Chromosomal and genic barriers to introgression in Helianthus

The sexual transfer of genes between taxa possessing different structural karyotypes must involve the passage of genes through a chromosomal sterility barrier. Yet little is known about the effects of structural differences on gene introgression within or adjacent to the rearranged chromosomal fragments or about the patterns of introgression in collinear regions. Here, we employ 197 mapped molecular markers to study the effects of chromosomal structural differences on introgression in backcrossed progeny of the domesticated sunflower, Helianthus annuus, and its karyotypically divergent wild relative, H. petiolaris. Forty percent of the genome from the seven collinear linkages introgressed, whereas only 2.4% of the genome from the 10 rearranged linkages was transferred. Thus, chromosomal rearrangements appear to provide an effective mechanism for reducing or eliminating introgression in rearranged chromosomal segments. On the other hand, observations that 60% of the markers from within the collinear portion of the genome did not introgress suggests that genic factors also resist introgression in Helianthus. That is, selection against H. petiolaris genes in concert with linkage may have reduced or eliminated parts of the genome not protected by structural changes. Thus, barriers to introgression in Helianthus appear to include both chromosomal structural and genic factors.