Interacting selection diversifies warning signals in a polytypic frog: an examination with the strawberry poison frog

Aposematic signals represent one of the most accessible traits to evaluate the interaction of natural and sexual selection on signal evolution. Here we investigate the contributions of these two selective forces on the aposematic signal evolution of the highly polytypic strawberry poison frog, Oophaga pumilio, of Bocas del Toro, Panama. Previous research has shown that the brightness of O. pumilio warning coloration can inform predators of the toxicity levels associated with different populations of the archipelago. Other studies suggest that sexual selection may be influencing warning signal brightness within populations via female mate choice (Isla Solarte, Isla Bastimentos, and Aquacate Peninsula populations) and male–male competition (Isla Solarte). Here we present two non-exclusive scenarios for how natural and sexual selection interact to drive phenotypic variation across this archipelago: (1) predators impose a selective regime whereby populations above a toxicity-brightness threshold are at liberty to diversify via sexual selection and below which populations are constrained to maintain a stricter resemblance to a more cryptic population mean, and (2) synergistic/additive effects of inter- and intrasexual selection drive the evolution of brighter males within populations above this toxicity threshold. We investigate whether aposematic patterns of divergence across the archipelago relative to the common mainland phenotype meet these predictions using existing data on O. pumilio morph toxicity measures and overall conspicuousness estimates to an avian predator. Using standardized z-scores to evaluate the range of trait values, we find that indeed the population representative of the common mainland phenotype (Almirante) represents an intermediate level of both toxicity and conspicuousness, and that derived Bocas del Toro populations vary in each of those components in directions predicted by the proposed scenarios. Furthermore, we find greater divergence towards conspicuousness than crypsis, a pattern suggestive of sexual and natural selection acting synergistically in morphs with high toxicity.     

INTRASPECIFIC REPRODUCTIVE CHARACTER DISPLACEMENT IN A POLYMORPHIC POISON DART FROG,DENDROBATES PUMILIO

Divergence in male mating signals and associated female preferences is often an important step in the process of speciation. Reproductive character displacement, the pattern of greater divergence of male signals and/or female preference in sympatry than in allopatry, has been observed in a variety of taxa with different degrees of postzygotic isolation. A number of selective processes, including reinforcement, have been proposed to cause such a pattern. Cases in which reproductive character displacement occurs among intraspecific variants are especially informative for understanding how selection acting within a species can lead to the evolution of reproductive barriers and speciation. This study tested the hypothesis that female strawberry poison dart frogs (Dendrobates pumilio) in polymorphic populations of the Bocas del Toro archipelago of Panama show stronger mating discrimination than do females from monomorphic populations, exhibiting an intraspecific pattern of reproductive character displacement. Our results contribute important insights into understanding selection's role in generating the striking diversity of Bocas del Toro's D. pumilio and provide a snapshot of what could be the early stages of reproductive isolation and speciation.     

Evidence for selection on coloration in a Panamanian poison frog: a coalescent‐based approach

Aim The strawberry poison frog, Oophaga pumilio, has undergone a remarkable radiation of colour morphs in the Bocas del Toro archipelago in Panama. This species shows extreme variation in colour and pattern between populations that have been geographically isolated for < 10,000 years. While previous research has suggested the involvement of divergent selection, to date no quantitative test has examined this hypothesis. Location Bocas del Toro archipelago, Panama. Methods We use a combination of population genetics, phylogeography and phenotypic analyses to test for divergent selection in coloration in O. pumilio. Tissue samples of 88 individuals from 15 distinct populations were collected. Using these data, we developed a gene tree using the mitochondrial DNA (mtDNA) d‐loop region. Using parameters derived from our mtDNA phylogeny, we predicted the coalescence of a hypothetical nuclear gene underlying coloration. We collected spectral reflectance and body size measurements on 94 individuals from four of the populations and performed a quantitative analysis of phenotypic divergence. Results The mtDNA d‐loop tree revealed considerable polyphyly across populations. Coalescent reconstructions of gene trees within population trees revealed incomplete genotypic sorting among populations. The quantitative analysis of phenotypic divergence revealed complete lineage sorting by colour, but not by body size: populations showed non‐overlapping variation in spectral reflectance measures of body coloration, while variation in body size did not separate populations. Simulations of the coalescent using parameter values derived from our empirical analyses demonstrated that the level of sorting among populations seen in colour cannot reasonably be attributed to drift. Main conclusions These results imply that divergence in colour, but not body size, is occurring at a faster rate than expected under neutral processes. Our study provides the first quantitative support for the claim that strong diversifying selection underlies colour variation in the strawberry poison frog.     

