Ovule abortion in white clover (Trifolium repens L.)

Plants of white clover ( Trifolium repens L.) cv. Olwen were grown in an open glasshouse maintained at a mean temperature of 20^oC and ovule growth and seed production measured. Differences in the rate of growth of ovules within ovaries were observed as early as 2 days after pollination. Ovules reached a maximum size after 8 days with the smallest only half the size of the largest. After 8 days, the smallest ovules became flaccid and shrivelled. Ovule position within the ovary had little effect on the frequency of seed set and although there was an apparently higher probability that central ovules produced a seed than those nearer the peduncle or style this was not statistically significant. Inflorescence position and floret position on the inflorescence had a significant effect on the number of seeds per floret and seed weight; the first formed inflorescences and the first florets to be pollinated on each inflorescence had more seeds per floret and heavier seeds and fewer florets with no seed than later pollinated florets. There were also differences between florets within the same whorl. The role of a number of factors which may influence floret site utilisation are discussed.     

Centromere‐associated meiotic drive and female fitness variation in Mimulus

Female meiotic drive, in which chromosomal variants preferentially segregate to the egg pole during asymmetric female meiosis, is a theoretically pervasive but still mysterious form of selfish evolution. Like other selfish genetic elements, driving chromosomes may be maintained as balanced polymorphisms by pleiotropic or linked fitness costs. A centromere‐associated driver (D) with a ～58:42 female‐specific transmission advantage occurs at intermediate frequency (32–40%) in the Iron Mountain population of the yellow monkeyflower, Mimulus guttatus. Previously determined male fertility costs are sufficient to prevent the fixation of D, but predict a higher equilibrium frequency. To better understand the dynamics and effects of D, we developed a new population genetic model and measured genotype‐specific lifetime female fitness in the wild. In three of four years, and across all years, D imposed significant recessive seedset costs, most likely due to hitchhiking by deleterious mutations. With both male and female costs as measured, and 58:42 drive, our model predicts an equilibrium frequency of D (38%) very close to the observed value. Thus, D represents a rare selfish genetic element whose local population genetic dynamics have been fully parameterized, and the observation of equilibrium sets the stage for investigations of coevolution with suppressors.     

A comparison of ovule abortion in self-incompatible and self-fertile lines of white clover (Trifolium repens L.)

Plants of white clover (Trifolium repens L.) cv. Olwen and three genotypes of each of three self-fertile inbred lines of white clover were grown from seed in a bee-proof glasshouse. When in flower, a series of crosses within and between the self-incompatible (SI) and self-fertile (SF) lines was made and ovule growth and seed production measured 7 and 28 days after pollination (DAP) respectively. At harvest, the SF plants had a lower proportion of ovules aborting and more seeds per floret than the SI plants, though the mean seed weight of the SI plants was greater. Ovule abortion was observed in both the SF and SI plants though the number of aborted ovules was greater in the SI plants. The stage when abortion occurred also differed. All of the ovule abortion in the SI plants occurred within a 7-day period after pollination. In the SF plants, some abortion was observed in this period but also between 7 and 28 DAP, when the seed was harvested. This suggested that abortion, up to 7 DAP, is due primarily to genetic factors whilst subsequent abortion, between 7 and 28 DAP, is due to non-genetic factors.