Ovule size variations and the relative sizes of ovules to seeds

Is there selection minimizing the costs of ovule production? Such selection should lead to a smaller ovule size in relation to seed size and, at the same time, smaller variation in ovule size within plants, the latter because the minimum structures and resources for functioning of ovules should be the same among ovules. Additionally, within species, ovule size should not depend on the plant’s resource status. To confirm these predictions, Itagaki et al. examined ovule and seed production for a variety of species.

The dependence of the species mean of the CV of plant ovule sizes on the ratio of the mean species seed size/mean species ovule size.
The dependence of the species mean of the CV of plant ovule sizes on the ratio of the mean species seed size/mean species ovule size. The line shows the phylogenetic linear regression obtained by phylom in the phylom package of R. Source: Itagaki et al. 2019.

Among the 27 species studied, they found a significant negative dependence of the species mean of the coefficient of variation for plant ovule size on the ratio of the mean species seed size/mean species ovule size. Thus, the smaller the ovule size as compared with seed size, the smaller the degree of variation in ovule size. Among the 49 species studied, only two species showed significant positive dependence of mean ovule size on plant size. Although larger plants should have greater resources for ovule production, selection has not enhanced the production of large ovules in most species.

Further reading

Itagaki, T., Mochizuki, J., Aoyagi Blue, Y., Ito, M., & Sakai, S. (2019). Evolution towards minimum ovule size? Ovule size variations and the relative sizes of ovules to seeds. Annals of Botany. https://doi.org/10.1093/aob/mcz031