Genomic structure and diversity of European polyploid spined loaches (Cypriniformes, Cobitidae) in a situation of genetic instability.

The European di-, tri- and tetraploid complex of spined loaches of the genus Cobitis is formed by hybridization of C. elongatoides and three vicarious species, С. taenia, C. tanaitica and C. pontica. It is represented in the form of populations in which parental or related species сoexist with polyploids that reproduce gynogenetically. Insufficient geographic coverage and the unclear genomic composition of certain polyploids and their genetic instability limit the understanding of the diversity and origin of polyploids. This study used allozyme markers, cytometry and karyotyping of spined loaches from six main Ukrainian river systems to reveal a zone of gene introgressions between vicarious species and the presence of di-, tri- and tetraploid hybrids that resulted from the hybridization of C. elongatoides with С. taenia, C. tanaitica and C. pontica vicarious species group representatives. In total, 11 allopolyploid biotypes were identified. Autochthonous biotypes coexist with parental species and partly arise de novo. Allochthonous biotypes form mixed populations with non-parental species and reproduce by cloning. Relict allochthons originated in the past from species whose areas are now separated. At the same time, they are the most abundant in the aquatic systems of Ukraine, except for the Danube basin. Besides the initial subbiotypes, which have allozyme sets corresponding to the gene pools of the parental species, there are also altered ones with derivative allozyme spectra and gene dosage effect anomalies. They result from genetic instability and appear sporadically or massively when their frequency among polyploids in certain populations reaches 97%. In the latter case, their occurrence should be attributed to the pollution of water systems. In addition, there are biotypes with the ambiguous genomic structure and consequently they are of unclear origin.