Katharina C. Wollenberg

Vast underestimation of Madagascar's biodiversity evidenced by an integrative amphibian inventory

Amphibians are in decline worldwide. However, their patterns of diversity, especially in the tropics, are not well understood, mainly because of incomplete information on taxonomy and distribution. We assess morphological, bioacoustic, and genetic variation of Madagascars amphibians, one of the first near-complete taxon samplings from a biodiversity hotspot. Based on DNA sequences of 2,850 specimens sampled from over 170 localities, our analyses reveal an extreme proportion of amphibian diversity, projecting an almost 2-fold increase in species numbers from the currently described 244 species to a minimum of 373 and up to 465. This diversity is widespread geographically and across most major phylogenetic lineages except in a few previously well-studied genera, and is not restricted to morphologically cryptic clades. We classify the genealogical lineages in confirmed and unconfirmed candidate species or deeply divergent conspecific lineages based on concordance of genetic divergences with other characters. This integrative approach may be widely applicable to improve estimates of organismal diversity. Our results suggest that in Madagascar the spatial pattern of amphibian richness and endemism must be revisited, and current habitat destruction may be affecting more species than previously thought, in amphibians as well as in other animal groups. This case study suggests that worldwide tropical amphibian diversity is probably underestimated at an unprecedented level and stresses the need for integrated taxonomic surveys as a basis for prioritizing conservation efforts within biodiversity hotspots.

Madagascar as a model region of species diversification

Tropical biotas provide excellent settings in which to explore mechanisms of evolutionary diversification, yet these processes remain poorly understood. Pioneering work on biodiversity patterns and diversification processes in other tropical regions has recently been complemented by studies in Madagascar. Here we review diversity models and diversification mechanisms proposed for the fauna of this island and the perspectives for testing them. Madagascar has a diverse biota that has evolved in isolation, and is characterised by regionally pronounced and locally steep environmental gradients, common patterns of microendemism across taxa and numerous evolutionary radiations. These characteristics establish Madagascar as a promising system for the study of pattern and process in species diversification.

Patterns of Endemism and Species Richness in Malagasy Cophyline Frogs Support a Key Role of Mountainous Areas for Speciation

Abstract Cophyline narrow-mouthed frogs (Anura: Microhylidae) are a diverse endemic radiation of Madagascar. Cophylines contain a high proportion of range restricted species and constitute a good model system to understand patterns of evolutionary diversification in tropical ecosystems. We combine spatial and phylogenetic analyses for a near-complete taxon sample to test competing explanations for patterns of species richness (SR) and endemism. Our reconstruction of the phylogeny of cophylines indicates the presence of 22 new species and several instances of nonmonophyly. We found a strong historical signal in current cophyline ranges indicating a high degree of spatial niche conservatism in clade diversification, with clades occurring in the North of Madagascar constituting the most derived in the phylogeny. We identified six positively correlated centers of SR and endemism that can neither be explained by stochastic models such as elevational or latitudinal mid-domain effect, nor by low-elevation river catchments. Instead, the locations of these centers in areas spanning a high altitudinal range in combination with specific climatic parameters support a key role of mountainous areas for speciation of these anurans, although we cannot exclude an influence of habitat loss due to human impact. High conservation priority is ascribed to these areas.

Speciation in little: the role of range and body size in the diversification of Malagasy mantellid frogs

