1 Zoological Museum, University of Zürich (Switzerland), 2 Department of Ecology, Ethology and Evolution, University of Illinois, Urbana (USA), 3 Polish Academy of Sciences, Poznan (Poland), 4 Biozentrum, University of Wien (Austria)
Data on genetic sex determination in animal groups lacking morphologically differentiated gonosomes may help to gain insights into the evolution of sex determining mechanisms. Results of crossing experiments indicate that European water frogs have a male heterogametic XX-XY sex determining mechanism. In backcross progeny of a male hybrid (Rana shqiperica x Rana lessonae) to Rana lessonae, 4 codominantly expressed enzyme loci were classically linked to gonadal sex: mannose-6-phosphate isomerase (MPI), lactate dehydrogenase-B (LDH-B), hexokinase-1, and peptidase B. A small fraction (3-12%) of recombinants between sex and each of the linked loci was found. The data confirm the XX-XY sex determination and suggest that the effective sex-determining factors are confined to a small region, possibly a single locus, of the sex chromosome. Using another taxon pair, sex linkage of MPI was confirmed in a male hybrid (Rana lessonae x an unnamed southern Italian taxon) backcrossed to Rana lessonae, and sex and MPI assorted independently, as expected for an XX-XY mechanism, in a backcross of a female hybrid, Italian taxon x Rana lessonae, to Rana lessonae. A second backcross of the male hybrid (Rana lessonae x Italian taxon) to another Rana lessonae female yielded 76 all-male progeny. MPI again evidenced that segregation of the respective chromosome occurred in the hybrid father, however, indicating that both XY and XX frogs were phenotypically males. Natural lineages of the hemiclonally reproducing hybrid Rana esculenta (Rana ridibunda x Rana lessonae) in the southeastern part of its range (Pannonian Basin) consist virtually only of females. Crossing experiments suggested that in this region both XX and XY hybrids are female. Tight sex-linkage of LDH-B in female and male progeny of a Rana esculenta female from Lake Neusiedl crossed with a heterozygous Rana lessonae male from Poland, and LDH-B segregation in all-female progeny of the same female crossed with an Austrian Rana lessonae male confirm femaleness of both XX and XY hybrids. These two cases of unisexuality apparently reflect disturbed sex determination occurring in specific genome combinations of hybrids. In contrast to several protein-coding marker loci tested that showed evolutionary conservation of linkages, sex has been assigned to a variety of different linkage groups in Holarctic frogs of the genus Rana. This multiplicity may reflect independent origins of sex determination genes, relaxed selective constraints on chromosomal translocations of a major sex determining locus or region, or determination of sex by a series of genes, with different genes of the set acting as the major sex determinant in different frog species.