Diversification and speciation in tree frogs from the Maghreb (Hyla meridionalis sensu lato), with description of a new African endemic

Morphometry of two cryptic tree frog species at their hybrid zone reveals neither intermediate nor transgressive morphotypes

Under incomplete reproductive isolation, secondary contact of diverged allopatric lineages may lead to the formation of hybrid zones that allow to study recombinants over several generations as excellent systems of genomic interactions resulting from the evolutionary forces acting on certain genes and phenotypes. Hybrid phenotypes may either exhibit intermediacy or, alternatively, transgressive traits, which exceed the extremes of their parents due to epistasis and segregation of complementary alleles. While transgressive morphotypes have been examined in fish, reptiles, birds, and mammals, studies in amphibians are rare. Here, we associate microsatellite-based genotypes with morphometrics-based morphotypes of two tree frog species of the Hyla arborea group, sampled across a hybrid zone in Poland, to understand whether the genetically differentiated parental species also differ in morphology between each other and their hybrids and whether secondary contact leads to the evolution of intermediate or transgressive morphotypes. Using univariate approaches, explorative multivariate methods (principal component analyses) as well as techniques with prior grouping (discriminant function analyses), we find that morphotypes of both parental species and hybrids differ from each other. Importantly, hybrid morphotypes are neither intermediate nor transgressive but found to be more similar to H. orientalis than to H. arborea.

Common patterns in the molecular phylogeography of western palearctic birds: a comprehensive review

A plethora of studies have offered crucial insights in the phylogeographic status of Western Palearctic bird species. However, an overview integrating all this information and analyzing the combined results is still missing. In this study, we compiled all published peer-reviewed and grey literature available on the phylogeography of Western Palearctic bird species. Our literature review indicates a total number of 198 studies, with the overwhelming majority published as journal articles (n = 186). In total, these literature items offer information on 145 bird species. 85 of these species are characterized by low genetic differentiation, 46 species indicate genetic variation but no geographic structuring i.e. panmixia, while 14 species show geographically distinct lineages and haplotypes. Majority of bird species inhabiting the Western Palearctic display genetic admixture. The glaciation cycles in the past few million years were pivotal factors in shaping this situation: during warm periods many species expanded their distribution range to the north over wide areas of Eurasia; whereas, during ice ages most areas were no longer suitable and species retreated to refugia, where lineages mixed.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10336-021-01893-x.

When Sex Chromosomes Recombine Only in the Heterogametic Sex: Heterochiasmy and Heterogamety in Hyla Tree Frogs

Sex chromosomes are classically predicted to stop recombining in the heterogametic sex, thereby enforcing linkage between sex-determining (SD) and sex-antagonistic (SA) genes. With the same rationale, a pre-existing sex asymmetry in recombination is expected to affect the evolution of heterogamety, for example, a low rate of male recombination might favor transitions to XY systems, by generating immediate linkage between SD and SA genes. Furthermore, the accumulation of deleterious mutations on nonrecombining Y chromosomes should favor XY-to-XY transitions (which discard the decayed Y), but disfavor XY-to-ZW transitions (which fix the decayed Y as an autosome). Like many anuran amphibians, Hyla tree frogs have been shown to display drastic heterochiasmy (males only recombine at chromosome tips) and are typically XY, which seems to fit the above expectations. Instead, here we demonstrate that two species, H. sarda and H. savignyi, share a common ZW system since at least 11 Ma. Surprisingly, the typical pattern of restricted male recombination has been maintained since then, despite female heterogamety. Hence, sex chromosomes recombine freely in ZW females, not in ZZ males. This suggests that heterochiasmy does not constrain heterogamety (and vice versa), and that the role of SA genes in the evolution of sex chromosomes might have been overemphasized.

