Ethological Differences and the Probability of Introgressive Hybridization Between Two Species of the Ninespine Stickleback, Pungitius Pungitius and Pungitius Platygaster, in Eurasia

Abstract

Two closely related allopatric species of the ninespine stickleback occur in Eurasia. One, Pungitius pungitius, is widespread within the arctic basin and the other, P. platygaster, ranges in the southern regions (Pontearal-Caspian province). The ranges of both species were affected in recent years by human activities and there now appears to be a strong likelihood that their ranges will come to overlap in the near future. The differences in reproductive behavior among species of the Gasterosteidae plays a leading role in the formation of interspecific barriers (WOOTTON, 1976). In anticipation of the eventual contact of the two forms, I conducted experiments to investigate the question: Does the prolonged adaptation of these forms to different ecological conditions result in sterility of hybrids or not? Some preference tests were made between sexual partners in aquaria under laboratory conditions. The methods I used (following FOSTER, 1977) increased the likelihood of free choice of conspecific or sexual partners. Females of both forms approximately equally chose to spawn with males of their own and the unfamiliar species. Some females of both species showed preference for males of P. pungitius, which used more striking and demonstrative postures in the nuptial dance. The main differences in the spawning behavior of the two forms actually amount to differences in the elements of twisting and leading, and to the faster completion of spawning in P. platygaster. It seems, that these differences are of only slight importance and, in case of sympatry of the two forms in the wild, probably would not prevent hybridization. Interspecific group crosses of P. pungitius x P. platygaster in controlled natural conditions (ponds) confirmed my assumption that hybrid populations would be viable. F1 and F2 hybrids and backcrosses were fertile and viable, and hybrid-pond populations were self-reproductive and increased their numbers during the two years that I observed them. There is an interesting peculiarity in the spawning of hybrid males. Instead of building their nests on stems of plants, like males of either pure-species form, they build their nests under rocks on the substrate, like the males of Gasterosteus aculeatus. In summary, the results of my experiments have led me to conclude that, in case of contact between the two species in the wild (Irtyshriver, West Siberia), it is highly probable that substantial amounts of introgressive hybridization will occur.