BackgroundThe rate and mode of lineage diversification might be shaped by clade-specific traits. In Madagascar, many groups of organisms are characterized by tiny distribution ranges and small body sizes, and this high degree of microendemism and miniaturization parallels a high species diversity in some of these groups. We here investigate the geographic patterns characterizing the radiation of the frog family Mantellidae that is virtually endemic to Madagascar. We integrate a newly reconstructed near-complete species-level timetree of the Mantellidae with georeferenced distribution records and maximum male body size data to infer the influence of these life-history traits on each other and on mantellid diversification.ResultsWe reconstructed a molecular phylogeny based on nuclear and mitochondrial DNA for 257 species and candidate species of the mantellid frog radiation. Based on this phylogeny we identified 53 well-supported pairs of sister species that we used for phylogenetic comparative analyses, along with whole tree-based phylogenetic comparative methods. Sister species within the Mantellidae diverged at 0.2-14.4 million years ago and more recently diverged sister species had geographical range centroids more proximate to each other, independently of their current sympatric or allopatric occurrence. The largest number of sister species pairs had non-overlapping ranges, but several examples of young microendemic sister species occurring in full sympatry suggest the possibility of non-allopatric speciation. Range sizes of species included in the sister species comparisons increased with evolutionary age, as did range size differences between sister species, which rejects peripatric speciation. For the majority of mantellid sister species and the whole mantellid radiation, range and body sizes were associated with each other and small body sizes were linked to higher mitochondrial nucleotide substitution rates and higher clade diversity. In contrast, small range sizes were unexpectedly associated with a slow-down of mitochondrial substitution rates.ConclusionsBased on these results we define a testable hypothesis under which small body sizes result in limited dispersal capabilities and low physiological tolerances, causing smaller and more strongly fragmented ranges. This can be thought to facilitate reproductive isolation and thus favor speciation. Contrary to the expectation of the faster speciation of such microendemic phenotype species, we only found small body sizes of mantellid frogs to be linked to higher diversification and substitution rates, but not small range sizes. A joint analysis of various species-rich regional anuran radiations might provide enough species with all combinations of range and body sizes for a more conclusive test of this hypothesis.

Small body size increases the regional differentiation of populations of tropical mantellid frogs (Anura: Mantellidae)

The processes affecting species diversification may also exert an influence on patterns of genetic variability within species. We evaluated the contributions of five variables potentially influencing clade diversification (body size, reproductive mode, range size, microhabitat and skin texture) on mtDNA divergence and polymorphism among populations of 40 species of frogs (Mantellidae) from two rainforest communities in Madagascar. We report an inverse association between body size and nucleotide divergence between populations but find no influence of other variables on genetic variation. Body size explained ca. 11% of the variation in nucleotide divergence between populations and was coupled with high FST levels and an absence of haplotype sharing in small‐bodied and medium‐sized frogs. Low dispersal ability is likely the proximate mechanism producing higher population differentiation in small mantellids. The lack of genetic cohesion among populations establishes regional genetic fragmentation which in turn has the potential to accelerate rates of allopatric speciation in small frogs relative to large species. However, there is little evidence of increased speciation rates in these or other small‐bodied organisms. We reconcile these contradictory observations by suggesting that lower dispersal ability also curbs colonization of new areas, decelerating diversification in weak dispersers. Our results imply that the intermediate dispersal model also applies to amphibians and may explain inconsistent previous results on the correlation of body size and speciation rate.

Molecular phylogeny and biogeography of Malagasy frogs of the genus Gephyromantis.

The genus Gephyromantis is a clade within the Malagasy-Comoroan family Mantellidae composed of rainforest frogs that live and breed to varying degrees independently from water. Based on DNA sequences of five mitochondrial and five nuclear genes we inferred the phylogeny of these frogs with full taxon coverage at the species level. Our preferred consensus tree from a partitioned Bayesian analysis of 5843 base pairs of 51 nominal and candidate species supports various major clades within the genus although the basal relationships among these remain unresolved. The data provide strong evidence for the monophyly of the subgenera Gephyromantis (after exclusion of Gephyromantis klemmeri), Laurentomantis, Vatomantis, and Phylacomantis. Species assigned to the subgenus Duboimantis belong to two strongly supported clades of uncertain relationships. G. klemmeri, previously in the subgenus Gephyromantis, was placed with high support sister to the Laurentomantis clade, and the Laurentomantis+G. klemmeri clade was sister to Vatomantis. A reconstruction of ancestral distribution areas indicates a diversification of several subgenera in the northern biogeographic regions of Madagascar and the dispersal out of northern Madagascar for several clades.

Why colour in subterranean vertebrates? Exploring the evolution of colour patterns in caecilian amphibians.

The proximate functions of animal skin colour are difficult to assign as they can result from natural selection, sexual selection or neutral evolution under genetic drift. Most often colour patterns are thought to signal visual stimuli; so, their presence in subterranean taxa is perplexing. We evaluate the adaptive nature of colour patterns in nearly a third of all known species of caecilians, an order of amphibians most of which live in tropical soils and leaf litter. We found that certain colour pattern elements in caecilians can be explained based on characteristics concerning above‐ground movement. Our study implies that certain caecilian colour patterns have convergently evolved under selection and we hypothesize their function most likely to be a synergy of aposematism and crypsis, related to periods when individuals move overground. In a wider context, our results suggest that very little exposure to daylight is required to evolve and maintain a varied array of colour patterns in animal skin.

