The biogeography, systematics and conservation of Phreatoicidean isopods in South Africa

[摘要] ENGLISH ABSTRACT: Historically, isopods of the suborder Phreatoicidea were thought to be represented in southernAfrica by four species belonging to the endemic genus Mesamphisopus. This taxonomy wasbased on poor collections and the extent of variation among and within populations werepoorly understood. In the present study, intensive sampling was undertaken to determine thediversity, distribution and biogeography of phreatoicidean isopods within South Africa.Analyses of allozyme data and mitochondrial DNA sequences (from the 12S rRNA andprotein-coding COI genes) were used to examine differentiation among populations, extricatespecies boundaries (in combination with morphometric and morphological data) and toelucidate the evolutionary relationships among taxa. Additionally, conservation units wereidentified among the sampled populations and conservation threats highlighted.First, genetic and morphometric differentiation was examined among populations identifiedmorphologically as M. capensis. Collection localities spanned two mountainous regions inthe Western Cape and these were separated by a coastal plain remnant. Fivemorphometrically and genetically distinct species were identified. These taxa are alsogeographically partitioned in two regions, which were regarded as Evolutionarily SignificantUnits. Differentiation among populations of the two regions, and similar patterns in othertaxa, was attributed to Cenozoic sea-level fluctuations.Second, populations, variably assigned to M. abbreviatus or M. depressus, were examined todetermine whether they were conspecific. A large geographic area was sampled to accountfor intraspecific differentiation. Limited morphometric discrepancies were observed, withindividual populations being either similar to the M. abbreviatus or the M. depressussyntypes. Genetic support for the recognition of a cryptic species complex among thesampled populations was equivocal. Substantial genetic differentiation and a lack of geneflow were observed among all populations. Clear patterns of isolation by distance were notdetected, and genetic structure appeared to be unrelated to geography or drainage systems.The mosaic pattern of relatedness among populations was best explained by stochasticdemographic processes, such as extinction-recolonization events or population bottlenecks.Thirdly, detailed taxonomic descriptions and illustrations of six new species, identifiedgenetically and morphometrically among the populations included in the above analyses, wereprovided. These species were largely distinguished from each other, and the four originalspecies, using a combination of setation, mouthpart, pleopod and uropod features.Lastly, phylogenetic relationships among all ten recognized Mesamphisopus species, and anadditional unresolved group of populations, were examined. MtDNA data partitions and arecoded allele frequency matrix were analysed independently and in combination. Topologiesindicated unrecognized species-diversity within an unresolved group of populations.Evolutionary relationships, the identification of six biogeographic centres, and the dating ofdivergences using a relaxed Bayesian clock suggested that differentiation and speciationwithin Mesamphisopus was largely allopatric or vicariant and driven by Mesozoic sea-leveland climate change. Chance long distance dispersal events would, in turn, explain spuriousphylogenetic relationships and distributions.This study contributes significantly to the understanding of the diversity and the conservationof the little-studied southern African freshwater invertebrates. Moreover, this study is the firstto investigate genetic and morphometric differentiation, and phylogenetic relationships, belowthe generic level within the Phreatoicidea; thus establishing a methodological and theoreticalframework for species delineation and the accurate determination of biodiversity withinindividual phreatoicidean genera.

[发布机构] Stellenbosch University