Figure WN22.4. Hybrid speciation in Helianthus. (A) The F1 hybrid, H. anomalus, derived from the parent species, H. annuus and H. petiolaris. (B) The ranges of H. annuus and H. petiolaris, together with that of the hybrid species H. anomalus. (C) The distribution across markers of the proportion of H. petiolaris alleles seen in experimental hybrids. There were three generations of crossing within the hybrid population, followed by two generations of backcrossing to H. annuus. Therefore, in the absence of selection, one expects 1/8 of the genes to derive from H. petiolaris, with a distribution concentrated in the 1–25% class. In regions of genome with the same gene order in H. petiolaris and H. annuus (red ), most markers fail to introgress, but some introgress more than expected. In regions of genome that differ in gene order as a result of chromosome rearrangements, there is almost no introgression (blue). (D) Patterns of introgression along the genomes are similar between experimental hybrids and the natural hybrid species, H. anomalus. Three of the 17 H. anomalus chromosomes are shown. The letters to the left (R, S, T, Q) indicate homology of these chromosomes to regions of the parental genomes. (The leftmost chromosome is rearranged, and combines linkage blocks R and S.) Arrows to the right indicate the genetic markers. The shading indicates the likelihood that the regions derived from H. annuus (blue) or H. petiolaris (yellow). (A, Courtesy USDA; B, redrawn from Rogers et al. 1982; C, data from Table 1 in Rieseberg et al. 1995a; D, redrawn from Fig. 3 in Rieseberg and Noyes 1998.)
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