No evidence for differential survival or predation between sympatric color morphs of an aposematic poison frog

Because variation in warning signals slows down the predator education process, aposematic theory predicts that animal warning signals should be monomorphic. Yet, warning color polytypisms are not uncommon in aposematic species. In cases where warning signal variants are separated geographically, adaptation to local predators could explain this variation. However, this cannot explain the persistence of sympatric polymorphisms in aposematic taxa. The strawberry poison frog (Oophaga pumilio) exhibits both allopatric and sympatric warning color variation in and around the Bocas del Toro archipelago of Panama. One explanation that has been proposed for the rapid diversification of O. pumilio coloration in this archipelago is low predation; if island populations have few predators, stabilizing selection would be relaxed opening the door for diversification via selection or genetic drift. Using a combination of mark-recapture and clay model studies, we tested for differences in survival and predation among sympatric red and yellow color morphs of O. pumilio from Bastimentos Island. We found no evidence for differential survival or predation in this population, despite the fact that one morph (red) is more common and widely distributed than the other (yellow). Even in an area of the island where the yellow morph is not found, predator attack rates were similar among morphs. Visual modeling suggests that yellow and red morphs are distinguishable and conspicuous against a variety of backgrounds and by viewers with different visual systems. Our results suggest that general avoidance by predators of red and yellow, both of which are typical warning colors used throughout the animal kingdom, may be contributing to the apparent stability of this polymorphism.     

Color Assortative Mating in a Mainland Population of the Poison Frog Oophaga pumilio

Assortative mating is a reproductive strategy used by a diversity of animals, in which individuals choose a mate that shares similar characteristics. This mating strategy has the potential to promote the evolution of various sexual signals and has been a proposed mechanism driving and maintaining color variation in the anuran family Dendrobatidae. Most studies have examined this reproductive strategy in the polytypic poison frog, Oophaga pumilio, in the Bocas del Toro archipelago in Panama. Little attention, however, has been given to ancestral populations across this species’ mainland range, where dramatic color polytypism appears to lack. Additionally, most studies are exclusively experimental and investigate mate choice between allopatric populations, neglecting the behaviors of naturally occurring mates. This study observed natural mating pairs within a population of O. pumilio on mainland Costa Rica and tested the prediction that color phenotype of mating females and males would be correlated. Naturally occurring pairs were found to share similar coloration, suggesting that color assortative mating operates in nature, and in a mainland population. Our results indicate that coloration is an important trait in driving the natural mate choices of female O. pumilio, which provides valuable insight into realistic mate selection tactics of this dendrobatid frog.     

Variation in spectral reflectance among populations of Dendrobates pumilio, the strawberry poison frog, in the Bocas del Toro Archipelago, Panama

Aim The goal of this study was to quantify levels of variation in spectral reflectance within and among populations of Dendrobates pumilio, the strawberry poison frog from the Bocas del Toro Archipelago. Location This study was carried out in the Bocas del Toro Archipelago, in the Republic of Panama, Central America. Methods Spectral reflectance was measured for samples of individuals from fifteen distinct island and mainland populations, using an Ocean Optics 2000 spectrometer and a BiLink portable computer. Results Our results provide quantitative evidence for extreme polymorphism among populations, and more limited levels of polymorphism within some populations. No obvious signs of sexual dimorphism were found. All the colour morphs appear to have relatively little reflectance in the ultraviolet part of the spectrum. There is some evidence for clinal variation in colour and pattern across some mainland populations. There is also at least one area where distinctly different morphs occur in sympatry, suggesting the possibility of incipient reproductive isolation. We argue that variation in coloration may have been enhanced by sexual selection.     

