Morphology and systematics of the Solomon Island Ranid frogs.

Abstract

The ranid frogs of the Solomon Islands have been known in the taxonomic literature for over 100 years. The Solomon Islands and surrounding islands contain a unique array of direct-developing ranids, most of which are endemic to the island masses on which they live. However, their phylogenetic relationships to each other and to other ranids in the southwest Pacific region have not been defined. Their assumed relationships are based on a restricted sample of osteological characters and the assumption of direct development, inferred from egg size and number. This study validates the Solomon Island taxa (using morphometrics) and explores the biology of the Solomon Island ranids, with detailed osteological descriptions, external morphology and karyology. Using characters from these data sets a cladistic analysis using parsimony reconstructed a phylogeny of these frogs. Morphometric analyses were undertaken on two levels, firstly using principal components analysis (PCA) to assess sexual dimorphism, and secondly using discriminant function analysis (DFA) to test the identity of the taxa. Sexual dimorphism was not recognised ill morphometric characters and so sexes were pooled for the DFA. For Batrachylodes 93.08% of individuals were successfully classified to their actual groups, Discodeles 89.77% and for Platymantis 84.55%. Osteological descriptions of Batrachylodes elegans, B. mediodiscus, B. trossulus, B. vertebralis, B. wolfi, Ceratobatrachus guentheri, Discodeles bufoniformis, D. guppyi, Palmatorappia solomonis, Platymantis guppyi, P. myersi, P. neckeri, P. parkeri, P. solomonis, P. weberi and Rana kreffti are provided. Karyotypes of eight species of ranid frogs from the Solomon Islands were compared and contrasted with the 2n = 26, FN = 52 karyotype of Rana, the typical karyotype of the subfamily Raninae. This karyotype was found in the Solomon Island species of Rana, R. kreffti. Ceratobatrachus guentheri, had an increased 2n of 30, a lower FN of 38, and altered relative lengths and centromere positions of pairs 1-5. These changes could have resulted from centric fissions and pericentric rearrangements which produced an increase in the number of telocentric chromosomes. The remaining seven species (Batrachylodes vertebralis, Discodeles bufoniformis, D. guppyi, Platymantis myersi, P. neckeri, P. solomonis and P. weberi) had reduced diploid numbers and FN. The means by which reduction in 2n and FN has occurred in these species is unknown, but may either involve centric fissions to produce telocentrics, followed by translocation onto other chromosomes, or a process involving pericentric rearrangements to produce telocentric chromosomes followed by fusions of these products. With the exception of Rana, the level of chromosome rearrangements in these frogs that are endemic to the Solomon Islands is high compared with that observed in the continental lineages of this subfamily. Phylogenetic analysis using maximum parsimony found three equally parsimonious trees. Subsequent character reanalysis (successive weighting) produced one parsimonious tree. The phylogenies indicate multiple invasion events into the Solomon Islands by these ranid frogs and despite the high level of endemism, monophyly is not supported.