First successful hybridization experiment between native European weatherfish (Misgurnus fossilis) and non-native Oriental weatherfish (M. anguillicaudatus) reveals no evidence for postzygotic barriers

The European weatherfish Misgurnus fossilis (Linnaeus, 1758) is a threatened freshwater species in large parts of Europe and might come under pressure from currently establishing exotic weatherfish species. Additional threats might arise if those species hybridize which has been questioned in previous research. Regarding the hybridization of M. fossilis × M. anguillicaudatus (Cantor, 1842), we demonstrate that despite the considerable genetic distance between parental species, the estimated long divergence time and different ploidy levels do not represent a postzygotic barrier for hybridization of the European and Oriental weatherfish. The paternal species can be easily differentiated based on external pigment patterns with hybrids showing intermediate patterns. No difference in standard metabolic rate, indicating a lack of hybrid vigour, renders predictions of potential threats to the European weatherfish from hybridization with the Oriental weatherfish difficult. Therefore, the genetic and physiological basis of invasiveness via hybridization remains elusive in Misgurnus species and requires further research. The existence of prezygotic reproductive isolation mechanisms and the fertility of F1 hybrids remains to be tested to predict the potential threats of globally invasive Oriental weatherfish species.

Defining and Disrupting Species Boundaries in Saccharomyces

The genus Saccharomyces is an evolutionary paradox. On the one hand, it is composed of at least eight clearly phylogenetically delineated species; these species are reproductively isolated from each other, and hybrids usually cannot complete their sexual life cycles. On the other hand, Saccharomyces species have a long evolutionary history of hybridization, which has phenotypic consequences for adaptation and domestication. A variety of cellular, ecological, and evolutionary mechanisms are responsible for this partial reproductive isolation among Saccharomyces species. These mechanisms have caused the evolution of diverse Saccharomyces species and hybrids, which occupy a variety of wild and domesticated habitats. In this article, we introduce readers to the mechanisms isolating Saccharomyces species, the circumstances in which reproductive isolation mechanisms are effective and ineffective, and the evolutionary consequences of partial reproductive isolation. We discuss both the evolutionary history of the genus Saccharomyces and the human history of taxonomists and biologists struggling with species concepts in this fascinating genus.

Transcriptomic Analysis Reveals Insect Hormone Biosynthesis Pathway Involved in Desynchronized Development Phenomenon in Hybridized Sibling Species of Tea Geometrids (Ectropis grisescens and Ectropis obliqua)

Ectropis grisescens and Ectropis obliqua are sibling species of tea-chewing pests. An investigation of the distribution of tea geometrids was implemented for enhancing controlling efficiency. E. grisescens is distributed across a wider range of tea-producing areas than Ectropis obliqua in China with sympatric distribution found in some areas. In order to explore reproductive isolation mechanisms in co-occurrence areas, hybridization experiments were carried out. Results showed they can mate but produce infertile hybrids. During experiments, the desynchronized development phenomenon was found in the hybridized generation of sibling tea geometrids. Furthermore, transcriptome analysis of those individuals of fast-growing and slow-growing morphs revealed that the insect hormone biosynthesis pathway was enriched in two unsynchronized development groups of hybrid offspring. More importantly, some genes regulating the synthesis of moulting hormone showed significantly up-regulated expression in fast-growing groups. Above all, metabolism of the juvenile hormone and synthesis of the ecdysone pathway were found to be crucially involved in the desynchronized development phenomenon. This research finding contributes to a better understanding of the mechanisms of insect development and reproductive isolation of two sibling species.

Impact of hybridisation in two Cossypha robin-chat species in southern Africa

Chorister Robin-Chat Cossypha dichroa, a South African forest endemic, and Red-capped Robin-Chat C. natalensis, a widely distributed species in African forest and woodland, are inferred to hybridise in areas of sympatry. DNA was extracted from blood samples of C. dichroa (n = 18), C. natalensis (n = 47), and two phenotypic hybrids. The mitochondrial cytochrome c oxidase I (COI) gene was amplified by PCR and sequenced. Phylogenetic analysis was performed on the sequence data to investigate taxonomic status and putative interspecific hybridisation. Phenotypic hybrids grouped with C. natalensis, suggesting maternal parentage from that species. Intra- and interspecific genetic and geographic distances were compared between C. dichroa and C. natalensis to assess genetic introgression. Seven of the thirteen microsatellite primer pairs developed for C. natalensis cross amplified in C. dichroa. These seven markers were then used for further analysis. STRUCTURE v2.3.4 was used to assign individuals to a particular genetic cluster and determine any admixture. NEWHYBRIDS v1.1 was used to assign hybrid status to samples beyond the F1 generation. Despite the hybridisation events recorded between C. dichroa and C. natalensis they still form two separate clusters as expected, and two genetic clusters (K=2) were identified using STRUCTURE. These two species are proficient vocal mimics and it is likely that reproductive isolation mechanisms are overcome through vocalisations. Genotypic hybrids are evident in the sampled population and hybridisation and backcrossing across a zone of sympatry is occurring. However, hybridisation is expected to have very little evolutionary influence on the integrity of recently diverged species which retain reproductive isolation across a wide region of sympatry through call distinctness.

