Sexual signal evolution and patterns of assortative mating across an intraspecific contact zone

Contact zones provide important insights into the evolutionary processes that underlie lineage divergence and speciation. Here, we use a contact zone to ascertain speciation potential in the red-eyed treefrog (Agalychnis callidryas), a brightly coloured and polymorphic frog that exhibits unusually high levels of intraspecific variation. Populations of A. callidryas differ in a number of traits, several of which are known sexual signals that mediate premating reproductive isolation in allopatric populations. Along the Caribbean coast of Costa Rica, a ~100 km contact zone, situated between two phenotypically and genetically divergent parent populations, contains multiple colour pattern phenotypes and late-generation hybrids. This contact zone provides the opportunity to examine processes that are important in the earliest stages of lineage divergence. We performed analyses of colour pattern variation in five contact zone sites and six parental sites and found complex, continuous colour variation along the contact zone. We found discordance between the geographic distribution of colour pattern and previously described genomic population structure. We then used a parental site and contact zone site to measure assortative mating and directional selection from naturally-occurring amplectant mating pairs. We found assortative mating in a parental population, but no assortative mating in the contact zone. Furthermore, we uncovered evidence of directional preference towards the adjacent parental phenotype in the contact zone population, but no directional preference in the parent population. Combined, these data provide insights into potential dynamics at the contact zone borders and indicate that incipient speciation between parent populations will be slowed.

Genetic isolation by distance underlies colour pattern divergence in red-eyed treefrogs (Agalychnis callidryas)

Investigating the spatial distribution of genetic and phenotypic variation can provide insights into the evolutionary processes that shape diversity in natural systems. We characterized patterns of genetic and phenotypic diversity to learn about drivers of colour-pattern diversification in red-eyed treefrogs (Agalychnis callidryas) in Costa Rica. Along the Pacific coast, red-eyed treefrogs have conspicuous leg colour patterning that transitions from orange in the north to purple in the south. We measured phenotypic variation of frogs, with increased sampling at sites where the orange-to-purple transition occurs. At the transition zone, we discovered the co-occurrence of multiple colour-pattern morphs. To explore possible causes of this variation, we generated a single nucleotide polymorphism data set to analyse population genetic structure, measure genetic diversity and infer the processes that mediate genotype-phenotype dynamics. We investigated how patterns of genetic relatedness correspond to individual measures of colour pattern along the coast, including testing for the role of hybridization in geographic regions where orange and purple phenotypic groups co-occur. We found no evidence that colour-pattern polymorphism in the transition zone arose through recent hybridization. Instead, a strong pattern of genetic isolation by distance indicates that colour-pattern variation was either retained through other processes such as ancestral colour polymorphisms or ancient secondary contact, or else it was generated by novel mutations. We found that phenotype changes along the Pacific coast more than would be expected based on genetic divergence and geographic distance alone. Combined, our results suggest the possibility of selective pressures acting on colour pattern at a small geographic scale.

An experimental test of disease resistance function in the skin-associated bacterial communities of three tropical amphibian species

Variation in the structure of host-associated microbial communities has been correlated with the occurrence and severity of disease in diverse host taxa, suggesting a key role of the microbiome in pathogen defense. However, whether these correlations are typically a cause or consequence of pathogen exposure remains an open question, and requires experimental approaches to disentangle. In amphibians, infection by the fungal pathogen Batrachochytrium dendrobatidis (Bd) alters the skin microbial community in some host species, whereas in other species, the skin microbial community appears to mediate infection dynamics. In this study, we completed experimental Bd exposures in three species of tropical frogs (Agalychnis callidryas, Dendropsophus ebraccatus, Craugastor fitzingeri) that were sympatric with Bd at the time of the study. For all three species, we identified key taxa within the skin bacterial communities that were linked to Bd infection dynamics. We also measured higher Bd infection intensities in D. ebraccatus and C. fitzingeri that were associated with higher mortality in C. fitzingeri. Our findings indicate that microbially-mediated pathogen resistance is a complex trait that can vary within and across host species, and suggest that symbiont communities that have experienced prior selection for defensive microbes may be less likely to be disturbed by pathogen exposure.

Genetic and phenotypic evidence of a contact zone between divergent colour morphs of the iconic red-eyed treefrog

Hybrid zones act as natural laboratories where divergent genomes interact, providing powerful systems for examining the evolutionary processes underlying biological diversity. In this study, we characterized patterns of genomic and phenotypic variation resulting from hybridization between divergent intraspecific lineages of the Neotropical red-eyed treefrog (Agalychnis callidryas). We found genetic evidence of a newly discovered contact zone and phenotypic novelty in leg colour-a trait suspected to play a role in mediating assortative mating in this species. Analysis of hybrid ancestry revealed an abundance of later-generation F_n individuals, suggesting persistence of hybrids in the contact zone. Hybrids are predominantly of southern ancestry but are phenotypically more similar to northern populations. Genome-wide association mapping revealed QTL with measurable effects on leg-colour variation, but further work is required to dissect the architecture of this trait and establish causal links. Further, genomic cline analyses indicated substantial variation in patterns of introgression across the genome. Directional introgression of loci associated with different aspects of leg colour are inherited from each parental lineage, creating a distinct hybrid colour pattern. We show that hybridization can generate new phenotypes, revealing the evolutionary processes that potentially underlie patterns of phenotypic diversity in this iconic polytypic frog. Our study is consistent with a role of hybridization and sexual selection in lineage diversification, evolutionary processes that have been implicated in accelerating divergence in the most phenotypically diverse species.

