Genetic variation in two economically important Aloe species (Aloaceae)

Four morphologically distinct and geographically separated populations of Aloe ferox and one population of A. marlothii were examined by horizontal starch gel electrophoresis to assess levels of genetic variation at 22 protein coding loci. Leaf extracts were surveyed for 26 proteins; gene products revealed no polymorphism in any of the populations studied in the former species, and genetic variation at one locus (4.55%) in the latter species. The percentage of polymorphic loci was 4.76%; a value of 1.05 (±0.05) was obtained for the mean number of alleles per locus, and the average heterozygosity per locus was calculated at 0.022 (±0.022) for A. marlothii. We propose the ecological theory to explain the low levels of genetic variation obtained (i.e. species that are well adapted to their environment need less genetic variation). Isozyme differences between species were encountered at two loci to produce a mean genotypic distance index of 0.056, indicating a high degree of differentiation between species. This is unexpected since the two species studied are closely related and known to produce natural hybrids with A. arborescens. Biochemical markers could not be found to distinguish different chemovars of the two species. The remarkably low levels of genetic variation may be related to the xerophytic habit of the plants, perhaps making them less sensitive to drought stress as a selection pressure.