Molecular characterization of a centromeric satellite DNA in the hemiclonal hybrid frog Rana esculenta and its parental species

Cytogenetics of the Hybridogenetic Frog Pelophylax grafi and Its Parental Species Pelophylax perezi

Hybrid taxa from the genus Pelophylax can propagate themselves in a modified way of sexual reproduction called hybridogenesis ensuring the formation of clonal gametes containing the genome of only one parental (host) species. Pelophylax grafi from South-Western Europe is a hybrid composed of P. ridibundus and P. perezi genomes and it lives with a host species P. perezi (P-G system). Yet it is unknown, whether non-Mendelian inheritance is fully maintained in such populations. In this study, we characterize P. perezi and P. grafi somatic karyotypes by using comparative genomic hybridization, genomic in situ hybridization, fluorescent in situ hybridization, and actinomycin D-DAPI. Here, we show the homeology of P. perezi and P. grafi somatic karyotypes to other Pelophylax taxa with 2n = 26 and equal contribution of ridibundus and perezi chromosomes in P. grafi which supports F1 hybrid genome constitution as well as a hemiclonal genome inheritance. We show that ridibundus chromosomes have larger regions of interstitial (TTAGGG)n repeats flanking the nucleolus organizing region on chromosome no. 10 and a high quantity of AT pairs in the centromeric regions. In P. perezi, we found species-specific sequences in metaphase chromosomes and marker structures in lampbrush chromosomes. Pericentromeric RrS1 repeat sequence was present in perezi and ridibundus chromosomes, but the blocks were stronger in ridibundus. Various cytogenetic techniques applied to the P-G system provide genome discrimination between ridibundus and perezi chromosomal sets. They could be used in studies of germ-line cells to explain patterns of clonal gametogenesis in P. grafi and broaden the knowledge about reproductive strategies in hybrid animals.

Karyotypes of water frogs from the Pelophylax esculentus complex: results of cross-species chromosomal painting

Amphibian species have the largest genome size enriched with repetitive sequences and relatively similar karyotypes. Moreover, many amphibian species frequently hybridize causing nuclear and mitochondrial genome introgressions. In addition, hybridization in some amphibian species may lead to clonality and polyploidization. All such events were found in water frogs from the genus Pelophylax. Among the species within the genus Pelophylax, P. esculentus complex is the most widely distributed and well-studied. This complex includes two parental species, P. ridibundus and P. lessonae, and their hybrids, P. esculentus, reproducing hemiclonally. Parental species and their hybrids have similar but slightly polymorphic karyotypes, so their precise identification is still required. Here, we have developed a complete set of 13 chromosome painting probes for two parental species allowing the precise identification of all chromosomes. Applying chromosomal painting, we identified homologous chromosomes in both parental species and orthologous chromosomes in their diploid hemiclonal hybrids. Comparative painting did not reveal interchromosomal exchanges between the studied water frog species and their hybrids. Using cross-specific chromosome painting, we detected unequal distribution of the signals along chromosomes suggesting the presence of species-specific tandem repeats. Application of chromosomal paints to the karyotypes of hybrids revealed differences in the intensity of staining for P. ridibundus and P. lessonae chromosomes. Thus, both parental genomes have a divergence in unique sequences. Obtained chromosome probes may serve as a powerful tool to unravel chromosomal evolution in phylogenetically related species, identify individual chromosomes in different cell types, and investigate the elimination of chromosomes in hybrid water frogs.

Genome elimination from the germline cells in diploid and triploid male water frogs Pelophylax esculentus

Hybridogenesis is a hemiclonal reproductive strategy in diploid and triploid hybrids. Our study model is a frog P. esculentus (diploid RL and triploids RLL and RRL), a natural hybrid between P. lessonae (LL) and P. ridibundus (RR). Hybridogenesis relies on elimination of one genome (L or R) from gonocytes (G) in tadpole gonads during prespermatogenesis, but not from spermatogonial stem cells (SSCs) in adults. Here we provide the first comprehensive study of testis morphology combined with chromosome composition in the full spectrum of spermatogenic cells. Using genomic in situ hybridization (GISH) and FISH we determined genomes in metaphase plates and interphase nuclei in Gs and SSCs. We traced genomic composition of SSCs, spermatocytes and spermatozoa in individual adult males that were crossed with females of the parental species and gave progeny. Degenerating gonocytes (24%–39%) and SSCs (18%–20%) led to partial sterility of juvenile and adult gonads. We conclude that elimination and endoreplication not properly completed during prespermatogenesis may be halted when gonocytes become dormant in juveniles. After resumption of mitotic divisions by SSCs in adults, these 20% of cells with successful genome elimination and endoreplication continue spermatogenesis, while in about 80% spermatogenesis is deficient. Majority of abnormal cells are eliminated by cell death, however some of them give rise to aneuploid spermatocytes and spermatozoa which shows that hybridogenesis is a wasteful process.

