Natural Interspecific Hybridization in Gunnera (Gunneraceae)

Natural interspecific hybridization between Gunnera bracteata and G. peltata (Gunneraceae) in the Juan Fernandez Islands, Chile, is analyzed morphologically and chemically. Parental types from isolated populations were compared with parents and intermediates occurring together in Quebrada Villagra on Masatierra. Two transects were made in that area, one in a relatively undisturbed site, and another along a disturbed path. Hybrid indexes and distance diagrams were used to analyze morphological relationships, and leafflavonoids revealed chemical affinities. Minor flavonoid divergence between parental species precluded detailed analysis of dynamics of hybridization. Morphological analysis revealed intermediacy in both transects, with intergradation back toward both parents. It is suggested that introgressive hybridization is occurring in Quebrada Villagra between these two wind-pollinated species, with more hybridization taking place in disturbed regions. Reduction in surface area and changes in the ecology of Masatierra during the past four million years may have brought the two species into closer contact and aided hybridization. NATURAL HYBRIDIZATION IS A frequent phenomenon among flowering plants (Knobloch 1972, Grant 1981). Although hybrids and hybrid swarms have been detected in nature in man y plant groups (Tanowitz and Adams 1986, Do yle and Doyle 1988, Grant and Wilken 1988, Tortosa 1988), the role of hybridization in plant evolut ion is still controversial (Levin 1979). Hybridization on one hand may be rejected as an evolutionarily important process because of low fitness and sterility of the hybrid derivatives. On the other hand , hybridization may be seen as an important process in the evolution of plant groups because it can be a source of recombination and therefore its importance can be as great as mutation and recombination (Grant 1981). A more realistic perspective is that hybridiza1 This paper represents a portion of a thesis submitted by the senior auth or in part ial fulfillment of the requirements for the Ph.D. degree at Ohio State University. Dissertation support provided by NSF grant INT8317088. Manuscript accepted I August 1990. 2 Department of Plant Biology, Ohio State University, 1735 Neil Avenue, Columbus , Ohio 43210. 3 Present address: Departamento de Biologia, Universidad de Taka, Taka, Chile. tion can have an evolutionarily significant effect if it is followed by establishment of advanced hybrid generations, with or without polyploid y, or by introgression (Grant 1981). Although the breakdown of isolating mechanisms is often the first factor in the appearance of natural interspecific hybrids, the environment determines whether these hybrids, advanced generation hybrids , or introgressants may become established successfully (Stebbins 1950). In a stable environment, hybrid s are at a disad vantage with respect to parental populations because all niches are filled and no " hybrid" habitats exist where the hybrids can survive (Anderson 1948). Disturbance of the environment disrupts stable habitats, creating new open areas and new ecological niches that can be exploited by hybrid derivatives. In these new niches, hybrid individuals, with an enriched gene pool (Crins et al. 1988), have higher fitness than the parental populations and can compete successfully with them. Oceanic islands, because of their recent origin, offer new habitats within a restricted geographical area. Hybridization might be favored in insular areas where segregates can exploit these new habitats and therefore con-