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The evolution of reproductive isolation between diploid and tetraploid Chamerion angustifolium (Onagraceae)

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Title: The evolution of reproductive isolation between diploid and tetraploid Chamerion angustifolium (Onagraceae)
Author: Sabara, Holly A.
Department: Department of Integrative Biology
Advisor: Husband, Brian C.
Abstract: Polyploidy is considered an instantaneous, sympatric speciation mechanism conferring immediate post-zygotic isolation between the polyploid and its progenitor. However, theoretical models show that pre-zygotic barriers are necessary for a rare polyploid to overcome the initial minority mating disadvantage and establish within the population. Few studies have examined the role of pre-versus post-zygotic barriers in facilitating reproductive isolation, or the role of natural selection versus genome duplication in strengthening these barriers. In this thesis, I examine the evolution of reproductive isolation in 'Chamerion angustifolium' through an analysis of reproductive barriers, a study of the effects of cytotype interactions on selection of reproductive traits, and a comparison of reproductive isolation in synthesized versus natural polyploids. Pre-zygotic barriers (geographic separation, flowering asynchrony and pollen deposition patterns) resulted in cumulative reproductive isolation values of 78% for diploids and 89% for tetraploids. Combined with post-zygotic barriers reproductive isolation between the cytotypes was essentially complete (99.4% for 2'x' and 98.6% for 4x). In contrast, triploids occur in natural mixed populations, although their fertility is low and their frequency is much lower than would be expected if mating was at random. To determine if cytotype mediated selection reinforced reproductive isolation between ploidies, I examined the strength and direction of phenotypic selection on reproductive traits in diploids and tetraploids in experimentally generated allopatric and sympatric populations. Although significant directional selection for increased display size and flowering duration was observed for all cytotypes, and in sympatric and allopatric populations, I did not observe selection for increased trait divergence when the two cytotypes co-occurred. These results indicate that selective mechanisms, other than to avoid hybridization, are influencing the reproductive phenotypes of both diploid and tetraploid plants. Finally, synthesized polyploids were used to quantify the immediate effects of genome duplication on the reproductive phenotype and ecology of polyploids. The phenotype of neopolyploids differed from that of diploids and established tetraploids for the 12 traits measured. However, neopolyploids exhibited strong pre- and post-zygotic reproductive isolation from diploids, similar in magnitude to that of established tetraploids. This research provides empirical support for the statement that polyploidy is an instantaneous speciation mechanism.
Date: 2008
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