THE GENETICS AND ECOLOGY OF SEED SIZE VARIATION IN A BIENNIAL PLANT, HYDROPHYLLUM-APPENDICULATUM (HYDROPHYLLACEAE)

The goal of this study was to elucidate the sources of seed size varia tion in Hydrophyllum appendiculatum, an outcrossing, biennial plant. T he genetic basis of seed size variation was examined with a diallel br eeding design. The analysis did not reveal any evidence for additive g enetic variance, suggesting that seed size could not evolve in respons e to natural selection. A series of greenhouse experiments was conduct ed to determine the sensitivity of seed weight to a number of ecologic al variables. Seed weight was affected by inbreeding depression: seeds produced by self-pollinations were significantly lighter than outcros sed seeds. Maternal plants did not differentially provision seeds that were the result of crosses between subpopulations (separated by 300 m ) or between populations (separated by 1.7 km). Mean seed size was ind ependent of the number of outcrossed pollen donors (one vs. many) that sired seeds on an inflorescence; however, the variance was greater on inflorescences pollinated by multiple donors. Direct manipulations of the abiotic environment showed that seed size was greater on plants g rowing under full sunlight compared to shaded plants. Seed size was un affected by soil type, fertilizer addition, or defoliation. Finally, I determined the effect of varying pollination intensity at the level o f a single inflorescence, and at the whole plant level. Seed weight wa s greatest on plants that had only 1 and 5 inflorescences pollinated, and least on those that had 10 and 20 pollinated. At the inflorescence level, seed weights were greatest on those where all flowers were pol linated, compared to inflorescences where only half of the flowers wer e pollinated. Perhaps the greatest contributor to variance in seed siz e in this species was the temporal decline within plants through the f lowering season. These results indicate that maternal plants are. not capable of producing uniform seed crops. Rather, the final distributio n of seed size produced by plants within a population will necessarily vary and be the result of pollination effects, heterogeneity in the a biotic environment, and developmental constraints.