This Trinidad and Tobago frog, formerly regarded as
Dedropsophis minutus is now Dendopsophus
goughi (Boulenger). JCM
Cryptic genetic diversity is now so commonly reported in molecular studies of amphibian species that the existence of nominally widespread tropical species has been called into question. However, supposedly widespread species occurring across multiple biomes and countries are rarely comprehensively sampled across their complete geographic range in screenings of genetic diversity or phylogeographic studies. Sampling of species from across vast continental areas and across political borders is often handicapped by financial, logistic and political factors.
In the Neotropics, nominal taxa such as the toad Rhinella margaritifera (Bufonidae), the thin-toed frog Leptodactylus fuscus (Leptodactylidae), and the tree frog Scinax ruber (Hylidae) are prominent examples of anuran species once considered to occur across nearly the entire tropical lowlands of South America. Evidence has accumulated that many such putatively widespread species could in fact be complexes of cryptic taxa. However, given limited genetic sampling and the difficulty in reviewing material from all countries hosting populations, their relationships and systematics remain in many cases as unclear as they were decades ago.
Dendropsophus minutus (Peters, 1872) is a small hylid frog, 21–28 mm snout-vent length, distributed in Cis-Andean South America, including the Andean slopes, the Amazon Basin, the Guiana Shield, down to the Atlantic Forests of southeastern Brazil, with an elevational record from near sea level up to 2,000 m. Variation in coloration, osteology, advertisement calls and larval morphology, along with molecular data from limited parts of the species' distribution suggest that the nominal D. minutus might represent a species complex. However, the sheer size of its supposed geographical range along with nomenclatural and taxonomic complexity (six junior synonyms) and unresolved relationships in the D. minutus species group have so far made these frogs inaccessible to revision.
In a new study in PLoS ONE Gehara and colleagues (2014) use D. minutus to understand to what degree a tropical, small-sized anuran has the potential to be continentally widespread with limited genetic structure within its range, as expected for a single species. In addition to conservation concerns, this question has important implications for South American biogeography in general and amphibian systematics and evolution in particular. Evidence is accumulating that body size in amphibians has a positive correlation with range size, but contrary to this trend many Holarctic amphibians occur with little genetic substructure across the vast ranges they colonized after the last glaciation, despite sometimes moderate to small body sizes. Whether such patterns also exist across vast ranges in tropical regions, with their distinct historical climatic dynamics, is an open question. Deciphering possible cryptic diversity within the nominal D. minutus would also help inform conservation assessments which typically use species' geographic distributions as criteria for conservation status.
The 16S tree containing all Dendropsophus for which sequences were available recovered the monophyly of the D. minutus species group. Within the group, the clade containing samples representing lineages 19–43 received a maximal posterior probability (1.0) and is defined here as the D. minutus complex, given that lineage 25 contains samples from the type locality of D. minutus.
Most of the mitochondrial lineages containing more than one sample received strong nodal support. The lineages splitting off from basal nodes of the tree (lineages 1–18) are distributed in the Guiana Shield, and in the Andean region of Peru, Ecuador and Colombia, with an eastern extralimital clade assembling disjunct localities in Mato Grosso and Pará.
The remaining lineages are in general more widely distributed in central and eastern South America Lineages are largely allopatric but several cases of sympatry were observed. The uncorrected pairwise distances between lineages for the 16S gene ranged from 0.7 to 13%, while within-lineage p-distances ranged from 0.0 to 1.8%.
Most of the lineages (45%) were found in only one or two localities. Fifty per cent of the lineages were only found in areas smaller than 10 km2, and more than 70% have known ranges smaller than 10,000 km2. Eight out of the 43 lineages have a distribution larger than 100,000 km2. Largest range sizes were found in northeastern Brazil (Caatinga domain; 997,262 km2, lineage 36), eastern Bolivia and western Brazil (Cerrado, Chaco and Dry Forest domains; 293,321 km2, lineage 33) and the Guiana Shield (269,741 km2, lineage 2).
Among the D. minutus species group members external to the D. minutus complex, lineages 1–6 are Guianan, while 7–18 are primarily distributed along the Andean foothills, and all show well-pronounced molecular differentiation and divergence. Among lineages 1–6, there is moderate genetic differentiation. Considering mitochondrial reciprocal monophyly and GMYC results as criteria, and being taxonomically conservative, one available name, Hyla goughi Boulenger, 1911 (type locality: Trinidad), should likely be removed from the synonymy of D. minutus and allocated to populations comprised by all or some of lineages 1–6. As a conservative estimate, the authors hypothesize that lineages 7–18 comprise seven distinct species, i.e., five named taxa and two undescribed species (lineages 9+10 and 11+12).
Data presented herein provide conclusive evidence for a strong genetic subdivision of the nominal species Dendropsophus minutus as currently understood. Current taxonomy conservatively assumes a putatively widespread species encompassing a vast area of South America (from approximately latitude 11.0°N to 35.0°S), distributed across several biomes. Our results, however, reveal high genetic diversity within D. minutus that would suggest the existence of numerous distinct species, leading to an important increase in number of species. If this hypothesis is confirmed through further studies, the existence of an increased number of species with decreased range sizes would have important consequences for the definition of centers of endemism and for assessing conservation status.
Despite revealing a substantial amount of cryptic genetic diversity within D. minutus sensu lato, our results also confirm the existence of widespread Neotropical species of anurans. While the authors cannot yet confirm which of the mitochondrial lineages within the D. minutus complex will merit a status as separate species, the authors suggest they can inversely conclude that in most cases, all samples assigned to one mtDNA lineage should be conspecific. Although cases of distinct amphibian species with low mtDNA differentiation exist and phenomena of mtDNA introgression can potentially blur species identities, such cases remain exceptional. Therefore, these factors are unlikely to substantially affect the calculation of range sizes according to which a total of eight mtDNA lineages have ranges >100,000 km2 (lineages 2, 19, 33, 34, 36, 39 41, 42). In the most inflationist taxonomic scenario, with each of these lineages representing separate species, the dataset still provides evidence for a species of lowland Neotropical amphibian (lineage 36) occupying an area of almost one million km2, encompassing multiple biomes across a distance of about 1,600 km between its two most distant populations.
The most widespread lineages within D. minutus sensu lato have distributions restricted to or centered in Brazil and occur within rather open habitats, while lineage 2 of the D. minutus group (with a range of almost 270,000 km2) occurs in rainforest. Several of the lineages known from only few or single sites (e.g., lineages 8, 11, 12, 13, 14, 15, 16) occur in the Andean foothills or on mountain slopes. Nevertheless, in the Andean area, a higher sampling density is needed before it can be concluded with certainty that those lineages are restricted to small ranges. Hence, the distribution of mitochondrial lineages in the D. minutus group indicate that in open lowland areas of South America, small-sized species of anurans can be widespread.
CitationGehara M, Crawford AJ, Orrico VGD, Rodríguez A, Lötters S, et al. (2014) High Levels of Diversity Uncovered in a Widespread Nominal Taxon: Continental Phylogeography of the Neotropical Tree Frog Dendropsophus minutus. PLoS ONE 9(9): e103958. doi:10.1371/