Hybridization as a Stimulus for the Evolution of Invasiveness in Plants
Ayres, Debra , Strong, Donald .
Hybridization as a Stimulus for the Evolution of Invasiveness in Spartina alterniflora x foliosa Hybrids.
SPARTINA hybrids originated shortly after the 1970s introduction of Atlantic smooth cordgrass, S. alterniflora, into the range of native California cordgrass, S. foliosa, in San Francisco Bay, CA. These perennial grasses are the lowest emergent vascular plants in tidal salt marshes. Evidence from nuclear and chloroplast DNA analyses, and chromosome counts has determined that hybridization is introgressive, bi-directional, and has not involved polyploidization or speciation. Since the 1980s, a morphologically and genetically diverse array of diploid hybrids has invaded native and restored marshes, and open tidal mudflats. In common garden, common greenhouse, and in situ assessments, we found that a subset of hybrids have greater ecological amplitude on the intertidal plane, are taller and expand laterally at a higher rate, produce larger infloresences, and have higher tolerance to salinity than either of the parental species and have a flowering phenology that overlaps that of the parental species. Genetic evidence suggests that transgressive phenotypes are due to the action of complementary genes from both species. Some hybrids are highly self-compatible, unlike the parental species. In spatial-genetic studies, we found that a few hybrids are driving the invasion of the vast open mudflats of the estuary, producing much selfed progeny that colonize local mudflats and marshes. Demographic trends indicate that the invasion rate is accelerating, resulting in faster-than-exponential population growth. A recent theoretical study, linking population dynamic and genetic factors, found dramatic effects on population growth rate and genetic structure when natural selection favored individuals with high rates of vegetative growth and sexual reproduction - population growth rates of hybrids became faster-than-exponential, and the less fit species became extinct. We propose that the formation of self-compatible hybrids be added to the list of mechanisms whereby hybridization stimulates the evolution of invasiveness.
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1 - University of California, Davis, Section of Evolution and Ecology, One Shields Ave., Davis, California, 95616, USA
Presentation Type: Symposium or Colloquium Presentation
Date: Wednesday, August 2nd, 2006
Time: 10:00 AM