Background. Acipenserid fishes may be divided into three groups in terms of their ploidy levels: 1) 120-chromosomes, 2) 250–270-chromosomes, and 3) about 370-chromosomes. The experimental crosses show that sturgeon species with the same ploidy often produce fertile hybrids, whereas the hybrids between species with different ploidy cannot reproduce because of female sterility or full sterility. To date, the only case of backcross hybrids obtained by hybridization of sturgeons with different ploidy is known: in these crosses, the sterlet, Acipenser ruthenus Linnaeus, 1758, with about 120 chromosomes and kaluga, Acipenser dauricus Georgi, 1775, with about 270 chromosomes were used as parental species; numerous progenies were obtained for females of both species and fertile hybrid males. The aims of this study were to determine the chromosome numbers in hybrid spermatozoa, involved in the production of viable backcross progenies, and to ascertain the chromosome numbers in backcrosses. Materials and Methods. The experiments for obtaining backcrosses were conducted in the Luchegorsk experimental station of the TINRO Centre in 2010. 16 one-year-old backcross hybrids sterlet × (sterlet × kaluga) and 14 one-year-old backcrosses kaluga × (sterlet × kaluga) were karyologically studied by using previously described method. From one to 23 metaphase plates were analyzed from every studied fish. Results. Average chromosome numbers in 14 backcross hybrids sterlet × (sterlet × kaluga) varied from 139 to 157. It means that hybrid males (sterlet × kaluga), participated in their origin, produced spermatozoa with 79–87 chromosomes. Two other backcrosses sterlet × (sterlet × kaluga) had about 178 and 184 chromosomes. Thus, they got 118–124 chromosomes from their fathers. Among backcrosses kaluga × (sterlet × kaluga), 11 specimens had 201–214 chromosomes, and three specimens: 219–223. It means that hybrid males, participating in their origin, should produce spermatozoa with 66–84 and 84–93 chromosomes, respectively. Conclusion. This study suggests non-random generation of chromosome sets in spermatozoa of fertile sterlet × kaluga hybrids. However, the mechanisms of meiosis, providing a non-random production of spermatozoa with similar numbers of chromosomes, are still unknown.
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