The high diversity of gametogenic pathways in amphispermic water frog hybrids from Eastern Ukraine

Interspecific hybridization can disrupt canonical gametogenic pathways, leading to the emergence of clonal and hemiclonal organisms. Such gametogenic alterations usually include genome endoreplication and/or premeiotic elimination of one of the parental genomes. The hybrid frog Pelophylax esculentus exploits genome endoreplication and genome elimination to produce haploid gametes with chromosomes of only one parental species. To reproduce, hybrids coexist with one of the parental species and form specific population systems. Here, we investigated the mechanism of spermatogenesis in diploid P. esculentus from sympatric populations of P. ridibundus using fluorescent in situ hybridization. We found that the genome composition and ploidy of germ cells, meiotic cells, and spermatids vary among P. esculentus individuals. The spermatogenic patterns observed in various hybrid males suggest the occurrence of at least six diverse germ cell populations, each with a specific premeiotic genome elimination and endoreplication pathway. Besides co-occurring aberrant cells detected during meiosis and gamete aneuploidy, alterations in genome duplication and endoreplication have led to either haploid or diploid sperm production. Diploid P. esculentus males from mixed populations of P. ridibundus rarely follow classical hybridogenesis. Instead, hybrid males simultaneously produce gametes with different genome compositions and ploidy levels. The persistence of the studied mixed populations highly relies on gametes containing a genome of the other parental species, P. lessonae.

Maintenance of pure hybridogenetic water frog populations: Genotypic variability in progeny of diploid and triploid parents

An intriguing outcome of hybridisation is the emergence of clonally and hemiclonally reproducing hybrids, that can sustain, reproduce, and lead to the emergence of polyploid forms. However, the maintenance of diploid and polyploid hybrid complexes in natural populations remains unresolved. We selected water frogs from the Pelophylax esculentus complex to study how diploid and triploid hybrids, which reproduce hemiclonally via hybridogenesis, are maintained in natural populations. During gametogenesis in diploid hybrids, one of the parental genomes is eliminated, and the remaining genome is endoreplicated. In triploid hybrids, the single-copy genome is typically eliminated, while genome endoreplication does not occur. To investigate how diploid and triploid hybrid frogs reproduce in populations without parental species, we crossed these hybrid animals from two separate pure hybrid populations located in Poland. Using cytogenetic analysis of tadpoles that emerged from the crosses, we established which gametes were produced by parental hybrids. The majority of hybrid females and hybrid males produced one type of gamete with the P. ridibundus genome. However, in both studied populations, approximately half of the diploid and triploid hybrids simultaneously produced gametes with different genome compositions and ploidy levels, specifically, the P. ridibundus and P. lessonae genomes, as well as diploid gametes with genomes of both parental species. Triploid hybrid males and females mostly produced haploid gametes with the P. lessonae genome; however, gametes with the P. ridibundus genome have also been observed. These results suggest that not all hybrids follow the classical hybridogenetic reproduction program and reveal a significant level of alterations in the gametogenesis pathways. In addition, we found a variable survival rate of particular progeny genotypes when we crossed hybrid females with different males suggesting the important role of postzygotic barriers on the maintenance of pure hybrid systems. We suggest that the observed variability in produced gametes and the different survival rate of the progeny with certain genotypes is crucial for the existence of pure hybrid systems.

