Mechanisms for introgression in allotriploid fish

Allotriploids (bearing two chromosome sets from one species + one chromosome set from another species) can be obtained from backcrosses between interspecific hybrids that produce unreduced gametes and one of the parental species. Sexual reproduction of allotriploids producing recombinant offspring has been recently described in the genus Salmo (Atlantic salmon, Salmo salar, and brown trout, Salmo trutta). Although they produce a high rate of abnormal spermatozoids, allotriploid male hybrids of Atlantic salmon (2n = 58) and brown trout (2n = 80) produce some functional reduced gametes that can fertilize salmon ova, generating salmon-like offspring. These F2 offspring bear some brown trout genes. When a telomeric probe was hybridized on synaptonemal complexes of allotriploid males only double or single signals were obtained, demonstrating that chromosome pairing in bivalents or univalents occurred in meiosis prophase I. This is parallel to homologous pairing described in plant allotriploid meiosis. Recombinant genotypes in offspring of allotriploid salmonids can be explained by monosomic inheritance of some trout chromosomes, not by heterologous pairing and recombination. The use of introgressive hybridization for transferring desired genes (or linkage groups) into cultivated species could be considered a subject of further exploration.