Nadia Talent: PhD Thesis abstract

Talent, Nadia. Gametophytic apomixis, hybridization, and polyploidy in Crataegus (Rosaceae), PhD thesis, University of Toronto, Department of Botany, October 2006. (Supervisor: Timothy A. Dickinson.)

Pseudogamous gametophytic apomixis in flowering plants can be a flexible type of asexual reproduction because pollination is required. Ploidy-level changes are possible if a meiotically reduced egg cell develops by parthenogenesis or if a meiotically unreduced egg cell is fertilized. In North American Crataegus, pseudogamous gametophytic apomixis exists among polyploids in several taxonomic sections of the genus, and many hybrids have been identified. Polyploidization and selection for apomixis have been attributed to diploid--diploid hybrids, but evidence for that evolutionary process is slight. Ploidy-level increases and hybridization may instead result from apomixis, which possibly occurs occasionally in diploids. This thesis explores the taxonomic distribution of ploidy levels in Crataegus and how the ploidy levels of seeds relate to the breeding systems of the parent plants.

Quantification of nuclear DNA amounts by flow cytometry revealed the ploidy levels of various tissues. The ploidy levels of the embryo and endosperm in seeds revealed whether sexual or apomictic reproduction and fertilization occurred. Inter-ploidy pollinations between different taxonomic series were compared with intra-taxon and open pollinations.

Some post-pollination crossing barriers apparently exist in the genus. Triploids formed from diploid mothers in some diploid--tetraploid inter-serial pollinations, and tetraploids formed from some triploid--diploid inter-serial pollinations. Tetraploid--diploid pollinations produced hybrids only if the mother plant was a largely sexual tetraploid. Fertilization of unreduced egg cells was also indicated by pentaploid and hexaploid embryos, and rare pentaploids and hexaploids occur as adults. A largely sexual tetraploid also produced diploids when pollinated from a diploid species, consistent with parthenogenesis in sexual embryo sacs.

These results greatly expand what is known about the formation of endosperm in Rosaceae, and the mechanism differs from what is thought to occur in related genera. Triploid hybrids may be a vital, although sometimes ephemeral, factor in maintaining gene flow from sexual diploids to apomictic tetraploids. Diploid--triploid--tetraploid--diploid gene flow appears to be possible, with pentaploid and hexaploid derivatives. One exceptional seed appears to indicate rare apomixis in a diploid, and a hypothetical model is proposed to explain the greater success of apomixis in polyploids of Rosaceae than in diploids.


Last modified 17 April 2014.