with both single-nucleotide polymorphisms (SNPs) and restriction fragment presence/absence (PA) markers in 30 perfectly sexed individuals of C. gariepinus. However, SNPs based markers were not found in this study. In total, 41 loci met the criteria for being moderately male-linked (with male vs. female ratios 80:20 and 70:30), while 25 loci were found to be moderately linked to female sex. No strictly male- or female-linked loci were detected. Seven moderately male-linked loci were partially homologous to some classes of transposable elements and three moderately male-linked loci were partially homologous to functional genes. Our data showed that the male heterogametic XX/XY sex determination system should co-exist with the ZZ/ZW system in C. gariepinus. Our finding of the co-existence of XX/XY and ZZ/ZW systems can be applied to benefit commercial breeding of this species in Thailand. This approach using moderately sex-linked loci provides a solid baseline for revealing sex determination mechanisms and identify potential sex determination regions in catfish, allowing further investigation of genetic improvements in breeding programs.

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The majority of lizards classified in the superfamily Iguanoidea have an XX/XY sexdetermination system in which sex-chromosomal linkage shows homology with chicken (Gallus gallus) chromosome 15 (GGA15). However, the genomics of sex chromosomes remain largely unexplored owing to the presence of homomorphic sex chromosomes in majority of the species. Recent advances in high-throughput genome complexity reduction sequencing provide an effective approach to the identification of sexspecific loci with both single-nucleotide polymorphisms (SNPs) and restriction fragment presence/absence (PA), and a better understanding of sex chromosome dynamics in Iguanoidea. In this study, we applied Diversity Arrays Technology (DArTseq TM) in 29 phenotypic sex assignments (14 males and 15 females) of green iguana (Iguana iguana). We confirmed a male heterogametic (XX/XY) sex determination mode in this species, identifying 29 perfectly sex-linked SNP/PA loci and 164 moderately sex-linked SNP/PA loci, providing evidence probably indicative of XY recombination. Three loci from among the perfectly sex-linked SNP/PA loci showed partial homology with several amniote sex chromosomal linkages. The results support the hypothesis of an ancestral super-sex chromosome with overlaps of partial sex-chromosomal linkages. However, only one locus among the moderately sex-linked loci showed homology with GGA15, which suggests that the specific region homologous to GGA15 was located outside the non-recombination region but in close proximity to this region of the sex chromosome

To better understand PBI-DdeI satellite DNA located in the centromeric region of python, molecular evolution analysis was conducted on 40 snake species. A ladder-like pattern of DNA bands with repetition of the 194–210 bp monomer was observed in 15 species using PCR. Molecular cloning was performed to obtain 97 AT-rich monomer sequences. Phylogenetic and network analyses showed three PBI-DdeI subfamilies with sequences grouped in species-specific clusters, suggesting rapid evolution. Slow evolution was found in eight species with shared PBI-DdeI sequences, suggesting recent species diversification, allowing PBI-DdeI no time to diverge, with limited homogenization and fixation processes. Quantitative real-time PCR showed large differences in copy number between Python bivittatus and other snakes, consistent with repeat scanning of whole genome sequences. Copy numbers were significantly higher in female Naja kaouthia than in males, concurring with chromosomal distribution of PBI-DdeI specifically localized to female W chromosomes. PBI-DdeI might act as an evolutionary driver with several repeats to promote W chromosome differentiation and heterochromatinization in N. kaouthia. Analysis revealed PBI-DdeI with a reduced copy number, compared to P. bivittatus, in most snakes studied, and it is possible that it subsequently dispersed and amplified on W chromosomes with different functional roles in N. kaouthia.

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