9419206 Rieseberg Biological species are often defined as groups of interbreeding organisms in populations that do not exchange genes with other groups. Any factor that impedes the exchange of genes between two species contributes to reproductive isolation and hence to the integrity and coherence of individual species. However, many physiological or reproductive barriers between species do not prevent gene flow completely but allow some small amount of hybridization to occur; and some of these barriers may actually limit gene flow to certain regions of the genome only. The purpose of research by Dr. Loren Rieseberg at Indiana University is to test empirically the effects of one of the most important reproductive barriers to gene flow in plants, chromosomal structural differences. This will be accomplished by surveying several hundred individual plants taken from two hybridizing populations (in California and Oklahoma) of sunflower species, genus Helianthus (Asteraceae), and by making use of 139 particular genomic markers of sunflower chromosomes already developed as a result of prior research support. Because the chromosome locations of these molecular markers are known in each of the parental sunflower species and differ in location in several instances, they can be used to trace the flow of particular genes or chromosomal segments across the species barrier in the two zones of hybridization. The results will yield data about the effectiveness of chromosomal structural differences as reproductive barriers and they will indicate whether such barriers limit interspecific gene flow to structurally similar regions of the genome only.