SEXUAL SELECTION CAN INCREASE THE EFFECT OF RANDOM GENETIC DRIFT—A QUANTITATIVE GENETIC MODEL OF POLYMORPHISM IN OOPHAGA PUMILIO,THE STRAWBERRY POISON‐DART FROG

The variation in color pattern between populations of the poison‐dart frog Oophaga pumilio across the Bocas del Toro archipelago in Panama is suggested to be due to sexual selection, as two other nonsexually selecting Dendrobatid species found in the same habitat and range do not exhibit this variation. We theoretically test this assertion using a quantitative genetic sexual selection model incorporating aposematic coloration and random drift. We find that sexual selection could cause the observed variation via a novel process we call “coupled drift.” Within our model, for certain parameter values, sexual selection forces frog color to closely follow the evolution of female preference. Any between‐population variation in preference due to genetic drift is passed on to color. If female preference in O. pumilio is strongly affected by drift, whereas color in the nonsexually selecting Dendrobatid species is not, coupled drift will cause increased between‐population phenotypic variation. However, with different parameter values, coupled drift will result in between‐population variation in color being suppressed compared to its neutral value, or in little or no effect. We suggest that coupled drift is a novel theoretical process that could have a role linking sexual selection with speciation both in O. pumilio, and perhaps more generally.     

Not all colors are equal: predation and color polytypism in the aposematic poison frog Oophaga pumilio

Aposematic organisms are not predicted to show high levels of warning signal diversity because they are expected to be under stabilizing selection to decrease costs of ‘educating’ predators about their unpalatability. However, systematic changes in warning signals (polytypism) can be expected if they represent adaptations to local predators. The aposematic strawberry poison frog (Oophaga pumilio) is red throughout its mainland distribution in Costa Rica and Panamá, but displays high levels of warning signal diversity in the Bocas del Toro Archipelago of Panamá. Both coloration and spot pattern vary in a polytypic sense. Sexual selection contributes to maintaining the polytypism, but little work has investigated the potential influence of predation. We used unspotted models of O. pumilio to determine if predation might help explain the color polytypism on Isla Colón in the Bocas del Toro Archipelago of Panamá. We tested whether attack rates differed among the red mainland morph, green/yellow Isla Colón morph, and the brown control. We found that frog color significantly predicted being attacked. The local green Isla Colón models were attacked more than foreign red or brown models. No difference in attack rate existed between red and brown control models. Our results suggest that the red mainland morph possesses a more effective warning signal, even when it is not the local morph. Honest signaling of unpalatability, neophobia, and the use of search images by local predators are potential explanations. Similarity of the brown model to other local poison frogs might explain the lower attack rate compared to previous work. The attack rate was lower on Isla Colón compared to mainland Costa Rica, which supports the hypothesis that less overall predation in the Bocas del Toro Archipelago may contribute to the overall warning signal diversity in O. pumilio there by relaxing selection for aposematic traits.     

Scorpions(Arachnida) from Bocas del Toro archipelago,Panamá

Four species of buthid scorpions (Ananteris platnicki Louren?o, 1993; Centruroides limbatus [Pocock, 1898]; Tityus pachyurus [Pocock, 1897]; and T. ocelote Francke and Stockwell, 1987) are recorded for the first time from some islands and cays of the Bocas del Toro Archipelago, Panama. Morphological variation and ecological data are given for all the species. This is the first Panamanian record for both C. limbatus and T. ocelote. The scorpion fauna of the Bocas de Toro Archipelago is related with the recent fauna of the Atlantic continental lowland region of Costa Rica and Panama and includes Amazonian-Guyanese (genera Ananteris and Tityus) as well as Mexican-North Central American (genus Centruroides) elements.     

