Many flowers have a radial design that looks similar no matter what angle you approach them from. But others have bilateral symmetry. They’re symmetrical, but with distinct left and right halves. Why should this be? Botanists have thought that it is a way of selecting specific pollinators, but a database study by Nicolay da Cunha and Marcelo Aizen indicates that it may have originated as a way of preventing self-fertilisation. Their results, published in Flora, have implications for understanding plant-pollinator coevolution.
You can find symmetry in nature in many different forms. Two types of symmetry in flowers are radial (actinomorphic) and bilateral (zygomorphic) symmetry. When studying flowers, researchers have found that zygomorphy has evolved repeatedly and independently in various orders and families. Why?
One idea is that flower symmetry aids animal perception, but the different symmetries mean that petals provide different information. Pollinators can process information from just one petal of a radially-shaped flower because they’re all much of a muchness. In contrast, a zygomorphic flower requires the pollinator to process information from at least one-half of the flower. This difficulty in recognition is important because it allows for a more precise coupling between the flower and the pollinator. Not all pollinators can be bothered spending the extra effort to learn these flowers, so bilateral flowers could be expected to attract more specialist pollinators.
Looking through previously published reports, da Cunha & Aizen find that bilateral flowers are indeed more specialised, with fewer species coming to visit. They can also have more specialised morphology so that not only do they get pollen onto a pollinator’s body but can target where on the body it hits. This way, they can reduce the risk of picking up pollen that they cannot use from other flowers. For this to be effective, though, the anthers and stigma should become active at different times (known as dichogamy) to avoid self-interference in sexual functions and reduce self-pollen transfer.
Botanists have conducted studies to explore whether conspecific pollen receipt differs between species with zygomorphic and actinomorphic flowers and to determine if self-compatibility is more often associated with flower zygomorphy. The results of these studies suggest that flower zygomorphy might be an adaptation with functional consequences to avoid self-fertilisation, which can increase female plant fitness.
Recently, a large dataset on pollen production and ovule number per flower for 1,392 animal-pollinated species of angiosperms was published by Cunha et al. (2022). It includes information on growth form (herbaceous versus woody) and flower size (largest linear dimension) for practically all species and on compatibility system (self-compatible versus self-incompatible) for more than half of species. To further analyse this dataset, they looked for articles reporting data on stigmatic pollen receipt, the number of conspecific pollen grains on the stigma.
To test the effect of flower symmetry (actinomorphic versus zygomorphic) on pollen production, ovule number, and stigmatic pollen receipt, da Cunha & Aizen used a phylogenetic generalised linear mixed model framework (PGLMM). They also tested for a differential incidence of self-compatibility among species with zygomorphic flowers using a PGLMM model.
da Cunha & Aizen argue their analysis reveals that the evolution of a bilateral flower morphology is likely intrinsically linked to the evolution of temporal separation between anther dehiscence and stigma receptivity. This temporal separation limits self-interference between the flower’s male and female functions. Phylogenetic analysis of a database that included 1458 species of asterid angiosperms found evidence of correlated evolution between protandry, a switch between male and female forms, and flower zygomorphy. If this is the case, then the move to bilateral flowers starts with avoiding self-pollination, and the specialisation of pollinators develops from that.
The evidence we gathered so far indicates that the evolution of zygomorphy allowed the relaxation and eventual disassemble of redundant self-avoidance mechanisms like self-incompatibility. However, in this case zygomorphy cannot be viewed as an isolated trait because selfing-avoidance will require a tighter integration between flower morphology and the staggered development of sexual organs, not only within flowers but also among flowers within inflorescences, and even among inflorescences within the same plant (Armbruster, 2022; Harder et al., 2019). In any event, our results and perspectives have allowed us to go deeper into the understanding of intriguing associations between reproductive traits like the one reported here between zygomorphy and self-compatibility.da Cunha & Aizen (2023)
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da Cunha, N.L. and Aizen, M.A. (2023) “Reduced pollination in bilateral flowers could reflect selfing avoidance,” Flora, (152220), p. 152220. Available at: https://doi.org/10.1016/j.flora.2023.152220.