Cytogenetic Characterization of Seven Novel satDNA Markers in Two Species of Spined Loaches (Cobitis) and Their Clonal Hybrids

Interspecific hybridization is a powerful evolutionary force. However, the investigation of hybrids requires the application of methodologies that provide efficient and indubitable identification of both parental subgenomes in hybrid individuals. Repetitive DNA, and especially the satellite DNA sequences (satDNA), can rapidly diverge even between closely related species, hence providing a useful tool for cytogenetic investigations of hybrids. Recent progress in whole-genome sequencing (WGS) offers unprecedented possibilities for the development of new tools for species determination, including identification of species-specific satDNA markers. In this study, we focused on spined loaches (Cobitis, Teleostei), a group of fishes with frequent interspecific hybridization. Using the WGS of one species, C. elongatoides, we identified seven satDNA markers, which were mapped by fluorescence in situ hybridization on mitotic and lampbrush chromosomes of C. elongatoides, C. taenia and their triploid hybrids (C. elongatoides × 2C. taenia). Two of these markers were chromosome-specific in both species, one had centromeric localization in multiple chromosomes and four had variable patterns between tested species. Our study provided a novel set of cytogenetic markers for Cobitis species and demonstrated that NGS-based development of satDNA cytogenetic markers may provide a very efficient and easy tool for the investigation of hybrid genomes, cell ploidy, and karyotype evolution.

Micronuclei in germ cells of hybrid frogs from Pelophylax esculentus complex contain gradually eliminated chromosomes

In most organisms, cells typically maintain genome integrity, as radical genome reorganization leads to dramatic consequences. However, certain organisms, ranging from unicellular ciliates to vertebrates, are able to selectively eliminate specific parts of their genome during certain stages of development. Moreover, partial or complete elimination of one of the parental genomes occurs in interspecies hybrids reproducing asexually. Although several examples of this phenomenon are known, the molecular and cellular processes involved in selective elimination of genetic material remain largely undescribed for the majority of such organisms. Here, we elucidate the process of selective genome elimination in water frog hybrids from the Pelophylax esculentus complex reproducing through hybridogenesis. Specifically, in the gonads of diploid and triploid hybrids, but not those of the parental species, we revealed micronuclei in the cytoplasm of germ cells. In each micronucleus, only one centromere was detected with antibodies against kinetochore proteins, suggesting that each micronucleus comprises a single chromosome. Using 3D-FISH with species-specific centromeric probe, we determined the role of micronuclei in selective genome elimination. We found that in triploid LLR hybrids, micronuclei preferentially contain P. ridibundus chromosomes, while in diploid hybrids, micronuclei preferentially contain P. lessonae chromosomes. The number of centromere signals in the nuclei suggested that germ cells were aneuploid until they eliminate the whole chromosomal set of one of the parental species. Furthermore, in diploid hybrids, misaligned P. lessonae chromosomes were observed during the metaphase stage of germ cells division, suggesting their possible elimination due to the inability to attach to the spindle and segregate properly. Additionally, we described gonocytes with an increased number of P. ridibundus centromeres, indicating duplication of the genetic material. We conclude that selective genome elimination from germ cells of diploid and triploid hybrids occurs via the gradual elimination of individual chromosomes of one of the parental genomes, which are enclosed within micronuclei.

Variation in hybridogenetic hybrid emergence between populations of water frogs from the Pelophylax esculentus complex

Many closely related species are capable of mating to produce hybrid offspring, which are usually sterile. Nevertheless, altering the gametogenesis of hybrid offspring can rescue hybrids from sterility by enabling asexual reproduction. Hybridogenesis is one of the most complicated asexual reproductive modes, and it includes drastic genome reorganization only in the germline; this is achieved through elimination of one parental genome and duplication of the remaining one to restore diploid chromosomal set and overcome blocks in meiotic progression. We investigated a model of hybridogenesis, namely, water frogs from the Pelophylax esculentus complex, for the emergence of asexual reproduction. Further, we assessed the impact of its asexual reproduction on the maintenance of interspecies hybrids from two populations on the western edge of the P. esculentus range, in which hybrids coexist with either both parental species or with only one parental species. After analysing tadpole karyotypes, we conclude that in both studied populations, the majority of diploid hybrid males produced haploid gametes with the P. ridibundus genome after elimination of the P. lessonae genome. Hybrid females exhibited problems with genome elimination and duplication; they usually produced oocytes with univalents, but there were observations of individual oocytes with 13 bivalents and even 26 bivalents. In some hybrid tadpoles, especially F1 crosses, we observed failed germ cell development, while in tadpoles from backcrosses, germ cells were normally distributed and contained micronuclei. By identifying chromosomes present in micronuclei, we estimated that the majority of tadpoles from all crosses were able to selectively eliminate the P. lessonae chromosomes. According to our results, hybridogenesis in hybrids can appear both from crosses of parental species and crosses between sexual species with hybrid individuals. The ability to eliminate a genome and perform endoreplication to ensure gamete formation differed between male and female hybrids from the studied populations. Some diploid hybrid females can rarely produce not only haploid gametes but also diploid gametes, which is a crucial step in the formation of triploid hybrids.

