Progress 10/01/03 to 09/30/04
Outputs
Chloroplast DNA (cpDNA) studies based on PCR/RFLP data have shown that North American Silene vulgaris populations are highly structured at the level of local populations and moderately structured among geographic regions. Further, pooling information from North American populations results in a measure of genetic diversity comparable to that found previously in Europe, suggesting multiple introductions to multiple localities. A companion study based on cpDNA sequencing of select individuals from North America and Europe has shown only modest regional geographic structuring in Europe, and that North American sequences can be found throughout a phylogenetic tree based on European cpDNA sequences. This again suggests multiple introductions from Europe to North America. Studies of the geographic structure of nuclear genes to date have been based on AFLP markers. Preliminary results suggest a nuclear population structure in Europe not seen in the cpDNA data. North American
multi-locus AFLP genotypes can be found throughout a neighbor joining tree created with European AFLP genotypes, once again suggesting multiple introductions to North America. Microsatellite nuclear genetic markers have been developed in anticipation of further studies of cyto-nuclear genetic structure. To date crossing studies have focused on crosses within and between North American populations. The major result is that crosses between some North American populations generate F1 offspring whose survivorship is significantly higher than is found in either parental population. Crosses between other combinations of populations result in F1 progeny with significantly reduced survivorship. Thus, there is evidence that gene flow could result in either heterosis or outbreeding depression, depending on which populations are involved. One notable result was that offspring of Giles County, VA plants who were crossed with individuals from any other region in the study display heterosis,
suggesting that inbreeding depression is a general property of the Virginia plants. One would predict that gene flow into the Virginia populations could increase their invasiveness. It was also shown that when these long distance outcross individuals are self-fertilized their offspring exhibit reduced inbreeding depression, when compared to the progeny of individuals outcrossed within their local population and then self-fertilized. Differently put, the advantage to gene flow persists beyond the first generation. A second result from the cross experiment was that long distance crosses increase the proportion of hermaphrodite offspring. S. vulgaris is gynodioecious with cyto-nuclear sex determination, and a change in offspring sex with outcrossing distance was considered to be an indication of the effect of population structure on cyto-nuclear interactions. The study of European populations was delayed by extreme heat and drought in Europe in the summer of 2003, which limited the
opportunity for collecting the necessary seeds. European seed collection in the summer of 2004 was very successful, however, and these studies are now proceeding well, as indicated above.
Impacts
Management strategies of invasive species depend on knowledge of population dynamics and population history. Studies of the geographic structure of genetic diversity in the native and introduced ranges can provide insight into the history of the invasion (e.g. one or multiple introductions into one or multiple sights) that can, in turn, demonstrate how population genetic processes influence invasion success. Crosses between plants from diverse locations can illustrate the degree to which the invasive species has undergone local adaptation or, conversely, suffers from inbreeding depression. Local adaptation implies that a management strategy must be tailored to local circumstances. Populations suffering from inbreeding depression could become considerably more aggressive, should conditions come to favor increased rates of gene flow. The results of this project have illustrated both the North American population structure of Silene vulgaris in the context of its
structure in Europe, and the consequences of gene flow among geographic populations.
Publications
- McCauley, DE, RA Smith, JD Lisenby, C Hsieh. 2003. The hierarchical spatial distribution of chloroplast DNA polymorphism across the introduced range of Silene vulgaris. Molecular Ecology 12: 3227-3235.
- Bailey, M.F. and D.E. McCauley. 2005 Offspring sex ratio under inbreeding and outbreeding in a gynodioecious plant. Evolution (in press)
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Progress 10/01/02 to 09/30/03
Outputs
Plant material for DNA extraction was collected from 56 North American Silene vulgaris populations ranging from Ontario to Virginia. Local populations were clustered into nine geographic regions. A survey of chloroplast DNA diversity identified 19 haplotypes distinguished by the PCR/RFLP method. Haplotype diversity was partitioned statistically into the following hierarchical components: among regions, among populations within regions, and among individuals within populations. The significant among region component, coupled with a lack of isolation by distance, suggests that S. vulgaris was introduced to North America in multiple episodes to different localities, and that subsequent gene flow by seed has not been sufficient to eradicate the genetic signature of these introductions. A large among population within region component suggests that recent patch dynamics drive local population structure. Comparison of total haplotype diversity across North American
populations to haplotype diversity calculated from previous studies of S. vulgaris chloroplast DNA in Europe suggests that the introduction of S. vulgaris to North America did not involve a severe population bottleneck. Additional studies of chloroplast DNA haplotype diversity and geographic structure in Europe are underway. Greenhouse crosses within and among North American populations were initiated to study the effects of mating system and population structure on viability. Cross treatments included self-fertilization, outcrossing with another individual from the same population, outcrossing with an individual from a nearby population, and outcrossing with an individual from a different geographic region. This is a two generation experiment. The F1 generation is complete with the following results. Self-fertilization reduces seed viability and post germination survivorship greatly, when compared to outcrossing within populations. Viability is increased further by outcrossing among
local populations and by outcrossing among regions. An interaction between geographic region of origin and cross treatment demonstrates that the magnitude of these effects is region dependent. Offspring gender was also noted. Self-fertilization produced a greater proportion of female offspring (vs. hermaphrodites) than did outcrossing within populations. The proportion of hermaphrodite offspring increased further with longer distance outcrossing though again there was a region x cross treatment interaction. The results of the cross experiment suggest that the viability of local populations depends in part on their ability to retain and accumulate genetic diversity. The fact that different geographic regions differ in their response to inbreeding/outbreeding suggests that the effects of genetic structure on viability depend in part on local history. Since one intent of the experiment was to distinguish between dominance and epistatic genetic effects on fitness, complete interpretation
of the results depends data from the second generation (F2). The F2 crosses are underway. Crosses among European populations and between European and North American populations will be initiated in Spring, 2004.
Impacts
Management stategies of invasive species depend on knowledge of population dynamics and population history. Studies of the geographic distribution of genetic diversity will provide information on the history and genetic consequences of the spread of the study species from Europe to North America. Crossing studies will indicate whether the persistence and vigor of natural populations is influenced by the magnitude of genetic variation within them, and whether this varies geographically across the range of the study species.
Publications
- McCauley D.E., Smith R.A., Lisenby J.D. and Hsieh, C. 2003. The hierarchical spatial distribution of chloroplast DNA polymorphism across the introduced range of Silene vulgaris. Molecular Ecology 12: 3227-3235.
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