Impact of hybridisation in two Cossypha robin-chat species in southern Africa

Chorister Robin-Chat Cossypha dichroa, a South African forest endemic, and Red-capped Robin-Chat C. natalensis, a widely distributed species in African forest and woodland, are inferred to hybridise in areas of sympatry. DNA was extracted from blood samples of C. dichroa (n = 18), C. natalensis (n = 47), and two phenotypic hybrids. The mitochondrial cytochrome c oxidase I (COI) gene was amplified by PCR and sequenced. Phylogenetic analysis was performed on the sequence data to investigate taxonomic status and putative interspecific hybridisation. Phenotypic hybrids grouped with C. natalensis, suggesting maternal parentage from that species. Intra- and interspecific genetic and geographic distances were compared between C. dichroa and C. natalensis to assess genetic introgression. Seven of the thirteen microsatellite primer pairs developed for C. natalensis cross amplified in C. dichroa. These seven markers were then used for further analysis. STRUCTURE v2.3.4 was used to assign individuals to a particular genetic cluster and determine any admixture. NEWHYBRIDS v1.1 was used to assign hybrid status to samples beyond the F1 generation. Despite the hybridisation events recorded between C. dichroa and C. natalensis they still form two separate clusters as expected, and two genetic clusters (K=2) were identified using STRUCTURE. These two species are proficient vocal mimics and it is likely that reproductive isolation mechanisms are overcome through vocalisations. Genotypic hybrids are evident in the sampled population and hybridisation and backcrossing across a zone of sympatry is occurring. However, hybridisation is expected to have very little evolutionary influence on the integrity of recently diverged species which retain reproductive isolation across a wide region of sympatry through call distinctness.

Modulation of Prdm9-controlled meiotic chromosome asynapsis overrides hybrid sterility in mice

Hybrid sterility is one of the reproductive isolation mechanisms leading to speciation. Prdm9, the only known vertebrate hybrid-sterility gene, causes failure of meiotic chromosome synapsis and infertility in male hybrids that are the offspring of two mouse subspecies. Within species, Prdm9 determines the sites of programmed DNA double-strand breaks (DSBs) and meiotic recombination hotspots. To investigate the relation between Prdm9-controlled meiotic arrest and asynapsis, we inserted random stretches of consubspecific homology on several autosomal pairs in sterile hybrids, and analyzed their ability to form synaptonemal complexes and to rescue male fertility. Twenty-seven or more megabases of consubspecific (belonging to the same subspecies) homology fully restored synapsis in a given autosomal pair, and we predicted that two or more DSBs within symmetric hotspots per chromosome are necessary for successful meiosis. We hypothesize that impaired recombination between evolutionarily diverged chromosomes could function as one of the mechanisms of hybrid sterility occurring in various sexually reproducing species.

Modulation of Prdm9-controlled meiotic chromosome asynapsis overrides hybrid sterility in mice

The infertility of hybrids between closely related species is one of the reproductive isolation mechanisms leading to speciation. Prdm9, the only known vertebrate hybrid sterility gene causes failure of meiotic chromosome synapsis and infertility in male hybrids between mouse strains derived from two mouse subspecies. Within species Prdm9 determines the sites of programmed DNA double-strand breaks and meiotic recombination hotspots. To investigate the relation between Prdm9-controlled meiotic arrest and asynapsis, we inserted random stretches of consubspecific homology on several autosomal pairs in sterile hybrids and analyzed their ability to form synaptonemal complexes and rescue male fertility. Twenty-seven or more Mb of consubspecific homology fully restored synapsis in a given autosomal pair and we predicted that two symmetric DSBs or more per chromosome are necessary for successful meiosis. We hypothesize that impaired recombination between evolutionary diverged homologous chromosomes could function as one of the mechanisms of hybrid sterility occurring in various sexually reproducing species.

Natural Hybridization between Persian Walnut and Chinese Walnut Revealed by Simple Sequence Repeat Markers

Hybridization between species of the genus Juglans is common because of weak reproductive isolation mechanisms between closely related species with sympatric distributions. In this research, we investigated the possibility of naturally occurring interspecific hybrids between two species in the genus Juglans: persian walnut (Juglans regia) and chinese walnut (Juglans cathayensis). We used 12 pairs of microsatellite markers to analyze introgression between the two species. All amplified microsatellites were polymorphic in the two species. The result of Bayesian admixture analyses showed that introgression between the two species is rare; only three of nine individuals tentatively identified as hybrids, based on intermediate morphological characteristics, were defined as mixed genotypes. The other six putative hybrids and 156 morphologically pure individuals showed no sign of introgression.