In southeastern Botswana, the freshwater anostracan Branchipodopsis wolfi is particularly abundant on rocky escarpments, which occur scattered and hold clusters of rock pools of a variable phenology. Branchipodopsis wolfi generates egg banks that hedge against drought catastrophes and potentially promote dispersal and gene flow. These topographical and biological properties in combination with ecological and genetic data promote the study of micro-evolutionary processes in these populations. Genetic differentiation was assessed in 17 populations across 3 metapopulations (KS, KR, and Th) on the basis of allozyme variation at 4 loci (PGM, PGI, APK, GOT) and variation in some ecologically relevant life history characteristics. Across all populations a high level of population differentiation (Pst = 0,3; (Nei’s) D = 0,378 ± 0,292) was measured. Clustering performed with UPGMA shows clearly differentiated metapopulations, but levels of differentiation did not correspond with geographic distance; metapopulations separated by less than 2 km were genetically more different than those 50 km apart. Within metapopulations, population differentiation was significant (p < 0.01) at KR (Pst = 0,03; D = 0,025 ± 0,022) and KS (Pst = 0,03 ; D = 0,018 ± 0,020), but at Th effective panmixis was observed (Pst = 0,02 ; D = 0,004 ± 0,006). Some effect of distance at the local scale was revealed only at KS. Mean temperature and depth (duration of hydrocycle) varied significantly (p < 0,05) among sites with Th pools having significantly higher temperatures and shorter hydro periods than the others. Within metapopulations significant variation between pools was measured for mean depth (KS and Th) and conductivity (Th). There was no clear trend between gene frequencies and pool water parameters. It seems therefore unlikely that the genetic divergence measured between sites and among KH and KS pools results from differences in the selective environments. After correction for female length (ANCOVA), a significant (p < 0,05) overall in situ differentiation in mean broodsize, egg diameter, and maturation rate was measured among metapopulations and local populations. Of all life history characteristics, only overall mean egg diameter was significantly (p < 0,05) correlated with some of the environmental variables (pH and depth). In individual metapopulations significant differentiation (p < 0,05) between pools was revealed for broodsize at KS and KH, but not at Th, while egg size varied significantly among pools at each site. No constant patterns of correlation between environmental variables and life history characteristics were revealed at the individual sites. Significant differentiation in allozyme frequencies between metapopulations and between populations at two sites and the absence of a clear association between environmental variables and allozyme and life history variation, point at tbe importance of stochastic processes in shaping the spatial genetic structure of B. wolfi. Additional 'common garden' experiments are required to reveal a possible genetic basis of the measured geographic variation in life history characteristics. |
