Isozyme expression and gene-centromere distances in diploid and triploid hybrid catfish

Allozyme expression of 14 loci was studied in diploid channel catfish Ictalurus punctatus, blue catfish I. furcatus, and diploid and triploid hybrid catfish (channel catfish X blue catfish). This allowed appropriate interpretation of allozyme expression and estimates of gene-centromere distances in triploid hybrid catfish and contributed to genetic characterization of the commercially important ictalurid catfishes. Five polymorphic loci in the parental female channel catfish were used to confirm the retention of an extra diagnostic maternal allele in triploid offspring. Fidelity of gene dosage and allozyme expression of three parental alleles were found at 13 of 14 loci of triploid progeny; however, expression of phosphoglucomutase (PGM) in 174 and 197 triploid hybrids was similar to that of diploid hybrids. Electrophoretic variation for PGM was observed when different buffers were used, indicating nongenetic alteration of expression. Two polymorphic loci in triploid hybrid catfish provided new gene-centromere distances of 14 centimorgans (cM) for aspartate aminotransferase (mAAT*) and 45 cM for glucose-6-phosphate isomerase (GPI-A*). No differences were found in recombination rates of isocitrate dehydrogenase (IDHP*) and peptidase-5 (PEPS-1*) loci of triploid hybrids at different ages. © Copyright by the American Fisheries Society 1996

Allozyme variation in natural and cultured populations in two tilapia species: Oreochromis niloticus and Tilapia zillii

We investigated the electrophoretic polymorphism at 30 protein loci in 27 wild and cultured populations of two tilapia species, Tilapia zillii and Oreochromis niloticus. Single and joint segregations were analysed at 12 polymorphic loci in full-sib families. All the loci showed single segregation in agreement with the Mendelian expectations, and two nonrandom joint segregations were found. In total, 26 loci were polymorphic and 12 were diagnostic between the two species. A pronounced differentiation was observed between the Ivory Coast and Nilo-Sudanian T. zillii populations (average Nei's standard genetic distance = 0.13). Within the Nilo-Sudanian region, the level of variation observed in O. niloticus and T. zillii was rather low when compared with other freshwater species analysed at similar macrogeographical scales. The same pattern of geographical differentiation was found between the west and east African populations in both species suggesting that this structuring mainly reflected the same palaeo-geographical events. Most of the geographical variation observed in the Nile tilapia populations analysed here was present in the cultured stocks. In contrast to fish-farmed stocks analysed in other countries, those from the Ivory Coast and Niger displayed neither evidence of loss of genetic diversity nor any trace of introgression with other cultured tilapia species, indicating that these stocks have been properly managed