Antimicrobial peptide and sequence variation along a latitudinal gradient in two anurans

BACKGROUND

While there is evidence of both purifying and balancing selection in immune defense genes, large-scale genetic diversity in antimicrobial peptides (AMPs), an important part of the innate immune system released from dermal glands in the skin, has remained uninvestigated. Here we describe genetic diversity at three AMP loci (Temporin, Brevinin and Palustrin) in two ranid frogs (Rana arvalis and R. temporaria) along a 2000 km latitudinal gradient. We amplified and sequenced part of the Acidic Propiece domain and the hypervariable Mature Peptide domain (~ 150-200 bp) in the three genes using Illumina Miseq and expected to find decreased AMP genetic variation towards the northern distribution limit of the species similarly to studies on MHC genetic patterns.

RESULTS

We found multiple loci for each AMP and relatively high gene diversity, but no clear pattern of geographic genetic structure along the latitudinal gradient. We found evidence of trans-specific polymorphism in the two species, indicating a common evolutionary origin of the alleles. Temporin and Brevinin did not form monophyletic clades suggesting that they belong to the same gene family. By implementing codon evolution models we found evidence of strong positive selection acting on the Mature Peptide. We also found evidence of diversifying selection as indicated by divergent allele frequencies among populations and high Theta k values.

CONCLUSION

Our results suggest that AMPs are an important source of adaptive diversity, minimizing the chance of microorganisms developing resistance to individual peptides.

Uniformity in premating reproductive isolation along an intraspecific cline

Premating reproductive isolation (RI) may reduce gene flow across populations that have differentiated in traits important for mate choice. Examining RI across genetic and phenotypic clines can inform the fundamental evolutionary processes that underlie population and lineage differentiation. We conducted female mate-choice studies across an intraspecific red-eyed treefrog cline in Costa Rica and Panama with 2 specific aims: (1) to characterize RI across the cline and examine the relationship between premating RI and genetic and phenotypic distance and (2) to evaluate our results within a broader evolutionary and taxonomic perspective through examination of other RI studies. We found that female red-eyed treefrogs prefer local males relative to non-local males, indicating that some premating RI has evolved in this system, but that preference strength is not associated with phenotypic or geographic distance. Our analysis of 65 other studies revealed no clear pattern between the strength of RI and geographic distribution (allopatry, parapatry, cline) or phenotypic distance, but revealed extreme variation and overlap in levels of intra- and interspecific levels of RI. This work contributes to a growing body of literature that examines intraspecific RI across a cline to understand the selective processes that shape evolutionary patterns at the earliest stages of divergence.

Variation in male spermiation response to exogenous hormones among divergent populations of Red-eyed Treefrogs

BACKGROUND

The non-lethal collection of sperm from live males is an important component for multiple captive-breeding techniques, including assisted reproductive technology (ART) protocols, sperm cryopreservation and in vitro fertilization. However, in amphibians, the type and amount of hormone necessary to induce spermiation can be highly variable, even among closely related species. We are unaware of any studies that have examined the spermiation response to exogenous hormones across highly differentiated populations within a species.

METHODS

We examined variation in sperm viability and production in response to the hormone LHRH among four divergent populations of the Red-eyed Treefrog (Agalychnis callidryas). We hypothesized that these highly differentiated populations would show variability in sperm count and viability in response to two dosages, 2 μg/g and 4 μg/g, of the hormone LHRH. We collected spermic urine 3 h post injection (PI). We then examined variation in spermiation at 3, 7, 12, and 24 h PI of LHRH for two allopatric populations that previously showed evidence of premating behavioral isolation.

RESULTS

One population of Red-eyed Treefrog exhibited reduced sperm viability but not count in response to the hormone LHRH compared to all other populations. In addition, we found peak viability at 3 h PI for the allopatric population comparison. There was no difference in sperm production within or between populations at 3, 7, 12, or 24 h PI. For both studies, intrapopulation variation was high.

CONCLUSION

ART often focuses on threatened species with small, isolated populations, which could evolve localized differences due to the evolutionary process of drift and selection. The high variation in response and the population-level differences in sperm viability we observed demonstrate that practitioners of ART should consider the possibility of divergent responses to hormones which may affect study design and animal receptivity to ART protocols.