Persistent effects of historical sea levels on the population structure of a temporary wetland copepod

Temporary wetland ecosystems are common in arid and semi-arid environments, and are inhabited by diverse invertebrate communities. Little is known about the dynamics of genetic connectivity in the geographically scattered populations of these wetland specialists. The current study investigated the spatial genetic structure and dispersal history of a recently described calanoid copepod, Lovenula raynerae, reported from temporary wetlands in the Eastern Cape province of South Africa. We tested whether the species represents a single, well-connected population or comprises different regional genetic groups, some of which may be rare or endangered. Mitochondrial COI sequences were generated for 365 specimens from 46 temporary wetlands spread across the species' known distribution range. Isolation-by-distance and isolation-by-environment patterns of partitioning genetic variations across the landscape were evaluated. In addition, the presence of historical impediments to gene flow between contemporary populations was investigated using a combination of Monmonier's algorithm and Bayesian reconstruction of phylogeographical diffusion in continuous space. The wetland populations were highly structured across the landscape and could be assigned to six distinct evolutionary lineages, potentially representing some level of cryptic speciation. Two distinct phases were identified in the dispersal history of these lineages. Initially, dispersal only occurred inland of a postulated barrier, but eventually the barrier disappeared and the species extended its range by spreading into regions close to the coastline. Molecular dating shows that the barrier represents the upper limit of the coastline during the Pliocene, and that its crossing was facilitated by Pliocene sea regression in southern Africa. Our finding shows that complex demographic histories can be preserved in the mitochondrial DNA of temporary wetland crustaceans because of limited effective gene flow after initial colonisation events. This makes them an interesting study system to explore the long-term effects of climate change on arid ecosystem communities.