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

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.

Natural Hybrid Zone of Aconitum Species in the Western Carpathians: Linnaean Taxonomy and ISSR Fingerprinting

“…It could not be explained by differences in phenology (Milne and Abbott, 2008) because both species flower at the same time, but studies on this are needed. Rusham et al (2011) reported a similar result in a hybrid swarm of Geum rivale and G. urbanum, where none of the individuals in the hybrid swarm identified by NEWHYBRIDS were classified as a backcross to G. urbanum. Hybrids between Cardamine enneaphyllos and C. glandulifera were placed by PCR-RFLP analysis in an intermediate position but they also largely overlapped with one of the parental species (Lihová et al, 2007).…”

Hybridization can facilitate species invasions, even without enhancing local adaptation

“…Intercrossing success (i.e., the production of viable seeds) is also known to be affected by the direction of crossing (28,29). The interplay between these preand/or postzygotic processes can result in unequal transfer of parental genetic material into the genome of hybrid progeny (28,29). Whereas the short-term demographic consequence of this asymmetry is facilitation of establishment, the long-term outcome can be species replacement as we see in Cakile species.…”

“…Pollinators rarely commit to random foraging bouts but rather discriminate among plant types (27); such behavior can result in assortative mating and produce the required asymmetry. Intercrossing success (i.e., the production of viable seeds) is also known to be affected by the direction of crossing (28,29). The interplay between these preand/or postzygotic processes can result in unequal transfer of parental genetic material into the genome of hybrid progeny (28,29).…”

Evolutionary consequences of self-fertilization in plants

“…An expected consequence is aberrant endosperm development upon hybridization across mating systems [72]. There is some support for this hypothesis [72], and it may contribute to patterns of elevated pollen gene flow from outcrossing to selfing lineages [73,74]. However, more microevolutionary studies addressing the extent and nature of post-zygotic isolation among selfing and outcrossing populations are needed to better understand the interplay between genomic conflicts, mating systems and reproductive isolation.…”