GENETIC-DIVERGENCE AND INTROGRESSIVE HYBRIDIZATION BETWEEN ALNUS-SINUATA AND A-CRISPA (BETULACEAE)

The actinorhizal genus Alnus contains numerous taxa that have been morphologically classified into different subgenera, species and subspecies. The genetic divergence has been evaluated within subg. Alnobetula between the parapatric taxa Alnus sinuata and A. crispa, using diversity of allozyme markers at 15 structural loci among 20 populations. Evidence for introgressive hybridization at the overlap of their ranges was noted in three populations. However, the width of the hybrid zone appeared tenuous. The average genetic distance derived from the comparisons of conspecific populations was much smaller than the interspecific distance (D = 0.047). This allelic divergence was also paralleled with larger amounts of allelic and genotypic diversity within and among populations of A. sinuata, which are occupying a more heterogenous ecological niche. It is proposed that the repeated advances and retreats of the ice sheet during the Pleistocene may have promoted the divergence and allopatric evolution of these subspecies, and that secondary contact may have occurred repeatedly during the interglacial periods. The dynamic-equilibrium model would predict in such cases that narrow hybrid zones, formed at the contact of parapatric ranges, would impede gene exchange between parental taxa by selection against hybrids. The results obtained in this study seemed concordant with this hypothesis, as they were also in agreement with the existent taxonomical treatment of these taxa based on morphology. © 1990 Springer-Verlag.