The programmed DNA elimination and formation of micronuclei in germ line cells of the natural hybridogenetic water frog Pelophylax esculentus

DNA elimination is a radical form of gene silencing and occurs both in somatic and germ cells. The programmed DNA elimination occurs during gametogenesis in interspecies hybrids that reproduce by hybridogenesis (stick insects, fishes, and amphibians) and concerns removal of whole genomes of one of the parental species and production of clonal gametes propagating the genome of the other species. The cellular mechanisms differ considerably in hybridogenetic insects and fishes but remains unknown in edible frogs Pelophylax esculentus, natural hybrids between Pelophylax lessonae and Pelophylax ridibundus. Here we report DNA elimination mechanism in early developing gonads of diploid and triploid hybrid frogs, studied by TEM, immunofluorescence, and cytochemistry. In gonocytes of both sexes (primary oogonia and prespermatogonia), micronuclei emerge as detached nuclear buds formed during interphase. We found depletion of nuclear pore complexes in micronuclear membrane and chromatin inactivation via heterochromatinization followed by degradation of micronuclei by autophagy. Micronuclei formation does not lead to apoptotic cell death showing that genome elimination is a physiological process. Chromatin elimination via micronuclei in P. esculentus is unique among hybridogenetic animals and contributes to broadening the knowledge about reproductive modes in animals.

Mutual maintenance of di- and triploid Pelophylax esculentus hybrids in R-E systems: results from artificial crossings experiments

Background

Interspecies animal hybrids can employ clonal or hemiclonal reproduction modes where one or all parental genomes are transmitted to the progeny without recombination. Nevertheless, some interspecies hybrids retain strong connection with the parental species needed for successful reproduction. Appearance of polyploid hybrid animals may play an important role in the substitution of parental species and in the speciation process.

Results

To establish the mechanisms that enable parental species, diploid and polyploid hybrids coexist we have performed artificial crossing experiments of water frogs of Pelophylax esculentus complex. We identified tadpole karyotypes and oocyte genome composition in all females involved in the crossings. The majority of diploid and triploid hybrid frogs produced oocytes with 13 bivalents leading to haploid gametes with the same genome as parental species hybrids usually coexist with. After fertilization of such gametes only diploid animals appeared. Oocytes with 26 bivalents produced by some diploid hybrid frogs lead to diploid gametes, which give rise to triploid hybrids after fertilization. In gonads of all diploid and triploid hybrid tadpoles we found DAPI-positive micronuclei (nucleus-like bodies) involved in selective genome elimination. Hybrid male and female individuals produced tadpoles with variable karyotype and ploidy even in one crossing owing to gametes with various genome composition.

Conclusions

We propose a model of diploid and triploid hybrid frog reproduction in R-E population systems. Triploid Pelophylax esculentus hybrids can transmit genome of parental species they coexist with by producing haploid gametes with the same genome composition. Triploid hybrids cannot produce triploid individuals after crossings with each other and depend on diploid hybrid females producing diploid eggs. In contrast to other population systems, the majority of diploid and triploid hybrid females unexpectedly produced gametes with the same genome as parental species hybrids coexist with.

Electronic supplementary material

The online version of this article (10.1186/s12862-017-1063-3) contains supplementary material, which is available to authorized users.

Unraveling the Sex Chromosome Heteromorphism of the Paradoxical Frog Pseudis tocantins

The paradoxical frog Pseudis tocantins is the only species in the Hylidae family with known heteromorphic Z and W sex chromosomes. The Z chromosome is metacentric and presents an interstitial nucleolar organizer region (NOR) on the long arm that is adjacent to a pericentromeric heterochromatic band. In contrast, the submetacentric W chromosome carries a pericentromeric NOR on the long arm, which is adjacent to a clearly evident heterochromatic band that is larger than the band found on the Z chromosome and justify the size difference observed between these chromosomes. Here, we provide evidence that the non-centromeric heterochromatic bands in Zq and Wq differ not only in size and location but also in composition, based on comparative genomic hybridization (CGH) and an analysis of the anuran PcP190 satellite DNA. The finding of PcP190 sequences in P. tocantins extends the presence of this satellite DNA, which was previously detected among Leptodactylidae and Hylodidae, suggesting that this family of repetitive DNA is even older than it was formerly considered. Seven groups of PcP190 sequences were recognized in the genome of P. tocantins. PcP190 probes mapped to the heterochromatic band in Wq, and a Southern blot analysis indicated the accumulation of PcP190 in the female genome of P. tocantins, which suggests the involvement of this satellite DNA in the evolution of the sex chromosomes of this species.

