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Title: Breeding systems, pollen flow and continuing evolution in Avon Gorge Sorbus (whitebeams, rowans and service trees)
Author: Ludwig, Shanna Cassel
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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Hybridisation, polyploidy and asexual (apomictic) reproduction have given rise to high genetic diversity and taxonomic complexity in Avon Gorge Sorbus. Sorbus aria (a sexual diploid species) in particular has been implicated as a primary contributor to Sorbus diversification through hybridisations with polyploid taxa. It may also play a major role in the reproductive success of apomictic taxa by acting as pollen parent for pseudogamy (the partly-sexual formation of endosperm in asexual seed). The hypothesis was that S. aria acts as the primary pollen parent for endemic pseudogamous Sorbus taxa and in doing so facilitates evolutionary diversification of Sorbus in the Avon Gorge through rare inter-taxon hybridisation. Observations of flowering periods and insect pollinators showed that intra-generic synchronous flowering and the use of generalist pollinators allow for the movement of pollen among Sorbus flowers and the occurrence of inter-taxon mating events between Sorbus taxa growing in the Gorge. Pollination experiments, combined with molecular marker analyses of adults and progeny, showed that outcrossing occurs in the sexual diploids, which may encourage inter-taxon hybridisation. The tetraploid taxa were found to reproduce by apomixis and pseudogamy with self-compatibility which together are advantageous in the perpetuation of these taxa. The triploids were also found to be apomictic and pseudogamous, but they are self-incompatible and rely on inter-taxon pollen for seed set. A lack of hybrid progeny from open pollinations indicated hybridisation occurs rarely in the natural populations. Genotyping and flow cytometric analyses of endosperm in seeds from triploid taxa provided conclusive evidence that diploid S. aria is involved in their reproductive success as a pseudogamous pollen parent. Seed viability testing showed low seed fertility in the polyploids compared to S. aria, suggesting new polyploids may struggle to establish populations. Finally, in light of current conservation concerns for local endemic Sorbus, research-based recommendations are made to provide a foundation for conservation strategies that aim to protect rare, endemic Sorbus taxa, Sorbus taxonomic complexity and the evolutionary processes that drive Sorbus diversification in the Avon Gorge.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available