GENOMES OF 2 WATER FROG SPECIES RESIST GERM-LINE EXCLUSION IN INTERSPECIES HYBRIDS

Abundant natural interspecies hybrids between the European water frog Rana ridibunda and at least three other taxa reproduce hemiclonally, b y hybridogenesis: the non-ridibunda genome is excluded in the germ lin e before meiosis, and the unrecombined ridibunda genome is transmitted to haploid gametes. In contrast, natural hybrids between Rana ridibun da and either of two Balkan species (Rana shqiperica and Rana epeiroti ca) do not show such genome exclusion. This plausibly results from fai lure of Balkan Rana ridibunda genomes to ''induce'' such exclusion in the germ line of hybrids, from ''resistance'' of Rana shqiperica and R ana epeirotica genomes to such exclusion in hybrids with an ''inducing '' Rana ridibunda genome, or both. We tested the second hypothesis by examining lampbrush chromosome patterns in oocytes of hybrids that in the soma contain one ''inducing'' ridibunda genome and one genome of e ither of the two Balkan species. Several lampbrush chromosome markers (e.g., presence and location of certain giant loops and conspicuousnes s and width of centromeres) discriminate sets of Rana ridibunda chromo somes from those of Rana shqiperica and Rana epeirotica. Based on such markers, nine diploid female hybrids between Rana ridibunda or Rana e sculenta from natural hybridogenetic lineages (Rana ridibunda x Rana l essonae, making ridibunda gametes) from central Poland and either Rana shqiperica or Rana epeirotica each contained both parental genomes in primary oocytes; the bivalents showed reduced numbers of chiasmata co mpared with parental species. It follows that none of these hybrids wa s hybridogenetic. This conclusion is confirmed, for two hybrids betwee n Rana epeirotica and either Rana ridibunda or Rana esculenta, by prot ein electrophoretic comparison of somatic tissues with primary oocytes , all of which evidenced allelic markers of both parental species. Bec ause Rana ridibunda genomes that are known to induce germ line genome exclusion when combined in hybrids with Rana lessonae genomes were use d, these data provide the first compelling evidence for resistance of Rana shqiperica as well as Rana epeirotica genomes to such exclusion. (C) 1997 Wiley-Liss, Inc.