Unexpected low genetic differentiation among allopatric species of section Algarobia of Prosopis (leguminosae).

The Chaqueña Biogeographic Province, in South America, is the main diversity centre of Prosopis. A group of sympatric species of Section Algarobia in this region constitutes a syngameon, characterised by frequent hybridization and introgression. These processes have been postulated as responsible for the low genetic differentiation observed among species within this group. In this study genetic variability and differentiation among geographically isolated species of the same section was analysed through isoenzyme electrophoresis. Variability parameters and fixation indices were estimated to determine the genetic structure of populations. Two Argentinean, P. ruscifolia ('vinal') and P. flexuosa ('algarrobo amarillo'), and one North American species, P. glandulosa (honey mesquite) were studied. All of them showed, similarly to other species of the same section, high genetic variability and exhibit homozygote excess, probably due to population substructure and low rates of selfing. In contrast to our hypothesis, genetic similarity among species is not related to geographic distance. Genetic distances between P. glandulosa and South American species are similar to those observed among species of this subcontinent. The results obtained suggest that the high genetic similarity among the species of the section Algarobia studied is not due to hybridization.