Biosystematics of Lobularia, Cruciferae, revisited: evidence from isozyme data

Abstract

To obtain better insight in evolutionary processes, data on genetic diversity are compared with data obtained by more traditional biosystematic methods. Altogether, ten isozymes in 25 populations of Lobularia, Cruceferac, from Spain, Morocco, the Salvage Islands, the Canary Islands and the Cape Verde Islands were screened by starch gel electrophoresis. Genetic identities between pairs of species were low: 0.582 for canariensislibyca, 0.603 for canariensis-maritima, and 0.403 for libyca-maritima. These species are biological species, i.e. isolated by strong internal crossing barriers. The isozyme data indicate that speciation is an old event in the genus. The nine subspecies of the L. canariensis complex are interfertile and isolated by ecogeographical barriers. Among the subspecies, the genetic identities ranged from 0.476 to 0.846, with a mean identity of 0.734 for all pairs. Among 22 populations of L. canariensis, irrespective of taxon, the identities ranged from 0.456 to 0.961, with a mean of 0.717. In comparison, the identity for two populations of L. maritima, one from Spain and one from Morocco, was 0.753. Thus genetic identities were higher within than among species but the L. canariensis complex showed much infraspecific genetic diversity. Genetic distances matched geographic distances to some extent, with a northern and a southern gene pool. In the Canaries, however, genetic distances did neither correspond to geographic distances nor to subspecies of L. canariensis, indicating some gene flow between populations, subspecies, and islands. The genetic diversity was high. The mean number of alleles per locus was 2.19, 67.6% of the loci were polymorphic and the mean heterozygosity was 0.268. The average total diversity was also high, F_IT=0.547. Among population diversity, F_ST=0.459 contributed much more to the total diversity than within population diversity, F_IS=0.163. My data thus seem to support the importance of the population as a main unit in evolution.