Genomic characterization of interspecific hybrids between the scallops Argopecten purpuratus and A. irradians irradians

The Peruvian scallop (Argopecten purpuratus) has been introduced to China and has successfully been hybridized with the bay scallop (A. irradians irradians). The F1 hybrids of these two scallops exhibited a large increase in production traits and some other interesting new characteristics. To understand the genetic basis of this heterosis, nuclear gene and partial mtDNA sequences, and genomic in situ hybridization (GISH) were employed to analyze the genomic organization of the hybrids. Amplification of the ribosomal DNA internal transcribed spacer (ITS) showed that the parental ITS sequences were present in all the hybrid individuals, illustrating that the hybrid offspring inherited nuclear DNA from both parents. Sequence analyses of the ITS region further confirmed that the hybrids harbored alleles from their parents; some recombinant variants were also detected, which revealed some alterations in the nuclear genetic material of the hybrids. The analysis of mitochondrial 16S rDNA showed that the hybrids possessed sequences that were identical to the 16S rDNA of the female parents, proving a matrilineal inheritance of mitochondrial genes in scallops. In addition, GISH clearly discriminated between the parental chromosomes and indicated a combination of haploid genomes of duplex parents in the hybrids. The genetic analyses in our study illustrated that the F1 hybrids inherited nuclear material from both parents and cytoplasmic genetic material maternally, and some variations occurred in the genome, which might contribute to a further understanding of crossbreeding and heterosis in scallop species.

Cytological maps of lampbrush chromosomes of European water frogs (Pelophylax esculentus complex) from the Eastern Ukraine

Background

Hybridogenesis (hemiclonal inheritance) is a kind of clonal reproduction in which hybrids between parental species are reproduced by crossing with one of the parental species. European water frogs (Pelophylax esculentus complex) represent an appropriate model for studying interspecies hybridization, processes of hemiclonal inheritance and polyploidization. P. esculentus complex consists of two parental species, P. ridibundus (the lake frog) and P. lessonae (the pool frog), and their hybridogenetic hybrid – P. esculentus (the edible frog). Parental and hybrid frogs can reproduce syntopically and form hemiclonal population systems. For studying mechanisms underlying the maintenance of water frog population systems it is required to characterize the karyotypes transmitted in gametes of parental and different hybrid animals of both sexes.

Results

In order to obtain an instrument for characterization of oocyte karyotypes in hybrid female frogs, we constructed cytological maps of lampbrush chromosomes from oocytes of both parental species originating in Eastern Ukraine. We further identified certain molecular components of chromosomal marker structures and mapped coilin-rich spheres and granules, chromosome associated nucleoli and special loops accumulating splicing factors. We recorded the dissimilarities between P. ridibundus and P. lessonae lampbrush chromosomes in the length of orthologous chromosomes, number and location of marker structures and interstitial (TTAGGG)_n-repeat sites as well as activity of nucleolus organizer. Satellite repeat RrS1 was mapped in centromere regions of lampbrush chromosomes of the both species. Additionally, we discovered transcripts of RrS1 repeat in oocytes of P. ridibundus and P. lessonae. Moreover, G-rich transcripts of telomere repeat were revealed in association with terminal regions of P. ridibundus and P. lessonae lampbrush chromosomes.

Conclusions

The constructed cytological maps of lampbrush chromosomes of P. ridibundus and P. lessonae provide basis to define the type of genome transmitted within individual oocytes of P. esculentus females with different ploidy and from various population systems.