Poison frog color morphs express assortative mate preferences in allopatry but not sympatry

The concurrent divergence of mating traits and preferences is necessary for the evolution of reproductive isolation via sexual selection, and such coevolution has been demonstrated in diverse lineages. However, the extent to which assortative mate preferences are sufficient to drive reproductive isolation in nature is less clear. Natural contact zones between lineages divergent in traits and preferences provide exceptional opportunities for testing the predicted evolutionary consequences of such divergence. The strawberry poison frog (Oophaga pumilio) displays extreme color polymorphism in and around the young Bocas del Toro archipelago. In a transition zone between red and blue allopatric lineages, we asked whether female preferences diverged along with coloration, and whether any divergent preferences persist in a zone of sympatry. When choosing among red, blue and phenotypically intermediate males, females from monomorphic red and monomorphic blue populations both expressed assortative preferences. However, red, blue, and intermediate females from the contact zone all preferred red males, suggesting that divergent preferences may be insufficient to effect behavioral isolation. Our results highlight the complexity of behavioral isolation, and the need for studies that can reveal the circumstances under which divergent preferences do and do not contribute to speciation.     

Non-gradual variation in colour morphs of the strawberry poison frog Dendrobates pumilio: genetic and geographical isolation suggest a role for selection in maintaining polymorphism

The relative roles that geographical isolation and selection play in driving population divergence remain one of the central questions in evolutionary biology. We approached this question by investigating genetic and morphological variation among populations of the strawberry poison frog, Dendrobates pumilio, in the Bocas del Toro archipelago, Panama. We found significant population genetic structure and isolation by distance based on amplified fragment length polymorphism markers. Snout vent length (SVL), coloration and the extent and size of dorsal black spots showed large variation among the studied populations. Differences in SVL correlated with genetic distance, whereas black spot patterns and other coloration parameters did not. Indeed, the latter characters were observed to be dramatically different between contiguous populations located on the same island. These results imply that neutral divergence among populations may account for the genetic patterns based on amplified fragment length polymorphism markers and SVL. However, selective pressures need to be invoked in order to explain the extraordinary variation in spot size and coverage, and coloration. We discuss the possibility that the observed variation in colour morphs is a consequence of a combination of local variation in both natural selection on an aposematic signal towards visual predators and sexual selection generated by colour morph-specific mate preferences.     

Size and sex matter: reproductive biology and determinants of offspring survival in Gazella marica

For prey, many behavioural traits are constrained by the risk of predation. Therefore, shifts between warning and cryptic coloration have been suggested to result in parallel changes in several behaviours. In the present study, we tested whether changes in chromatic contrast among eight populations of the strawberry poison‐dart frog, Dendrobates pumilio, co‐vary with behaviour, as expected if selection is imposed by predators relying on visual detection of prey. These eight populations are geographically isolated on different island in the Bocas del Toro region of Panama and have recently diverged morphologically and genetically. We found that aggression and explorative behaviour were strongly correlated and also that males tended to be more aggressive and explorative if they belonged to populations with conspicuously coloured individuals. We discuss how evolutionary switches between predator avoidance strategies and associated behavioural divergence between populations may affect reproductive isolation. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ●● , ●●–●●.     

Coral reefs of Bocas del Toro, Panamá: IV. Distribution, structure and conservation state of continental reefs of Peninsula Valiente]

This is the fourth and last contribution describing the individual structure, distribution and conservation status of coral reefs in the Province of Bocas del Toro. Here we describe 14 new reefs along 129 km of coast from Peninsula Valiente to Río Calovébora. Average live coral coverage for this region was 17.1% (+/- 3.6%), mainly in the western region of the peninsula (Bahia Bluefield and Ensenada Tobobe). Coral cover increases with depth (> 5 m) for most species at several reefs and the corals Porites furcata and Acropora palmata dominated shallow waters. Acropora palmata was found abundant in 43% of the studied reefs and toward the regions of the Ensenada Tobobe and Punta Valiente. Coral recruitment rates were similar in distribution to those reefs with greater coral coverage, with average densities of 4 recruit/m2 (maximum 9 recruits/m2) and mainly Agaricia spp., Porites astreoides and Siderastrea siderea. The greater diversity of corals and sponges was recorded toward the western side of the peninsula, with a total of 55 coral species in the study area, including two new records for Bocas del Toro (59 species in total), Dichocoenia stellaris and Madracis luciphila and increasing the diversity of corals of Panama to 65 species. We found 24 species of octocorals and Gorgonia mariae, Muriceopsis sulphurea and Muricea laxaoosens, are informed for the first time to the area, increasing in 10% the diversity for Bocas del Toro (32 in total). We recorded 48 sponges, including five new species for the area and representing an increase of 9% in the total number (58). Large populations of Acropora palmata were found in the Ensenada Tobobe, what justifies once again the need for modifying the existing protected area, so that this new region is incorporated within the conservation plans.     

