Population genetics and phylogenetic placement of the endangered Knysna seahorse, Hippocampus capensis

[摘要] ENGLISH ABSTRACT:The aims of this study were to investigate genetic issues pertaining to theconservation of the Knysna seahorse, Hippocampus capensis, and to determine thephylogenetic placement of this endangered estuarine species among marine seahorses.This was accomplished by focusing on three aspects of the taxonomy: the interspecificlevel; the inter-population level; and the intra-population level. To determinewhich species are closely related to H. capensis, and how the evolutionary history ofthis lineage relates to that of other seahorses, sequence data derived from four genefragments (the nuclear RPI and Aldolase and the mitochondrial 16S rRNA andcytochrome b genes) were used to determine the phylogenetic relationships among 30species belonging to the genus Hippocampus. There were marked differences in therate of evolution among these gene fragments, with Aldolase evolving the slowest andthe mtDNA cytochrome b gene the fastest. Among individual partitions, the RPIgene recovered the highest number of nodes supported by >70% bootstrap valuesfrom parsimony analysis, and >95% posterior probabilities from Bayesian inference.The combined analysis based on 2317 nucleotides resulted in the most robustphylogeny. A distinct phylogenetic split was identified between the pygmy seahorse,H. bargibanti, and a clade including all other species. Three species from the westernPacific Ocean included in this study, namely H. bargibanti, H. breviceps, and H.abdominalis, occupy basal positions in the phylogeny. This and the high speciesrichness in the region suggest that the genus probably originated in this region. Thereis also fairly strong molecular support for the remaining species being subdivided intothree main evolutionary lineages: two West Pacific clades and a clade of speciespresent in both the Indo-Pacific and the Atlantic Ocean, which includes H. capensis.The phylogeny obtained herein suggests that seahorses belonging to the latter cladecolonised the Atlantic Ocean at least twice, once before the closure of the TethyanSeaway, and once afterwards. Phylogenies reconstructed using mitochondrial DNAgene fragments (l6S rRNA, cytochrome band 382 bp of the rapidly evolving controlregion) indicate that H. capensis is closely related to an Indian Ocean lineage of H.kuda and a Red Sea lineage of H. fuscus. Other lineages closely associated withthese taxa include H. kuda from the West Pacific, the East Atlantic species H. algiricus, the West Atlantic species H reidi, the East Pacific species H ingens, andthe Hawaiian species H fisheri. No control region alleles were shared among Hcapensis and any of the marine seahorses, suggesting that the Knysna seahorse isphylogenetically distinct. The evolutionary history of H capensis, and the extent ofgene flow between its three known populations, were investigated using controlregion sequences from 138 specimens. Most samples were obtained by taking finclips; this method was studied on captive seahorses and no negative effects werefound. Similarly high levels of genetic diversity were found in two of the wildpopulations (Knysna and Keurbooms Estuaries), whereas diversity in the thirdpopulation (Swartvlei Estuary) was lower. Although most haplotypes are sharedamong at least two populations, based on the haplotype frequency distributions thethree assemblages constitute distinct management units. The extant populationstructure of H capensis suggests that the Knysna seahorse originated in the largeKnysna Estuary. The presence of seahorses in the two smaller estuaries is either theresult of a vicariance event at the beginning of the present interglacial period, orcolonisation of the estuaries via the sea, or a combination of the two. Populationgenetic parameters of the Knysna population and those of two populations of closelyrelated marine seahorses (H kuda from the Philippines and H fuscus from the RedSea) were similar, suggesting that the Knysna population is not geneticallyimpoverished, despite its comparatively small area of occupancy.

[发布机构] Stellenbosch University