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Early evolution in a hybrid swarm between outcrossing and selfing lineages in Geum - PubMed

Early evolution in a hybrid swarm between outcrossing and selfing lineages in Geum

M Ruhsam et al. Heredity (Edinb). 2011 Sep.

Abstract

Although often considered as evolutionary dead ends, selfing taxa may make an important contribution to plant evolution through hybridization with related outcrossing lineages. However, there is a shortage of studies examining the evolutionary dynamics of hybridization between outcrossing and selfing taxa. On the basis of differential pollinator attractiveness, production and competitive ability of pollen, as well as levels of inbreeding depression, we predict that the early products of hybridization between outcrossing and selfing lineages will be F1s and first-generation backcrosses sired mainly by the outcrossing lineage, together with selfed F2s containing a limited genetic contribution from the outcrosser. These predictions were tested using amplified fragment length polymorphism and chloroplast markers to analyze the composition of a recent hybrid swarm between predominantly outcrossing Geum rivale and predominantly selfing Geum urbanum. In line with predictions, the hybrid swarm comprised both parental species together with F1s and first-generation backcrosses to G. rivale alone. Chloroplast data suggested that G. rivale was the pollen parent for both observed hybrid classes. However, there was no evidence for F2 individuals, despite the fact that the F1 was fully self-compatible and able to auto-pollinate. The pollen fertility of F1s was only 30% lower than that of the parental taxa, and was fully restored in backcross hybrids. Predicting future evolution in the hybrid swarm will require an understanding of the mating patterns within and among the mix of parental, F1 and backcross genotypes that are currently present. However, these results support the hypothesis that introgression is likely to be asymmetrical from selfing to outcrossing lineages.

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Figures

Figure 1
Figure 1

The PCO plot of individuals from a hybrid swarm in Edinburgh (n=34, open symbols) based on variation at 202 AFLP markers. Reference samples (filled symbols) of G. urbanum (n=12), G. rivale (n=12) and F1 individuals from controlled crosses (n=15) are also included. Hybrid classes are identified according to NEWHYBRIDS (Anderson and Thompson, 2002).

Figure 2
Figure 2

The PCA plot based on 9 morphological characters measured in a common environment for 34 individuals collected from a hybrid swarm in Edinburgh (open symbols), as well as reference samples (filled symbols) from pure G. urbanum (n=39), pure G. rivale populations (n=32) and controlled F1 crosses (n=32). The genetic group into which individuals are classified by the NEWHYBRIDS program is indicated on the PCA plot.

Figure 3
Figure 3

The flowering schedule measured in 2008 for five genetic groups within a hybrid Geum swarm in Edinburgh. Groups were identified on the basis of flower morphology. The ‘Uncertain' group comprises a mixture of G. rivale and backcrosses to G. rivale. The overall contribution of flowers by the five genetic groups throughout the period was 42% for G. urbanum, 29% for G. rivale, 21% for F1s, 5% for backcrosses to G. rivale, 0% for backcrosses to G. urbanum and 3% for the uncertain group. A full color version of this figure is available at the Heredity journal online.

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