No abstract

Diploid F1 hybrids between Rana ridibunda from Adriatic SW Yugoslavia and Polish R. lessonae, and between Polish R. ridibunda and an unnamed species from SW Yugoslavia are shown by electrophoresis and examination of lampbrush chromosomes to contain both parental genomes in their diploid oocytes I. In contrast, central European R. ridibunda genomes in diploid hybrids, whether Fls or from natural hemiclonal lineages, induce

Mitotic chromosomes of the European water frogs Rana ridibunda and Rana lessonae, the parental species of Rana esculenta, differ significantly in their centromeric regions: when C-banded or when made fluorescent, the centromeres of R. ridibunda (and of ridibunda chromosomes in R. esculenta) are visible as a conspicuous dark granule or as a conspicuous fluorescent spot; the centromeres of R. lessonae (and of the lessonae chromosomes in R. esculenta) are inconspicuous or not fluorescent. Lampbrush chromosomes of these three taxa are described in detail for the first time; those of R. ridibunda and R. lessonae differ significantly in morphostructural characters such as conspicuousness of centromeres and number, form, and location of giant loops as well as in chiasma frequency. Chromosomes of the two parental species can thus be distinguished when present in lampbrush complements of hybrids. Reproduction in both sexes of natural R. esculenta lineages is hemiclonal: only the unrecombined genome of one parental species, usually R. ridibunda, is transmitted to haploid gametes (hybridogenesis). In 18 hybrids from natural populations of Poland, somatic tissues had allodiploid complements with chromosomes from each parental species. In contrast, spermatocytes I of five males and oocytes I of seven of eight females (221 of 222 oocytes) were autodiploid and contained only R. ridibunda chromosomes that formed n bivalents. These 12 hybrids thus were hybridogenetic. A single female hybrid had oocytes I (33 of 34) with genomes of both parental species; they showed various disturbances including tetraploidy, reduced number of chiasmata, and incomplete synapsis resulting in univalents. This individual thus was not hybridogenetic. The irregular lampbrush patterns indicate that such hybrids will have severely reduced fertility and most of their successful gametes will result in allotriploid progeny.

Hybrid water frogs Rana esculenta reproduce by hybridogenesis: one parental genome (of Rana lessonae) is excluded in the germ line, the other (of Rana ridibunda) is clonally transmitted to haploid gametes. The two parental species differ in that the amount of centromeric heterochromatin revealed by differential staining is much higher in Rana ridibunda. An abundant, tandemly arrayed, centromeric satellite DNA, designated RrS1, is revealed in Rana ridibunda genomes by the restriction endonuclease Stul, which generates a major repetitive sequence fragment of 300 and a minor one of 200 bp. This AT-rich (68%) satellite family is located at the centromeres of the five largest chromosomes (1-5) and of a medium to small heterobrachial one (8 or 9); it thus constitutes only part of the centromeric heterochromatin that characterizes all Rana ridibunda chromosomes. RrS1 represents about 2.5% of the genome of Rana ridibunda; it may represent as little as 0.2% of the genome of Rana lessonae, and cannot be detected in Xenopus laevis frogs or Salamandra salamandra and Triturus carnifex salamanders. Segments of the satellite sequence are similar to sequences of yeast centromeric DNA element CDEIII and of the mammalian CENP-B box. A role for RrS1 and other centromeric satellite DNAs in the germ line genome exclusion of the hybridogenetic frog hybrids, although suggested, has not yet been demonstrated.

European water frog hybrids Rana esculenta (R. ridibundarR. lessonae) reproduce hemiclonally, by hybridogenesis : in the germ line they exclude the genome of one parental species and produce haploid gametes with an unrecombined genome of the other parental species. In the widespread L-E population system, both sexes of hybrids (E) coexist with R. lessonae (L). They exclude the lessonae genome and produce ridibunda gametes. In the R-E system, hybrid males coexist with R. ridibunda (R); they exclude either their ridibunda or their lessonae genome and produce sperm with a lessonae or with a ridibunda genome or a mixture of both kinds of sperm. We examined 13 male offspring, 12 of which were from crosses between L-E system and R-E system frogs. All were somatically hybrid. With one exception, they excluded the lessonae genome in the germ line and subsequently endoreduplicated the ridibunda genome. Spermatogonial metaphases contained a haploid or a diploid number of ridibunda chromosomes, identified through in situ hybridization to a satellite DNA marker, and by spermatocyte I metaphases containing a haploid number of ridibunda bivalents. The exception, an F1 hybrid between L-E system R. lessonae and R-E system R. ridibunda, was not hybridogenetic, showed no genome exclusion, and evidenced a disturbed gametogenesis resulting from the combination of two heterospecific genomes. None of the hybridogenetic hybrids showed any cell lines excluding the ridibunda genome, the pattern most frequent in hybrids of the R-E system, unique to that system, and essential for its persistence. A particular combination of R-E system lessonae and R-E system ridibunda genomes seems necessary to induce the R-E system type of hemiclonal gametogenesis. Gametogenesis of intergroup hybrids of hemiclonal frogs SummaryEuropean water frog hybrids Rana esculenta (R. ridibundarR. lessonae) reproduce hemiclonally, by hybridogenesis : in the germ line they exclude the genome of one parental species and produce haploid gametes with an unrecombined genome of the other parental species. In the widespread L-E population system, both sexes of hybrids (E) coexist with R. lessonae (L). They exclude the lessonae genome and produce ridibunda gametes. In the R-E system, hybrid males coexist with R. ridibunda (R) ; they exclude either their ridibunda or their lessonae genome and produce sperm with a lessonae or with a ridibunda genome or a mixture of both kinds of sperm. We examined 13 male offspring, 12 of which were from crosses between L-E system and R-E system frogs. All were somatically hybrid. With one exception, they excluded the lessonae genome in the germ line and subsequently endoreduplicated the ridibunda genome. Spermatogonial metaphases contained a haploid or a diploid number of ridibunda chromosomes, identified through in situ hybridization to a satellite DNA marker, and by spermatocyte I metaphases containing a haploid number of ridibunda bivalents. The exception, an F1 hybrid between L-E system R. lessonae and R-E system R. ridibunda, w...