Abstract -- Abundant natural interspecies hybrids between the European water
frog Rana ridibunda and at least three other taxa reproduce hemiclonally,
by hybridogenesis: the non-ridibunda genome is excluded in the germ line
before meiosis, and the unrecombined ridibunda genome is transmitted to
haploid gametes. In contrast, natural hybrids between Rana ridibunda and
either of two Balkan species (Rana shqiperica and Rana epeirotica) do not
show such genome exclusion. This plausibly results from failure of Balkan
Rana ridibunda genomes to "induce" such exclusion in the germ line of
hybrids, from "resistance" of Rana shqiperica and Rana 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 either of the two Balkan species.
Several lampbrush chromosome markers (e.g., presence and location of
certain giant loops and conspicuousness and width of centromeres)
discriminate sets of Rana ridibunda chromosomes from those of Rana
shqiperica and Rana epeirotica. Based on such markers, nine diploid female
hybrids between Rana ridibunda or Rana esculenta from natural
hybridogenetic lineages (Rana ridibunda x Rana lessonae, 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 compared with parental species. It
follows that none of these hybrids was hybridogenetic. This conclusion is
confirmed, for two hybrids between Rana epeirotica and either Rana
ridibunda or Rana esculenta, by protein electrophoretic comparison of
somatic tissues with primary oocytes, all of which evidenced allelic
markers of both parental species. Because Rana ridibunda genomes that are
known to induce germ line genome exclusion when combined in hybrids with
Rana lessonae genomes were used, these data provide the first compelling
evidence for resistance of Rana shqiperica as well as Rana epeirotica
genomes to such exclusion.
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