DELESALLE, VERONIQUE A1* and SUSAN J MAZER2. 1Dept of Biology, Gettysburg College, Gettysburg, PA 17325; 2Dept of Ecology, Evolution and Marine Biology, University of California at Santa Barbara, Santa Barbara, CA 93106. - Comparing pollen:ovule ratios of early versus late flowers in Clarkia species with contrasting mating system.
Despite early recognition that the mating system of flowering plants
will profoundly influence the evolution of gender-related traits, few
studies have examined in detail the sex allocation patterns of
congeners with contrasting mating systems. Numerous studies have shown
that selfers produce flowers with lower pollen:ovule (P:O) ratios and
smaller petals than outcrossers. However, the variances within and
among genotypes in P:O ratio and in flower size, which should differ
between selfers and outcrossers, have rarely been documented. Our
research is employing a series of field surveys and greenhouse
experiments to address predictions made by Mazer and Delesalle (1998)
regarding these questions. In particular, we predicted that
stabilizing selection against genotypes with inefficient P:O ratios
should result in greater canalization of the P:O ratio in selfers than
in outcrossers. To test this prediction, we collected buds from one
population each of Clarkia xantiana parviflora (a selfer) and
its outcrossing sister species, C. xantiana xantiana in spring
2000. From ~50 plants/population, we collected the first flower, two
"early" buds (usually buds two and three) and two
"late" buds (buds produced just before each individual
stopped flowering). In the lab, undehisced stamens were placed in
eppendorf tube for future pollen counts, while ovaries were frozen for
future ovule counts. The number of ovules has been counted for all
these buds. Based on only one population per species, the sister taxa
differed in the number of ovules per ovary (parviflora: mean
=110.8, SE= 1.04, n = 169; xantiana: mean = 76.5, SE = 0.74, n
= 209). In both species, late flowers had fewer ovules per ovary than
early flowers, but similar reduction in ovule number (% of ovules in
late buds compared to early buds in parviflora versus
xantiana: 93% versus 91%, t = 1.034, p = 0.15, df = 86). Pollen
counts will be completed this July and data on changes in pollen
production and P:O ratios presented.
Key words: Clarkia, mating system, Onagraceae, ontogeny, pollen:ovule ratios