PARENT SESSION
Oral Session # 46: Evolutionary Ecology I: Plants.
Presiding: P Minchin
Wednesday, August 6. 8:00 AM to 11:30 AM, SITCC Meeting Room 201.

Factors affecting the mating system of an inbreeding conifer, Thuja plicata (Cupressaceae).

O'Connell, Lisa^*,1, Ritland, Kermit¹, Russell, John², ¹ University of British Columbia, Vancouver, British Columbia² British Columbia Ministry of Forest, Mesachie Lake, British Columbia

ABSTRACT- Conifers are predominantly outcrossing, but in a review of the literature 25% of 52 species showed selfing rates above 20%. For a conifer, western redcedar (Thuja plicata) shows high self-fertility, and population outcrossing rates estimated using one isozyme locus varied significantly among six natural populations (weighted mean = 71.5%). We used microsatellite assays of bulked DNA to obtain fine-scale estimates of outcrossing and study the variation in selfing within and among trees. Outcrossing rates did not vary significantly among crown positions within trees, but decreased with tree height in all four populations studied. Taller trees probably receive higher proportions of self-pollen because they contribute a larger proportion of self-pollen to the surrounding pollen cloud. The occurrence of multiple embryos within the same ovule (polyembryony) is a mechanism in conifers that can potentially increase the proportion viable and/or outcrossed seeds. Using controlled crosses, we found evidence that with high self-pollination rates, embryo competition can decrease the proportion of selfed seeds. However, pre-pollination mechanisms and the degree of self-fertility are more important than embryo competition in determining selfing rates.

Key words: mating system, inbreeding, conifer, Thuja plicata