Fertilization and cytogenetic examination of interspecific reciprocal hybridization between the scallops, Chlamys farreri and Mimachlamys nobilis

Crossbreeding is a powerful tool for improving productivity and profitability in aquaculture. We conducted a pilot study of an artificial cross between two important cultivated scallops in China, Chlamys farreri and Mimachlamys nobilis, to test the feasibility of interspecific hybridization. Reciprocal hybridization experiments were performed using a single-pair mating strategy (M. nobilis ♀ × C. farreri ♂ and C. farreri ♀ × M. nobilis ♂). The fertilization of each pair was tracked using fluorescence staining of the gametes, and the chromosomes of the F1 hybrid larvae were examined via conventional karyotyping and genomic in situ hybridization (GISH). We observed moderate fertilization success in both interspecific crosses, although the overall fertilization was generally less rapid than that of intraspecific crosses. Conventional karyotyping showed that 70.4% of the viable F1 larvae in M. nobilis ♀ × C. farreri ♂ and 55.4% in C. farreri ♀ × M. nobilis ♂ comprised hybrid karyotypes (2n = 35 = 6m+5sm+11st+13t), and the results were further confirmed by GISH. Interestingly, we detected a few F1 from the M. nobilis ♀ × C. farreri ♂ cross that appeared to have developed gynogenetically. In addition, chromosome fragmentations, aneuploids and allopolyploids were observed in some F1 individuals. Our study presents evidence that the artificial cross between M. nobilis and C. farreri is experimentally possible. Further investigations of the potential heterosis of the viable F1 offspring at various developmental stages should be conducted to obtain a comprehensive evaluation of the feasibility of crossbreeding between these two scallop species.

Evidence for integrity of parental genomes in the diploid hybridogenetic water frog Pelophylax esculentus by genomic in situ hybridization

The Western Palearctic water frogs Pelophylax ridibundus and P. lessonae were identified as parental (sexual) species and P. esculentus as their interspecific, hybridogenetically reproducing hybrid with hemiclonal heredity. We used genomic in situ hybridization (GISH) to identify parental chromosomes of P.lessonae and P.ridibundus in diploid P. esculentus karyotypes (2n = 26). GISH probes were made by fluorochrome labeling of total genomic DNA extracted from the sexual progenitors. The labeled probe from one species was hybridized to chromosomes of P. esculentus in the presence of excess of unlabeled genomic DNA from the other species. Thus, the P. lessonae probe was blocked by P. ridibundus unlabeled DNA, and vice versa. We successfully discriminated each of the 13 respective parental chromosomes in metaphase complements of the hybrids according to species-specific hybridization signals. GISH enabled us to confirm additional differences between parental chromosomes in size (smaller chromosomes belong to P. lessonae) and in the presence of DAPI-positive centromeric heterochromatin (detected in chromosomes of P. ridibundus, but not in P. lessonae). The fact that no visible intergenomic exchanges were found in metaphase chromosomes of diploid P. esculentus provides important information on the genomic integrity of hemiclonal transmission and supports hybridogenesis as a reproductive mode at the chromosome level for the specimens examined.

RrS1-like sequences of water frogs from Central Europe and around the Aegean Sea: chromosomal organization, evolution, possible function

RrS1-like sequences of water frogs (genus Pelophylax) display varied genomic organization, whereas the centromeric hybridization pattern reveals species-specific differences. Using fluorescent in situ hybridization, Pelophylax cf. bedriagae, Pelophylax kurtmuelleri, and Pelophylax ridibundus showed a hybridization signal at centromeres of chromosomes 1-5, but in P. kurtmuelleri the medium-small chromosome labeled was 10 rather than 8. Pelophylax cretensis had almost 16 of 26 centromeres labeled, as did Pelophylax lessonae from Poland when its chromosomes are hybridized with a homologous probe. When StuI-digested genomic DNA was hybridized with RrS1 probe, hybridization ladders for P. ridibundus from Poland have evenly spaced steps (about 100 bp) of uniform intensity from about 200 bp upward. Steps in hybridization ladders from circum-Aegean taxa vary in intensity: larger, odd-numbered steps are often fainter. A strong double band (800/900 bp) in Anatolian P. cf. bedriagae, emphasized by a weak 700 bp band, distinguishes them from P. kurtmuelleri from the Peloponnisos, in which the 900 bp band is almost absent. The ladder in P. cretensis lacks odd-numbered steps. A and B repeats, observed originally within the RrS1 satellite of P. ridibundus, occur also in the circum-Aegean frogs and in P. lessonae, Pelophylax epeiroticus, Pelophylax saharicus, and Pelophylax shqipericus. It is plausible that AB dimers or ABB trimers rather than A or B monomers correspond to functional/evolutionary units. The presence of regions similar to yeast CDEs and mammalian CENP-B boxes suggests a role for RrS1 sequences in centromere organization.

