Pollen-Ovule Ratios and Breeding System Evolution in Solanum Section Basarthrum (Solanaceae)

Abstract

Pollen-ovule ratio, pollen grains per flower, pollen grain volume, pollen biomass, ovules per ovary, seeds per fruit, seed volume, seed biomass, and corolla size were studied to reveal patterns of variation within a monophyletic group. Solanum sect. Basarthrum includes 22 neotropical species of two clades, one having small, few-seeded red fruits, and the other having larger, many-seeded green fruits. The former includes self-incompatible species and a dioecious species, while the latter includes self-incompatible species, self-compatible (and autogamous) species, and a domesticate (the “pepino”). Although the pollen quantity and ovule quantity of the self-incompatible species are significantly higher in the green-fruited subgroup than in the red-fruited subgroup, the pollen-ovule ratios of the self-incompatible species are not significantly different between the two subgroups, suggesting parallel evolution of the pollen-ovule ratio. Furthermore, the pollen-ovule ratio tracks the breeding system: self-incompatible species have significantly higher pollen-ovule ratios than self-compatible species, resulting both from fewer pollen grains and more ovules of the latter. The pollen-ovule ratio of the dioecious species is among the highest of all, resulting only from fewer ovules. The pepino is self-compatible but has a pollen-ovule ratio like its wild self-incompatible progenitors and shows wide variation in seed production and pollen quality, but not in pollen quantity. Among all species, pollen size and quantity are inversely proportional, as are seed size and quantity.