The evolution of polymorphism in the warning coloration of the Amazonian poison frog Adelphobates galactonotus

Distribution and diversification of Adelphobates, emblematic poison frogs from Brazilian Amazonia

Adelphobates contains three species, and the inaccurate identification of A. quinquevittatus and the scarcity of records of A. castaneoticus complicate inference of their distributions; the latter species occurs in sympatry with A. galactonotus. Our objective was to revise the distributions of Adelphobates by compiling data and modeling habitat suitability, as range limits may be shaped by landscape features and biotic interactions. We initially analyzed the existence of operational taxonomic units within the nominal species and subsequently inferred the observed and potential distributions, taking into account the possible independent lineages for the three species, and we also generated a molecular timetree to understand the chronology of interspecific diversification events. Adelphobates quinquevittatus was found to have a more easterly distribution than previously described, and specimens with phenotypic variation were found to occur in areas inconsistent with the modeling, and A. castaneoticus was concentrated in the Tapajós-Xingu interfluve, surrounded by A. galactonotus. Models indicated that the right bank of the Xingu River is suitable for both species, indeed, both were found there. Despite Adelphobates species having their distributions delimited by major Amazonian rivers, estimated divergence times predate the formation of the modern river network, suggesting that other mechanisms were involved in their diversification.

Anti-predator defences are linked with high levels of genetic differentiation in frogs

Predator-prey interactions have been suggested as drivers of diversity in different lineages, and the presence of anti-predator defences in some clades is linked to higher rates of diversification. Warning signals are some of the most widespread defences in the animal world, and there is evidence of higher diversification rates in aposematic lineages. The mechanisms behind such species richness, however, are still unclear. Here, we test whether lineages that use aposematism as anti-predator defence exhibit higher levels of genetic differentiation between populations, leading to increased opportunities for divergence. We collated from the literature more than 3000 pairwise genetic differentiation values across more than 700 populations from over 60 amphibian species. We find evidence that over short geographical distances, populations of species of aposematic lineages exhibit greater genetic divergence relative to species that are not aposematic. Our results support a scenario where the use of warning signals could restrict gene flow, and suggest that anti-predator defences could impact divergence between populations and potentially have effects at a macro-evolutionary scale.

Colour, location and movement: what do models tell us about predation on colour morphs of a poison frog from eastern Amazonia?

Many dendrobatid frogs are known to be aposematic: brightly coloured and unpalatable to predators. To deceive predators, frog models used to test for predatory colour bias must be similar in size, colour, shape, and movement to frogs. We carried out an experiment with moving models of the species Adelphobates galactonotus, in two localities. A. galactonotus is a polytypic frog and each population of the species has a distinct colour. Birds and mammals were the vertebrates responsible for the marks on the models, but there was no difference in frequency of attacks on local-, non-local- and cryptic-colour models. Only invertebrates avoided cryptic models. Different populations of the species seem to be under different predation pressure, but colour differentiation in this species is probably related to other mechanisms, such as sexual selection.

Contrasting environmental drivers of genetic and phenotypic divergence in an Andean poison frog (Epipedobates anthonyi)

Phenotypic and genetic divergence are shaped by the homogenizing effects of gene flow and the differentiating processes of genetic drift and local adaptation. Herein, we examined the mechanisms that underlie phenotypic (size and color) and genetic divergence in 35 populations (535 individuals) of the poison frog Epipedobates anthonyi along four elevational gradients (0-1800 m asl) in the Ecuadorian Andes. We found phenotypic divergence in size and color despite relatively low genetic divergence at neutral microsatellite loci. Genetic and phenotypic divergence were both explained by landscape resistance between sites (isolation-by-resistance, IBR), likely due to a cold and dry mountain ridge between the northern and southern elevational transects that limits dispersal and separates two color morphs. Moreover, environmental differences among sites also explained genetic and phenotypic divergence, suggesting isolation-by-environment (IBE). When northern and southern transects were analyzed separately, genetic divergence was predicted either by distance (isolation-by-distance, IBD; northern) or environmental resistance between sites (IBR; southern). In contrast, phenotypic divergence was primarily explained by environmental differences among sites, supporting the IBE hypothesis. These results indicate that although distance and geographic barriers are important drivers of population divergence, environmental variation has a two-fold effect on population divergence. On the one hand, landscape resistance between sites reduces gene flow (IBR), while on the other hand, environmental differences among sites exert divergent selective pressures on phenotypic traits (IBE). Our work highlights the importance of studying both genetic and phenotypic divergence to better understand the processes of population divergence and speciation along ecological gradients.

Evolutionary history and genetic connectivity across highly fragmented populations of an endangered daisy

Conservation management can be aided by knowledge of genetic diversity and evolutionary history, so that ecological and evolutionary processes can be preserved. The Button Wrinklewort daisy (Rutidosis leptorrhynchoides) was a common component of grassy ecosystems in south-eastern Australia. It is now endangered due to extensive habitat loss and the impacts of livestock grazing, and is currently restricted to a few small populations in two regions >500 km apart, one in Victoria, the other in the Australian Capital Territory and nearby New South Wales (ACT/NSW). Using a genome-wide SNP dataset, we assessed patterns of genetic structure and genetic differentiation of 12 natural diploid populations. We estimated intrapopulation genetic diversity to scope sources for genetic management. Bayesian clustering and principal coordinate analyses showed strong population genetic differentiation between the two regions, and substantial substructure within ACT/NSW. A coalescent tree-building approach implemented in SNAPP indicated evolutionary divergence between the two distant regions. Among the populations screened, the last two known remaining Victorian populations had the highest genetic diversity, despite having among the lowest recent census sizes. A maximum likelihood population tree method implemented in TreeMix suggested little or no recent gene flow except potentially between very close neighbours. Populations that were more genetically distinctive had lower genetic diversity, suggesting that drift in isolation is likely driving population differentiation though loss of diversity, hence re-establishing gene flow among them is desirable. These results provide background knowledge for evidence-based conservation and support genetic rescue within and between regions to elevate genetic diversity and alleviate inbreeding.

The Influence of Environmental Variation on the Genetic Structure of a Poison Frog Distributed Across Continuous Amazonian Rainforest

Biogeographic barriers such as rivers have been shown to shape spatial patterns of biodiversity in the Amazon basin, yet relatively little is known about the distribution of genetic variation across continuous rainforest. Here, we characterize the genetic structure of the brilliant-thighed poison frog (Allobates femoralis) across an 880-km-long transect along the Purus-Madeira interfluve south of the Amazon river, based on 64 individuals genotyped at 7609 single-nucleotide polymorphism (SNP) loci. A population tree and clustering analyses revealed 4 distinct genetic groups, one of which was strongly divergent. These genetic groups were concomitant with femoral spot coloration differences, which was intermediate within a zone of admixture between two of the groups. The location of these genetic groups did not consistently correspond to current ecological transitions between major forest types. A multimodel approach to quantify the relative influence of isolation-by-geographic distance (IBD) and isolation-by-environmental resistance (IBR) nevertheless revealed that, in addition to a strong signal of IBD, spatial genetic differentiation was explained by IBR primarily linked to dry season intensity (r2 = 8.4%) and canopy cover (r2 = 6.4%). We show significant phylogenetic divergence in the absence of obvious biogeographical barriers and that finer-scaled measures of genetic structure are associated with environmental variables also known to predict the density of A. femoralis.