An odyssey out of Africa: an integrative review of past and present invasions by the Mediterranean tree frog (Hyla meridionalis)

An illustration of the human footprint on biodiversity are the faunal movements that have accompanied commercial and cultural exchanges between civilizations throughout history. In this article, we provide an integrative review of biogeographical and archaeological knowledge to understand these processes for the Mediterranean tree frog (Hyla meridionalis), an African species that has reached several regions of south-western Europe and associated islands. By re-analysing molecular (multilocus mitochondrial and genome-wide markers) and bioclimatic (ecological niche modelling) data, we give a comprehensive picture of the genetic diversity, structure and environmental suitability of H. meridionalis across its natural and exotic ranges, which in turn offer specific clues to the putative routes of colonization and associated events. Long-term monitoring efforts suggest northwards shifts of local range margins, potentially due to global warming, and we further demonstrate that this species is the most frequent amphibian to travel via the food supply chain in Western Europe. High dispersal ability, ecological tolerance and proximity to human settlements have made H. meridionalis a recurrent witness to the complexity and diversity of the civilizations that ruled the Western Mediterranean.

The effect of phylogeographic history on species boundaries: a comparative framework in Hyla tree frogs

Because it is indicative of reproductive isolation, the amount of genetic introgression across secondary contact zones is increasingly considered in species delimitation. However, patterns of admixture at range margins can be skewed by the regional dynamics of hybrid zones. In this context, we posit an important role for phylogeographic history: hybrid zones located within glacial refugia (putatively formed during the Late-Pleistocene) should be better defined than those located in post-glacial or introduced ranges (putatively formed during the Holocene and the Anthropocene). We test this hypothesis in a speciation continuum of tree frogs from the Western Palearctic (Hyla), featuring ten identified contacts between species spanning Plio-Pleistocene to Miocene divergences. We review the rich phylogeographic literature of this group and examine the overlooked transition between H. arborea and H. molleri in Western France using a multilocus dataset. Our comparative analysis supports a trend that contacts zones resulting from post-glacial expansions and human translocations feature more extensive introgression than those established within refugial areas. Integrating the biogeographic history of incipient species, i.e. their age since first contact together with their genetic divergence, thus appears timely to draw sound evolutionary and taxonomic inferences from patterns of introgression across hybrid zones.

A river runs through it: tree frog genomics supports the Dead Sea Rift as a rare phylogeographical break

Phylogeographical breaks can be viewed as regional hotspots of diversity where the genetic integrity of incipient species is put to the test. We focus on an understudied species transition from the Middle East, namely the Dead Sea Rift in the Levant region, which presumably divided the tree frogs Hyla savignyi and H. felixarabica. Combining multilocus genetic analyses (mitochondrial DNA and RAD-sequencing) with ecological niche modelling, we test whether the rift effectively acts as a biogeographical barrier preventing this pair from admixing and merging. The answer is yes: despite weak signs of introgression, all parapatric populations were assigned to either species without cyto-nuclear discordance. Yet, the projected distributions under present and glacial conditions largely overlapped in the area, meaning their current parapatric ranges do not represent an ecological transition. Instead, we hypothesize that H. savignyi and H. felixarabica are maintained apart by limited opportunities for dispersal across the barren Jordan Valley, combined with advanced reproductive isolation. Therefore, the Dead Sea Rift may represent a rare phylogeographical break, and we encourage international efforts to assess its contribution to the rich biodiversity of the Middle East.

Call a spade a spade: taxonomy and distribution of Pelobates, with description of a new Balkan endemic

The genomic era contributes to update the taxonomy of many debated terrestrial vertebrates. In an accompanying work, we provided a comprehensive molecular assessment of spadefoot toads (Pelobates) using genomic data. Our results call for taxonomic updates in this group. First, nuclear phylogenomics confirmed the species-level divergence between the Iberian P.cultripes and its Moroccan relative P.varaldii. Second, we inferred that P.fuscus and P.vespertinus, considered subspecies until recently, feature partial reproductive isolation and thus deserve a specific level. Third, we evidenced cryptic speciation and diversification among deeply diverged lineages collectively known as Pelobatessyriacus. Populations from the Near East correspond to the Eastern spadefoot toad P.syriacus sensu stricto, which is represented by two subspecies, one in the Levant (P.s.syriacus) and the other in the rest of the range (P.s.boettgeri). Populations from southeastern Europe correspond to the Balkan spadefoot toad, P.balcanicus. Based on genetic evidence, this species is also polytypic: the nominal P.b.balcanicus inhabits the Balkan Peninsula; a new subspecies P.b.chloeaessp. nov. appears endemic to the Peloponnese. In this paper, we provide an updated overview of the taxonomy and distribution of all extant Pelobates taxa and describe P.b.chloeaessp. nov.