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Characterization of clonal structure and mating patterns in Maianthemum dilatatum Wilson, Amy


Maianthemum dilatatum is a liliaceous, rhizomatous herb, which adopts a guerrilla mode of lateral spread. The central focus of this study was to characterize the clonal structure of Maianthemum dilatatum using AFLP markers. A subpopulation was selected where 21 patches, covering a total area of 3236 m2 were mapped and sampled. Within these patches, 116 ramets were sampled and assigned to 74 distinct genets. Patches were genetically heterogeneous and moderately differentiated (ΦST =0.291, p=0.001). Multivariate Mantel tests detected a genetic patch width of approximately 55-m, to which clonal growth contributed up to 25-m. This genetic patch was concluded to be the cumulative result of seed and pollen dispersal distances. Evidence of genet natality was present with the detection of five yearlings. Therefore, within M. dilatatum populations, clonality is a significant factor but the spatial structuring of genetic variation suggests that both low levels of restricted gene flow and repeated recruitment of genets occur. It has often been predicted that clonal plants will suffer increased geitonogamy (withingenet self pollination) as a genet increases in size. The prerequisites for this relationship to exist are: a high clonemate encounter probability and highly leptokurtic pollinator visitation, which have not been tested in a guerrilla morphology such as Maianthemum dilatatum. Clonemate encounter probability was measured at the ramet level, and was determined to account for 80% of all ten near-neighbours. Pollinators did preferentially forage among near-neighbours but they typically left the plots after visiting an average of only two floral ramets, which lowers the expected rate of geitonogamy based on genet arrangement alone. Therefore, the prerequisites for clonemate geitonogamy did exist in M. dilatatum, namely the high frequency of clonemate encounter and leptokurtic pollinator visitation patterns. Secondly, the contribution of geitonogamy to female reproductive success was estimated through hand pollinations and pollinator exclusion. Hand pollinations confirmed that M. dilatatum's self-incompatibility and pollen limited. An interaction was found between pollination and pollinator exclusion treatments (χ2=92.399, p<0.001), where excluding pollinators from outcrossed individuals increased fruit set. This supports the self-interference hypothesis, which is a predicted consequence of geitonogamy within self-incompatible plants. Emasculation did not have a significant effect on seed set (χ2=l .0991, p=0.2945) which implies that ramet-level geitonogamy is not a significant contributor to self-interference. Therefore, in a guerrilla species such as Maianthemum dilatatum, clonemate geitonogamy is a factor constraining female reproductive success, but to a lesser degree than would be expected in a species with a phalanx morphology. In conclusion, clonal growth makes a significant contribution to the spatial structure and to mating patterns within M. dilatatum populations. Yet, repeated genet recruitment and genet intermingling allows for the high genotypic diversity, which reduces but does not eliminate the influence of geitonogamous pollination.

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