African catfish (Clarias gariepinus) is a promising food fish species with significant potential and growing mass of production in freshwater aquaculture. Male African catfish possess improved production characteristics over females, therefore the use of monosex populations could be advantageous for aquaculture production. However, our knowledge about the sex determination mechanism of this species is still limited and controversial. A previously isolated male-specific DNA marker (CgaY1) was validated using offspring groups from targeted crosses (n=630) and it was found to predict the sex of 608 individuals correctly (96.43% accuracy). Using the proportion of recombinants, we estimated the average genetic distance between the potential sex determination locus and the sex-specific marker to be 2.69 cM. As an earlier study suggested that both XX/XY and ZZ/ZW systems coexist in this species, we tested the applicability of their putative moderately sex-linked loci and found that no sex-specific amplification could be detected for any of them. In addition, temperature-induced masculinization suggested by others was also tested, but no such effect was detected in our stocks when the published parameters were used for heat treatment. Altogether, our results support an exclusive XX/XY sex determination system in our African catfish stock and indicate a good potential for the future use of this male-specific DNA marker in research and commercial production.

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Background Invasive gibel carp, Carassius gibelio (Bloch, 1782) has become well-established in the Hungarian waters and now are spreading in the European waters. On major concern now is the potential hybridization between gibel carp and the other invasive species in the Carassius auratus complex (CAC), which may further accelerate the spread of the whole invasive species complex. The identification of gibel carp and their hybrids is difficult because of its morphological similarity to the other species in CAC. Here we carry out a genomic assessment to understand the history of gibel carp invasion and its phylogenetic relationship with the other species in CAC. Three loci of the mitochondrial genome (D-loop, CoI, Cytb) were used to determine the phylogenetic origin of individuals and relarionship among six gibel carp populations and the other species in the CAC. Methodolgy A total of 132 gibel carp samples from six locations in Southern Transdanubia (Hungary) were collected after phenotypic identification to measure the genetic diversity within and among gibel carp populations of Southern Transdanubia (Hungary). The genetic background was examined by the sequences of the mitochondrial genome: D-loop, Cytochrome c oxidase I (CoI) and Cytochrome b (Cytb). Mitochondrial genetic markers are excellent tools for phylogenetic studies because they are maternally inherited. Successfully identified haplotypes were aligned and with reference sequences in nucleotide databases (i.e., NCBI-BLAST: National Centre for Biotechnology Information and BOLD: Barcode of Life Data System). The phylogenetic relationships among gibel carp populations were then analyzed together with the reference sequences to understand the relationship and the level of hybridization with the species in CAC. Results Among the 132 aligned D-loop sequences 22 haplotypes were identified. Further examination of representative individuals of the 22 haplotypes, six Cytb and four CoI sequences were detected. The largest number of haplotypes of all three loci were found in Lake Balaton, the largest shallow lake in Central Europe. Based on the NCBI-BLAST alignment of the D-loop, haplotypes of Carassius auratus auratus and Carassius a. buergeri in CAC were identified in the C. gibelio samples. Further analysis of haplotypes with the other two mitochondrial markers confirmed the occurrence of intragenus hybridization of C. gibelio in the Hungarian waters. Conclusion By using three mitochondrial markers (D-loop, Cytb, CoI), we genomically characterized a gibel carp-complex in Hungarian waters and assessed the C. gibelio phylogenetic status between them. Hybrid origin of locally invasive Carassius taxon was detected in Hungary. It points out that invasive species are not only present in Hungary but reproduce with each other in the waters, further accelerating their spread.