Molecular phylogeny and geographic variation of Malagasy iguanas (Oplurus and Chalarodon).

The iguanid subfamily Oplurinae consists of seven species of small to medium-sized, arboreal, sand-dwelling or rock-dwelling lizards endemic to Madagascar and the Comoros, belonging to the genera Oplurus and Chalarodon. We here present the first complete molecular species-level phylogeny for Oplurinae based on DNA sequences (865 bp) of the mitochondrial 16S rRNA gene and the nuclear c-mos gene. Our study is based on 52 specimens sampled from different populations in Madagascar and includes the geographically isolated population from Grande Comore, hitherto considered as subspecies Oplurus cuvieri comorensis or even as a separate species O. comorensis. Our results confirm that, within the genus Oplurus, the largely arboreal O. cuvieri and O. cyclurus form the sister clade to the remaining, more rock-dwelling species. Within the latter lineage, Oplurus quadrimaculatus is placed most basal, O. fierinensis and O. grandidieri are closely related sister species with high support, and O. saxicola is sister to the fierinensis/grandidieri lineage. Within the arboreal Oplurus, the Comoran sample shows no genetic differentiation relative to O. cuvieri populations from the North West and Sambirano regions of Madagascar, indicating that this population should not be considered as a separate species. In the monotypic genus Chalarodon, we discovered deep genetic divergences among populations of C. madagascariensis indicating the presence of a previously unrecognized cryptic species and the need for taxonomic revision.

Nidicolous tadpoles rather than direct development in Malagasy frogs of the genus Gephyromantis

Frogs in the genus Gephyromantis from Madagascar were assumed to have a direct developmental mode, i.e. the complete embryonic and larval development within the egg, but recently free-swimming, exotrophic tadpoles of a few species have been found. Herein we provide detailed morphological descriptions of the tadpoles of five more species of this genus, indicating a developmental mode other than direct development in species of Gephyromantis. Tadpoles of Gephyromantis granulatus, G. sculpturatus, G. tschenki, and G. ventrimaculatus were found free-swimming in streams, and tadpoles of G. sp. aff. blanci were raised after hatching from clutches found in the leaf litter. All tadpoles were identified by DNA barcoding. The oral discs of all five species are characterized by the lack of many typical morphological traits of exotrophic tadpoles (such as oral papillae and keratodonts). This indicates that these tadpoles are either non-feeding (endotrophic) or only facultatively feeding tadpoles. We classify these tadpoles as nidicolous based on the observation that the larvae of G. sp. aff. blanci stayed after hatching in the jelly nest until metamorphosis. It remains unclarified whether all species have strictly nidicolous tadpoles, and the larvae of the four species found in the streams were just accidentally washed into these streams; alternatively, some of these tadpoles might be nidicolous at first but in some species need to live in free water at later stages.

DETERMINISM IN THE DIVERSIFICATION OF HISPANIOLAN TRUNK-GROUND ANOLES (ANOLIS CYBOTES SPECIES COMPLEX)

The evolutionary processes that produce adaptive radiations are enigmatic. They can only be studied after the fact, once a radiation has occurred and been recognized, rather than while the processes are ongoing. One way to connect pattern to process is to study the processes driving divergence today among populations of species that belong to an adaptive radiation, and compare the results to patterns observed at a deeper, macroevolutionary level. We tested whether evolution is a deterministic process with similar outcomes during different stages of the adaptive radiation of Anolis lizards. Using a clade of terrestrial–scansorial lizards in the genus Anolis, we inferred the adaptive basis of spatial variation among contemporary populations and tested whether axes of phenotypic differentiation among them mirror known axes of diversification at deeper levels of the anole radiation. Nonparallel change associated with genetic divergence explains the vast majority of geographic variation. However, we found phenotypic variation to be adaptive as confirmed by convergence in populations occurring in similar habitats in different mountain ranges. Morphological diversification among populations recurs deterministically along two axes of diversification previously identified in the anole radiation, but the characters involved differ from those involved in adaptation at higher levels of anole phylogeny.