Science for all: Most flowers usually pick one father and stick with him

Conflicts lurk inside every flower with multiple seeds. The embryos jostle for food, the maternal and paternal genomes bargain over control, and pollen grains compete to be fertilised. Scientists have therefore wondered whether natural selection encourages one-parent broods that keep such quarrels to a minimum and, in so doing, make plant flowers unexpectedly monogamous, much like many animal families.

Scientists have also long believed that most large fruits mix the genes of several parents, a view already under fire from smaller case-studies that hinted at widespread single paternity.

In challenging that orthodoxy, a new study — including scientists from the Ashoka Trust for Research in Ecology and the Environment, Bengaluru and the Nature Conservancy and the Swaniti Initiative in New Delhi — provides a unifying picture of how kin conflict, pollinator behaviour, and flower design shape reproduction across the plant kingdom.

The scientists searched the research literature, focusing on papers published between 1984 and 2024 and selected 102 candidate studies. They finally shortlisted 63 species representing many flowering-plant families. For each of these species, they tracked down genetic studies that compared the DNA fingerprints of sibling seeds and converted the resulting “correlated paternity” values into a number of pollen donors per fruit.

Upon analysis, the scientists found that the headline numbers overturned the textbook story. Among the 63 species, 15 (or 24%) had strictly single paternity and another 18 (28%) averaged fewer than 1.5 fathers per fruit. Taken together, 52% of the sample displayed de facto monogamy at the flower level. The remaining 48% did allow multiple fathers yet even here most fruits harboured only two or three donors, a far cry from the genetic free-for-all that scientists once assumed was the case.

The patterns became clearer when the scientists split the species by mating system. In plants that couldn’t be mated with others of the same species, i.e. which must receive pollen from other individuals, 59% of fruits were sired by a single donor. In self-compatible plants on the other hand fruits had a single donor in only 41% of instances. Statistical tests also confirmed that the self-incompatible group consistently hosted fewer fathers per fruit.

The seed number also mattered less than expected. Although very large fruits sometimes had several donors, no overall rise in pollen parents accompanied an increase from tens to hundreds of seeds. Indeed, across all species, the link between seed count and paternity vanished after the scientists controlled for evolutionary relatedness.

The team also found that the breeding system, not the ancestry, best predicted paternity patterns, implying that kin conflict and pollinator precision evolve quickly when selection demands it. As a result, the plant world may resemble animals more closely than once thought: single fathers dominate, with true genetic polyandry the exception rather than the rule.

In their paper, published in Proceedings of the National Academy of Sciences on August 5, the scientists have urged more fieldwork, especially measurements of how many individual pollinators contribute to a single pollen load, to reveal exactly when and how plants shift from monogamy to polyandry. But for now their message is clear: most flowers, even crowded ones, usually pick one father and stick with him.

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