Phylogeography and connectivity of molluscan parasites: Perkinsus spp. in Panama and beyond

Panama is a major hub for commercial shipping between two oceans, making it an ideal location to examine parasite biogeography, potential invasions, and the spread of infectious agents. Our goals were to (i) characterise the diversity and genetic connectivity of Perkinsus spp. haplotypes across the Panamanian Isthmus and (ii) combine these data with sequences from around the world to evaluate the current phylogeography and genetic connectivity of these widespread molluscan parasites. We collected 752 bivalves from 12 locations along the coast of Panama including locations around the Bocas del Toro archipelago and the Caribbean and Pacific entrances to the Panama Canal, from December 2012 to February 2013. We used molecular genetic methods to screen for Perkinsus spp. and obtained internal transcribed spacer region (ITS) ribosomal DNA (rDNA) sequences for all positive samples. Our sequence data were used to evaluate regional haplotype diversity and distribution across both coasts of Panama, and were then combined with publicly available sequences to create global haplotype networks. We found 26 ITS haplotypes from four Perkinsus spp. (1–12 haplotypes per species) in Panama. Perkinsus beihaiensis haplotypes had the highest genetic diversity, were the most regionally widespread, and were associated with the greatest number of hosts. On a global scale, network analyses demonstrated that some haplotypes found in Panama were cosmopolitan (Perkinsus chesapeaki, Perkinsus marinus), while others were more geographically restricted (Perkinsus olseni, P. beihaiensis), indicating different levels of genetic connectivity and dispersal. We found some Perkinsus haplotypes were shared across the Isthmus of Panama and several regions around the world, including across ocean basins. We also found that haplotype diversity is currently underestimated and directly related to the number of sequences. Nevertheless, our results demonstrate long-range dispersal and global connectivity for many haplotypes, suggesting that dispersal through shipping probably contributes to these biogeographical patterns.     

A new species of Lepidodasys (Gastrotricha, Macrodasyida) from Panama with a description of its peptidergic nervous system using CLSM, anti-FMRFamide and anti-SCPB

A new species of Lepidodasys (Gastrotricha: Macrodasyida: Lepidodasyidae) is described from sublittoral sediments in the Bocas del Toro archipelago in Panama and represents the first species of Lepidodasyidae described from the Caribbean. The new species possesses keeled scales that form a crossed-helical pattern across its dorsal and lateral surfaces and ventral scales that form a herringbone pattern between and lateral to the ciliary columns. A bilateral pair of three ventral adhesive tubes at the posterior end further differentiates this new species from its seven congeners. A confocal laser scanning microscope examination of the nervous system using antibodies to small cardioactive peptide B (SCP_B) and FMRFamides reveals a dumbbell-shape cerebral ganglion, paired pharyngeal neurites and paired posterior nerve cords. Expression patterns of immunoreactivity to both classes of neuropeptides show a high degree of similarity. Only within the lateral somata of the cerebral ganglion and a single median pharyngeal neurite is there a difference in immunoreactivity to FMRFamide (positive) compared to SCP_B (negative). Results from this investigation reveal that neuropeptides, among other neuronal markers, might provide phylogenetically informative characters in macrodasyidan gastrotrichs, especially regarding the topology of the cerebral ganglion.     

