Ovule and seed production patterns in relation to flower size and floral symmetry

Animal pollinated flowers show two types of floral symmetry, actinomorphy (radial symmetry) with several planes of symmetry or zygomorphy (bilateral symmetry) with just one symmetry plane. Although these floral types evolved separately, to a large degree they show functionally similar pollination patterns. However, zygomorphic flowers are often visited by a narrower range of pollinator groups. This may be because zygomorphic flowers often have complex structures to which only certain pollinators can respond. Actinomorphic flowers, on the other hand, have more simple structures that many pollinator groups can visit. This has resulted in zygomorphic species showing less variation in flower size than their actinomorphic counterparts. Yet it remains unclear whether this difference has also brought about an evolutionary change in the number and size of ovules and seeds these species produce.

A zygmomorphic flower used in the study.
A zygomorphic flower of Impatiens noli-tangere, one of the studied species. Image credit: Tsungam / Wikipedia.

In a recent study published in AoBP, Mochizuki et al. investigated whether ovule and seed production is affected by floral symmetry in 49 naturally growing plant species of Japan. They found that variations in mean ovule sizes and ovule numbers of flowers increased with increase in variations of flower sizes in the actinomorphic flower species but not in the zygomorphic flower species. Thus, the degrees in variations in ovule size and number of flowers were influenced by the interaction of floral symmetry type and flower size variation, suggesting that floral symmetry has also brought an evolutionary association in ovule production by flowers. The authors state that such differences have not been observed previously, and this finding may bring new insight to studies of the evolution of ovule and seed production in relation to floral symmetry.