Reproductive Isolation in the Snails, Pomatiopsis Lapidaria and P. Cincinnatiensis

Abstract

Even though their ranges overlap, two species of the American snail Pomatiopsis are usually ecologically isolated, with Pomatiopsis lapidaria (Say) inhabiting seepage areas and P. cincinnati- ensis (Lea) occurring only on river banks. A site was found on the River Raisin in Michigan where both species occurred together. Inten- sive studies over an extended period indicated that these species do not interbreed under natural, sympatric conditions. Previous to World War II, when the interest in human blood fluke was revived, Bartsch (1936) had grouped the Oriental snails which serve as intermediate hosts of Schistosoma japonicum under three gen- era: Katayama, Schistosomophora and Oncomelania. These are now all included in the single genus Oncomelania (Abbott, 1948). The status of the four species (0. quadrasi, 0. nosophora, 0. formo- sana, and 0. hupensis) within this genus is now also open to question. In this connection, Pesigan and co-workers (unpublished data) were able to intercross three species (0. quadrasi, 0. nosophora and 0. for- mosana) in any combination. These crosses were later confirmed by Wagner and Chi Wong (1959); in addition, they were able to cross 0. hupensis with the other three species. The hybrids of all these On- comelania crosses produced fertile offspring. Furthermore, it has been possible to cross males of the North American species, Pomatiopsis lapidaria, with females of Oncomelania quadrasi and 0. formosana (van der Schalie, et al., in press). The offspring of at least the former cross proved to be fertile. Attempted crosses of another North Ameri- can species, Pomatiopsis cincinnatiensis, with the four species of On- comelania and P. lapidaria were all unsuccessful. All of the nominal species of Oncomelania are allopatric. The tax- onomic status ascribed to, them depends on the definition of a species when dealing with allopatric populations. Judgments regarding the relationship between Oncomelania and Pomatiopsis also involve groups widely separated in nature. Although the ranges of P. lapidaria and P. cincinnatiensis overlap throughout an extensive area (van der Schalie and Dundee, 1955; Dundee, 1957), these species are usually ecological- ly isolated. Studies over an extended period indicate that P. cincin- natiensis occurs only on the banks of rivers, while P. lapidaria normally lives in seepage areas in the vicinity of streams and lakes. However, the latter species may occasionally appear on river banks (probably in