Bitten down to size: Fish predation determines growth form of the Caribbean coral reef sponge Mycale laevis

Interactions between organisms add complexity to ecosystem function, particularly on coral reefs. The Caribbean orange icing sponge Mycale laevis is semi-cryptic, often growing under coral colonies or between coral branches. This association is reportedly a mutualism, with the sponge deterring boring sponges from invading the coral skeleton and the coral providing an expanding surface for sponge growth. But is there an alternative explanation for the proximity of sponge and coral? We examined the importance of fish predation on the growth of the sponge. While the semi-cryptic growth form of M. laevis predominates on reefs off the Florida Keys and the Bahamas Islands, M. laevis grows with a non-cryptic, erect morphology off Bocas del Toro, Panama. Surveys revealed that sponge-eating fishes were rare or absent at Bocas del Toro compared to sites in the Florida Keys. Because past studies were inconsistent about the palatability of M. laevis to fish predators, we conducted feeding experiments with sponges from all three sites. Crude organic extracts of M. laevis from all three sites were palatable to generalist fish predators in aquarium assays, and field feeding assays and caging experiments conducted in the Florida Keys confirmed that spongivorous fishes readily ate exposed fragments of M. laevis. Our results suggest that M. laevis is restricted to its semi-cryptic growth form by spongivorous predators, with corals providing a physical refuge from predation. This alternative explanation supports the broader hypothesis that Caribbean reef sponges can be categorized on the basis of chemical defense into defended, palatable, and preferred species, the last of which are restricted to refugia.     

A new species of Blakea (Melastomataceae) from Panama with foliaceous sepal appendages and zygomorphic flowers

The new species Blakea bocatorena is described from Bocas del Toro province in Panama. It is only the third species to be described in the genus that has foliaceous appendages on the sepals. The other two species with these sepal appendages are B. calycosa and B. tuberculata. Blakea bocatorena differs from the latter two species in that it has white petals and a type of herkogamy in which the style is opposite the stamens and results in a zygomorphic flower. In B. calycosa and B. tuberculata, the larger petals are pink-magenta and tuberculate with the stamens encircling the exserted style resulting in a radially symmetric herkogamous flower.     

Distribution and Diversity of Archaeal Ammonia Monooxygenase Genes Associated with Corals

Corals are known to harbor diverse microbial communities of Bacteria and Archaea, yet the ecological role of these microorganisms remains largely unknown. Here we report putative ammonia monooxygenase subunit A (amoA) genes of archaeal origin associated with corals. Multiple DNA samples drawn from nine coral species and four different reef locations were PCR screened for archaeal and bacterial amoA genes, and archaeal amoA gene sequences were obtained from five different species of coral collected in Bocas del Toro, Panama. The 210 coral-associated archaeal amoA sequences recovered in this study were broadly distributed phylogenetically, with most only distantly related to previously reported sequences from coastal/estuarine sediments and oceanic water columns. In contrast, the bacterial amoA gene could not be amplified from any of these samples. These results offer further evidence for the widespread presence of the archaeal amoA gene in marine ecosystems, including coral reefs.     

Genetic diversity and differentiation processes in the ploidy series of Olea europaea L.: a multiscale approach from subspecies to insular populations

Geographical isolation and polyploidization are central concepts in plant evolution. The hierarchical organization of archipelagos in this study provides a framework for testing the evolutionary consequences for polyploid taxa and populations occurring in isolation. Using amplified fragment length polymorphism and simple sequence repeat markers, we determined the genetic diversity and differentiation patterns at three levels of geographical isolation in Olea europaea : mainland-archipelagos, islands within an archipelago, and populations within an island. At the subspecies scale, the hexaploid ssp. maroccana (southwest Morocco) exhibited higher genetic diversity than the insular counterparts. In contrast, the tetraploid ssp. cerasiformis (Madeira) displayed values similar to those obtained for the diploid ssp. guanchica (Canary Islands). Geographical isolation was associated with a high genetic differentiation at this scale. In the Canarian archipelago, the stepping-stone model of differentiation suggested in a previous study was partially supported. Within the western lineage, an east-to-west differentiation pattern was confirmed. Conversely, the easternmost populations were more related to the mainland ssp. europaea than to the western guanchica lineage. Genetic diversity across the Canarian archipelago was significantly correlated with the date of the last volcanic activity in the area/island where each population occurs. At the island scale, this pattern was not confirmed in older islands (Tenerife and Madeira), where populations were genetically homogeneous. In contrast, founder effects resulted in low genetic diversity and marked genetic differentiation among populations of the youngest island, La Palma.