Climate-induced shifts in the niche similarity of two related spadefoot toads (genus Pelobates)

The eastern boundary of the geographic range of the Pallas’ spadefoot Pelobates vespertinus (Anura, Amphibia) is limited by overwintering temperatures

We studied the relationship between the geographic distribution of the Pallas’ spadefoot (Pelobates vespertinus) and the soil temperature regime at the eastern boundary of this species’ range (south-western West Siberia and northwest Kazakhstan). This species overwinters underground, burrowing down to 1.5 m or deeper and has poor frost tolerance – it is unable to withstand temperatures below 0 °C, therefore, the temperature at its overwintering depth has to be above zero. A cartographic approach to identifying the distribution of zero isotherms was used. Winter soil temperatures at depths of 80 cm, 120 cm, 160 cm and 240 cm were plotted, based on deep soil thermometer data and the outline of the spadefoot’s range was compared with close-to-zero isotherms in deep soil. Within the range of this species, the depth of the position of zero isotherms increases eastwards: from 80 cm in European Russia, to more than 160 cm in the south-west of Western Siberia. The eastern boundary of the species’ range lies in the forest-steppe, steppe and semi-desert zones in the Tobol-Ishim interfluve and further south, on the left bank area of the Turgai River. This boundary clearly coincides with the zero isotherm, which lies here at a depth of 160 cm, forming an impermeable barrier to the spadefoot’s distribution.

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.

Phylogeography of a cryptic speciation continuum in Eurasian spadefoot toads (Pelobates)

Cryptic phylogeographic diversifications provide unique models to examine the role of phylogenetic divergence on the evolution of reproductive isolation, without extrinsic factors such as ecological and behavioural differentiation. Yet, to date very few comparative studies have been attempted within such radiations. Here, we characterize a new speciation continuum in a group of widespread Eurasian amphibians, the Pelobates spadefoot toads, by conducting multilocus (restriction site associated DNA sequencing and mitochondrial DNA) phylogenetic, phylogeographic and hybrid zone analyses. Within the P. syriacus complex, we discovered species-level cryptic divergences (>5 million years ago [My]) between populations distributed in the Near-East (hereafter P. syriacus sensu stricto [s.s.]) and southeastern Europe (hereafter P. balcanicus), each featuring deep intraspecific lineages. Altogether, we could scale hybridizability to divergence time along six different stages, spanning from sympatry without gene flow (P. fuscus and P. balcanicus, >10 My), parapatry with highly restricted hybridization (P. balcanicus and P. syriacus s.s., >5 My), narrow hybrid zones (~15 km) consistent with partial reproductive isolation (P. fuscus and P. vespertinus, ~3 My), to extensive admixture between Pleistocene and refugial lineages (≤2 My). This full spectrum empirically supports a gradual build up of reproductive barriers through time, reversible up until a threshold that we estimate at ~3 My. Hence, cryptic phylogeographic lineages may fade away or become reproductively isolated species simply depending on the time they persist in allopatry, and without definite ecomorphological divergence.

Food availability influences postmetamorphic growth in two spadefoot toad species (genus Pelobates)

Understanding how major life history traits such as body size and mass and growth change in response to resource availability is crucial in explaining life history trade-offs. We conducted a laboratory experiment with three (high, medium and low) feeding intensity treatments using metamorphs of two spadefoot toads species,Pelobates syriacusandP. fuscus, from syntopic populations. We tested how total food consumption, final body size and mass, body mass increase, body mass and length growth rates and growth efficiency are influenced by food availability. The responses to food availability differed significantly between the species with respect to the total food consumption, body mass increase, body mass growth rate and growth efficiency (i.e. the ratio between total amount of food consumed during the experiment divided by the increase in body mass).P. syriacusmetamorphs had higher growth rates and growth efficiency thanP. fuscusjuveniles. Also,P. syriacusjuveniles responded to differences in food level by increasing growth efficiency with decreasing food levels. OverallP. syriacusseems better adapted to shortages in food availability thanP. fuscus. Our results clearly indicate that the differences in body size between the two species originate between metamorphosis and sexual maturity.