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Genetic structure and mating patterns of diploid and polyploid Easter daisies (Townsendia hookeri, asteraceae) Thompson, Stacey Lee

Abstract

Reproduction in the Rocky Mountain genus Townsendia involves a complex interplay of polyploidy and apomixis. Molecular markers were used to assess genetic structure in relation to patterns of mating mode and ploidy at four hierarchical levels: within the genus, within T. hookeri, within populations of this species, and among progeny from these populations. Phylogenetic analyses of rDNA repeats indicated that polyploid apomixis has evolved multiple times within the genus. Pollen studies and analyses of cpDNA demonstrated that sexual diploids of T. hookeri are found in both northern and southern unglaciated regions, polyploid apomicts have evolved at least once in the north and thrice in the south, and these polyploid apomicts have colonized the post-glacial landscape from two refugia, suggesting that glaciation and not latitude influenced the distribution of apomicts. Pollen studies, flow cytometry, and multilocus tests on AFLP marker genotypes from four Yukon standing populations of T. hookeri indicated sexuality in one male-fertile diploid population, clonality in two male-sterile tetraploid populations, and a combination of sexual and clonal reproduction in one male-sterile polyploid population. This latter population of mixed mating mode included triploids and tetraploids and showed that evidence of cryptic sex may linger in the genomes within a morphologically asexual population. Finally, a new method for mating system analysis, which jointly estimates the rates of outcrossing, selfing, automixis and apomixis, was developed and applied to dominant AFLP marker genotypes from progeny whose mothers arose from three Yukon populations of T. hookeri, one consisting of male-fertile diploids, the other two of male-sterile tetraploids. Despite indications of sexuality in some standing populations, progeny analyses revealed that apomixis is the predominant mating mode in all populations. Levels of outcrossing were moderate in the diploid population and very low in the tetraploids. Selfing/automixis was absent in the diploids and moderate in tetraploids. These findings suggest that the correlation between ploidy and apomixis is not strict when observed on a fine scale, that polyploidy alone does not induce apomixis, and perhaps it is asexuality that selects for polyploidy within this system.

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