NATURAL HYBRIDIZATION BETWEEN IRIS FULVA AND IRIS HEXAGONA: PATTERN OF RIBOSOMAL DNA VARIATION

Genetic variation associated with the natural hybridization of Iris fulva and I. hexagona was investigated to test for the occurrence of introgression. These species have been viewed as a classic example of the process of introgressive hybridization (Anderson, 1949). However, more recent studies have concluded that there has not been an exchange of genetic material between I. fulva and I. hexagona (Randolph et al., 1967). Our analysis has involved the examination of both allopatric and parapatric populations of I. fulva and I. hexagona with reference to diagnostic ribosomal DNA markers. The pattern of variation in the parapatric population indicates the presence of the repeated backcrossing necessary to the process of introgressive hybridization. Indeed, in the region of parapatry, we suggest that localized introgression of ribosomal sequences has occurred into both I. fulva and I. hexagona. Significantly, we have also detected the presence of the diagnostic ribosomal markers from each species in allopatric populations of the alternate species. Our findings suggest that not only is introgressive hybridization presently occurring in parapatry between I. fulva and I. hexagona, but that past hybridization between these species has resulted in introgression into areas of allopatry.