Speciation by hybridization in 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.

Successful mating and hybridisation in two closely related flatworm species despite significant differences in reproductive morphology and behaviour

Reproductive traits are some of the fastest diverging characters and can serve as reproductive barriers. The free-living flatworm Macrostomum lignano, and its congener M. janickei are closely related, but differ substantially in their male intromittent organ (stylet) morphology. Here, we examine whether these morphological differences are accompanied by differences in behavioural traits, and whether these could represent barriers to successful mating and hybridization between the two species. Our data shows that the two species differ in many aspects of their mating behaviour. Despite these differences, the species mate readily with each other in heterospecific pairings. Although both species have similar fecundity in conspecific pairings, the heterospecific pairings revealed clear postmating barriers, as few heterospecific pairings produced F1 hybrids. These hybrids had a stylet morphology that was intermediate between that of the parental species, and they were fertile. Finally, using a mate choice experiment, we show that the nearly two-fold higher mating rate of M. lignano caused it to mate more with conspecifics, leading to assortative mating, while M. janickei ended up mating more with heterospecifics. Thus, while the two species can hybridize, the mating rate differences could possibly lead to higher fitness costs for M. janickei compared to M. lignano.

Comparative cytogenetics of Auchenorrhyncha (Hemiptera, Homoptera): a review

A comprehensive review of cytogenetic features is provided for the large hemipteran suborder Auchenorrhyncha, which currently contains approximately 42,000 valid species. This review is based on the analysis of 819 species, 483 genera, and 31 families representing all presently recognized Auchenorrhyncha superfamilies, e.i. Cicadoidea (cicadas), Cercopoidea (spittle bugs), Membracoidea (leafhoppers and treehoppers), Myerslopioidea (ground-dwelling leafhoppers), and Fulgoroidea (planthoppers). History and present status of chromosome studies are described, as well as the structure of chromosomes, chromosome counts, trends and mechanisms of evolution of karyotypes and sex determining systems, their variation at different taxonomic levels and most characteristic (modal) states, occurrence of parthenogenesis, polyploidy, B-chromosomes and chromosome rearrangements, and methods used for cytogenetic analysis of Auchenorrhyncha.

Origin and evolution of animal hybrid species

The increasing number of hybrid species, discovered in both vertebrates and invertebrates by the combined use of chromosome, allozyme and molecular markers, calls for a reevaluation of hybrid speciation and reticulate evolution In animals. The array of reproductive strategies recently detected In phylogenetically related stick Insects allows us to Investigate, using a comparative approach, questions such as the relationship between hybridization and unisexuality, and short- versus long-term evolutionary success of hybrid species. Unexpected similarities are now apparent in hybrid evolution of animals as varied as insects, snails, fish, frogs and lizards. Hybrid species may combine, to some extent, the main advantage of sex (genetic diversity) with those of clonal reproduction. This explains why these species are often so successful, and indicates a potential use of some hybrid species in experimental biology and resource management (e.g. mass production of animal proteins).

Genome exclusion and gametic DAPI-DNA content in the hybridogenetic Bacillus rossius-grandii benazzii complex (Insecta Phasmatodea)

Among Sicilian stick insects, two hybridogenetic complexes have been discovered: Bacillus rossius-grandii benazzii and B. rossius-grandii grandii, which also produce androgenetic offspring. The egg maturation of the former is analyzed here through DAPI fluorometry, which, besides the assessment of the meiotic stages, also allows their DNA measurements and the analysis of sperm-head evolution into male pronuclei in these polyspermic eggs. Hybridogenetic eggs undergo an extrasynthesis of chromosomes, because two groups of n autobivalents (4C each) are segregated at metaphase 1st; the two groups must correspond to the pure parental species haplosets. Then the grandii chromosomes degenerate (1st polar body), while the rossius chromosomes divide further to produce two groups of n autodiads (2C each); one of them degenerates (2nd polar body), and the other is ready to perform syngamy (female pronucleus). Meanwhile, several B. grandii sperm evolve into male pronuclei by doubling their DNA (from 1C to 2C content) and assuming an interphase nucleus appearance. If regular mixis occurs, the F1 hybrid constitution is restored but, if it fails, a fusion between two sperms may occur, originating fully paternal descendants (natural androgenesis). The genome exclusion mechanism of stick-insect hybridogens appears to be more primitive than those observed in the already known hybridogenetic complexes of Poeciliopsis and Rana esculenta. Unfertilized eggs of hybridogens are capable of self-activation, but the cytology of the related clonally reproducing B. whitei indicates that its parthenogenetic mechanism stems from the hybridization event (hybrid theory) rather than from tychoparthenogenetic potentialities (spontaneous theory).