Abstract
Summary Seed size shapes plant evolution and ecosystems, and may be driven by plant size and architecture, dispersers, habitat and insularity. How these factors influence the evolution of giant seeds is unclear, as are the rate of evolution and the biogeographical consequences of giant seeds.We generated DNA and seed size data for the palm tribe Borasseae (Arecaceae) and its relatives, which show a wide diversity in seed size and include the double coconut (Lodoicea maldivica), the largest seed in the world. We inferred their phylogeny, dispersal history and rates of change in seed size, and evaluated the possible influence of plant size, inflorescence branching, habitat and insularity on these changes.Large seeds were involved in 10 oceanic dispersals. Following theoretical predictions, we found that: taller plants with fewer‐branched inflorescences produced larger seeds; seed size tended to evolve faster on islands (except Madagascar); and seeds of shade‐loving Borasseae tended to be larger.Plant size and inflorescence branching may constrain seed size in Borasseae and their relatives. The possible roles of insularity, habitat and dispersers are difficult to disentangle. Evolutionary contingencies better explain the gigantism of the double coconut than unusually high rates of seed size increase.
Highlights
Seed size is a major determinant of seed dispersal and seedling establishment (Moles, 2018)
Seven DNA loci were chosen for phylogenetic inference from among regions that had already proven to be informative for palms (Baker et al, 2009)
Borassus aethiopum was recovered as paraphyletic, with accessions from Burkina Faso, Cameroon, Kenya and Madagascar intermixed with B. madagascariensis and B. akeassii
Summary
Seed size is a major determinant of seed dispersal and seedling establishment (Moles, 2018). Knowing the rates and drivers of seed size evolution is essential to improve predictions of species’ responses to changing ecological conditions. Seed size varies by orders of magnitude in almost all angiosperm orders (Linkies et al, 2010). This lability has been related to many factors, such as disperser availability, plant size and habitat shadiness (Willson & Traveset, 2010; Leishman et al, 2000; Moles et al, 2006). The challenge of disentangling these factors impedes understanding of seed size variation within lineages and is an obstacle to a predictive understanding of seed size evolution. The evolution of extreme seed sizes, which define the boundaries of the global seed size distribution, remains unclear
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