Genetic variability of South African fiscal shrikes (<i>Lanius collaris</i>) across an altitudinal gradient

Abstract

An electrophoretic allozyme study of the fiscal shrike Lanius collaris, a widely distributed sub-Saharan passerine, was undertaken to investigate altitudinal variation in genetic structure within this species. It is a sedentary bird with limited dispersal so that subpopulations may be expected to show genetic isolation by distance, which may have been enhanced by Pleistocene glaciation effects. The study area was a linear altitudinal transect of four localities in the eastern half of South Africa, which ranged from sea level to an altitude of 1800 m, with the geographic distance between the furthest subpopulations being 300 km. Fourteen (40%) of the 35 electrophoretic loci examined were polymorphic. The mean percentage of polymorphic loci within populations, mean heterozygosity and mean number of alleles per locus were 0.519, 0.272 and 1.60, respectively. These values were much higher than those reported for most other avian species. The negative F_ISand F_IT values indicated a small excess of heterozygotes. The populations are only weakly differentiated genetically (F_ST = 0.076). Rogers’ (1972) mean genetic distance (D) was 0.094, and Nei’s (1978) mean unbiased genetic distance was 0.019. A phenogram (unweighted pair group method) and a phylogenetic tree (distance Wagner network), both based on Rogers’ (1972) distance, showed some degree of geographical subgrouping. The high mean value for the effective number of migrants (Nm) among demes indicated that 3.04 individuals were exchanged among demes per generation. The mean value for local genetic differentiation (F_ST) and Nm indicated levels of gene flow between the different localities of fiscal shrike subpopulations. The Mantel test indicated that there was no significant relationship between the F_ST values and the geographical distances between sample pairs. This study showed that the isolation-by-distance model of subpopulation structure is inappropriate for these birds. Juvenile natal dispersal distances, female divorce rates (every three years) and the distance of travel of females (although low) appear to be sufficient to maintain high levels of polymorphism and heterozygosity. These levels of polymorphism and heterozygosity, together with large amounts of gene flow and the general lack of spatial autocorrelation imply that the populations of L. collaris may be subject to high rates of random drift. High values of sample heterozygosity suggest that fiscal shrike subpopulations have been stable for a long period and that genetic diversity has accumulated within populations. The rates of migration suggest that the ‘subpopulations’ of fiscal shrike act more or less as a single population extending over at least 300 km.

Keywords: Lanius collaris, fiscal shrike, allozyme electrophoresis, altitudinal variation, genetic variation, population structure