Molecular evidence for the early history of living amphibians.

The evolutionary relationships of the three orders of living amphibians (lissamphibians) has been difficult to resolve, partly because of their specialized morphologies. Traditionally, frogs and salamanders are considered to be closest relatives, and all three orders are thought to have arisen in the Paleozoic (>250 myr). Here, we present evidence from the DNA sequences of four mitochondrial genes (2.7 kilobases) that challenges the conventional hypothesis and supports a salamander-caecilian relationship. This, in light of the fossil record and distribution of the families, suggests a more recent (Mesozoic) origin for salamanders and caecilians directly linked to the initial breakup of the supercontinent Pangaea. We propose that this single geologic event isolated salamanders and archaeobatrachian frogs on the northern continents (Laurasia) and the caecilians and neobatrachian frogs on the southern continents (Gondwana). Among the neobatrachian frog families, molecular evidence supports a South American clade and an African clade, inferred here to be the result of mid-Cretaceous vicariance.

Phylogeny and biogeography of ratite birds inferred from DNA sequences of the mitochondrial ribosomal genes.

The origin of the flightless ratite birds of the southern continents has been debated for over a century. Whether dispersal or vicariance (continental breakup) best explains their origin depends largely on their phylogenetic relationships. No consensus has been reached on this issue despite many morphological and molecular studies. To address this question further we sequenced a 2.8-kb region of mitochondrial DNA containing the ribosomal genes in representative ratites and a tinamou. Phylogenetic analyses indicate that Struthio (Africa) is basal and Rhea (South America) clusters with living Australasian ratites. This phylogeny agrees with transferrin and DNA hybridization studies but not with sequence analyses of some protein-coding genes. These results also require reevaluation of the phylogenetic position of the extinct moas of New Zealand. We propose a new hypothesis for the origin of ratites that combines elements of dispersal and vicariance.

Phylogenetic relationships of the hoatzin, an enigmatic South American bird.

The hoatzin (Opisthocomus hoazin) lives in the humid lowlands of northern and central South America, often in riparian habitats. It is a slender bird approximately 65 cm in length, brownish with lighter streaks and buffy tips to the long tail feathers. The small head has a ragged, bristly crest of reddish-brown feathers, and the bare skin of the face is bright blue. It resembles a chachalaca (Ortalis, Cracidae) in size and shape, but its plumage and markings are similar to those of the smaller guira cuckoo (Guira guira). The hoatzin (pronounced Watson) has been a taxonomic puzzle since it was described in 1776. It usually has been viewed as related to the gallinaceous birds, but alliances to other groups have been suggested, including the cuckoos. We present DNA sequence evidence from the 12S and 16S rRNA mitochondrial genes, and from the nuclear gene that codes for the eye lens protein, alpha A-crystallin. The results indicate that the hoatzin is most closely related to the typical cuckoos and that the divergence occurred at or near the base of the cuculiform phylogenetic tree.

Origin of West Indian populations of the geographically widespread boa Corallus enydris inferred from mitochondrial DNA sequences.

Corallus enydris (Serpentes: Boidae: Boinae) is an arboreal snake with an extremely wide mainland distribution from southern Costa Rica to southeastern Brazil and is one of two boine species that has invaded the Lesser Antilles (Grenada Bank and St. Vincent). Mitochondrial DNA sequences of samples from seven geographically disparate localities provided evidence of phylogenetic relationships. The monophyly of C. enydris is corroborated and a major dichotomy between northern samples (Panama and Trinidad) and southern samples (Guyana, Perú, southeastern Brazil) was found and corresponds to the two currently recognized subspecies. Unexpectedly, the two samples from the West Indies (southern Lesser Antilles) cluster with the southern rather than the geographically closer northern samples (e.g., Trinidad). The results imply a fairly recent Guianan-Amazonian origin of West Indian populations.