Examining the genetic structure among populations of the common Cape river crab Potamonautes perlatus from river systems in South Africa reveals hydrographic boundaries

The genetic structure of the common Cape river crab Potamonautes perlatus (Decapoda: Potamonautidae) was assessed among 21 populations from major drainage systems in South Africa using allozymes and sequence data from two mitochondrial genes (16S rRNA and COT). Protein gel electrophoresis of 15 loci encoding 11 enzyme systems revealed limited genetic variation in the Berg and Olifants drainages. However, in the Gamtoos and Bre river systems, and over all populations, moderate levels of variation were evident from the FST values. Gene-flow estimates revealed high levels of dispersal within and between drainages. In contrast, phylogenetic analysis of the 16S rRNA mtDNA sequence data from 10 representative populations revealed two distinct clades. One consisted of all the rivers that drain the western escarpment, and another consisted of rivers that drain the southern escarpment. The COI mtDNA and a combined sequence data analysis consistently placed the western flowing rivers in a distinct clade. The split into two clades coincided with the western and southern drainages with the Cape Fold mountains being the barrier that separates the drainage basins. These results are discussed in light of possible geological factors that could have sculpted the present genetic patterns. Allozyme and sequence data were generally complementary at a small spatial scale (within drainages), where both markers indicated the absence of genetic structure. However, at larger spatial scales, the sequence data proved superior at detecting genetic variation.