A Global Analysis of the Distribution Patterns of Style-Length Polymorphisms Across Angiosperms

Unraveling the evolutionary and biogeographical factors that drive the widespread occurrence of outcrossing mechanisms in plants is key to understanding their prevalence. Style-length polymorphisms (SLP) are labile outcrossing traits that have evolved repeatedly in several angiosperm lineages sharing moderately specialized pollination features, reflecting a convergent strategy to optimize outcrossing efficiency. Notably, over 50% of known genera exhibiting SLP belong to the Rubiaceae family, which, beyond harboring floral architectures that promote pollination precision, is characterized by a predominantly tropical distribution and high species diversity. This fact suggests that the evolution and distribution of SLP could be associated with specialized pollinator interactions prevalent in tropical regions (tropicality-driven hypothesis) and with high species richness, potentially boosting the chance of random appearance of SLP (diversity-driven hypothesis). We examined the biogeographical correlates of SLP across all angiosperm genera on a global scale to test the tropicality- and diversity-driven hypotheses. We conducted phylogenetic logistic regression models to assess the prevalence of SLP genera in the tropics and biodiversity hotspots (areas of exceptional species richness), accounting for phylogenetic conservatism, genus-level species richness, and evaluating the role of the Rubiaceae family. Our analyses found no support for either the tropicality- or the diversity-driven hypotheses once phylogenetic relatedness was accounted for. Despite the occurrence of SLP being significantly associated with tropical habitats, this pattern relied on the Rubiaceae family. No positive association emerged between SLP and biodiversity hotspots. Instead, phylogenetic conservatism predominantly shaped SLP distribution. Also, there was a clear positive relationship between genus-level species richness and SLP. Our findings challenge tropicality- and diversity-driven hypotheses, suggesting that SLP distribution is primarily shaped by conserved evolution. Future studies should explore further the role of SLP in species richness by explicitly assessing its potential influence on diversification patterns.