Plasma sex steroid and thyroid hormones profile in male water frogs of the Rana esculenta complex from agricultural and pristine areas

Some chemical compounds used in intensive agriculture have been found to induce estrogenic effects; therefore a histological analysis of the testes and an evaluation of plasma levels of sex steroid, thyroid hormones, and vitellogenin were carried out in adult male water frogs of two coexisting taxa (Rana lessonae and the hemiclonal hybrid Rana esculenta) sampled in agricultural and pristine areas. Differences in seasonal profiles of hormones were found in water frogs living in the agricultural area where the presence of endocrine disrupting compounds was suspected on the basis of a previous study. In R. esculenta, sampled in the pristine area, high androgen levels were found in May; the opposite trend was found for R. esculenta sampled in agricultural areas in which the highest androgen levels were found in September, significantly lower compared with those found in R. esculenta sampled in the pristine area. Low androgen levels were also recorded in R. lessonae males sampled both in pristine and agricultural areas, while the highest levels were found in September. Regarding the trend of estradiol-17beta, an increase of this hormone was found in July both in esculenta and lessonae sampled in the agricultural area, and in the same month an estradiol-17beta peak, even though lower, was also found both in esculenta and lessonae males captured in the pristine area; detectable vitellogenin was found neither in males captured in the agricultural area, nor in those sampled in the pristine one. Moreover, while no significant changes of thyroid hormones were found either in the esculenta or lessonae males sampled in the pristine area, increased T3 and T4 titers were found in July in both esculenta and lessonae captured in the agricultural area. Morphological differences of the testes in males of parental species captured in the agricultural area were also observed. These findings indicate alterations in endocrine and reproductive function in frogs in the agricultural area, that could suggest the presence of endocrine disrupting compounds.

Molecular characterization of the first satellite DNA with CENP-B and CDEIII motifs in the bat Pipistrellus kuhli

The centromere is an essential structure in the chromosomes of all eukariotes and is central to the mechanism that ensures proper segregation during mitosis and meiosis. The comparison of DNA sequence motifs, organization and kinetocore components from yeast to man is beginning to indicate that, although centromeres are highly variable DNA elements, a conserved pattern of sequence arrangement and function is emerging. We have identified and characterized the first satellite DNA (P.k.SAT) from microbat species Pipistrellus kuhli. The presence of mammalian CENP-B box and yeast CDEIII box could indicate the participation of P.k.SAT in centromere organization.

Identification of xenopus CENP-A and an associated centromeric DNA repeat

Kinetochores are the proteinaceous complexes that assemble on centromeric DNA and direct eukaryotic chromosome segregation. The mechanisms by which higher eukaryotic cells define centromeres are poorly understood. Possible molecular contributors to centromere specification include the underlying DNA sequences and epigenetic factors such as binding of the centromeric histone centromere protein A (CENP-A). Frog egg extracts are an attractive system for studying centromere definition and kinetochore assembly. To facilitate such studies, we cloned a Xenopus laevis homologue of CENP-A (XCENP-A). We identified centromere-associated DNA sequences by cloning fragments of DNA that copurified with XCENP-A by chromatin immunoprecipitation. XCENP-A associates with frog centromeric repeat 1 (Fcr1), a 174-base pair repeat containing a possible CENP-B box. Southern blots of partially digested genomic DNA revealed large ordered arrays of Fcr1 in the genome. Fluorescent in situ hybridization with Fcr1 probes stained most centromeres in cultured cells. By staining lampbrush chromosomes, we specifically identified the 11 (of 18) chromosomes that stain consistently with Fcr1 probes.

A satellite DNA containing CENP-B box-like motifs is present in the antarctic scallop Adamussium colbecki

The DNA of the Antarctic scallop Adamussium colbecki was found to contain a highly repeated sequence identifiable upon restriction with endonuclease BglII. The monomeric unit - denominated pACS (about 170bp long) - was cloned. Southern blot hybridization yielded a ladder-like banding pattern, indicating that the repeated elements are tandemly arranged in the genome and therefore represent a sequence of satellite DNA. Sequence analysis of five different clones revealed the presence of various subfamilies, some of which showed a high degree of divergence. In each clone, regions homologous to the mammalian CENP-B box were observed. A region homologous to the CDEIII centromeric sequence of yeast was also found in one of the clones. These observations suggest a relationship of the pACS family to the centromeric area in A. colbecki.

Genomes of two 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, 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.