Evolution of dispersal strategies in conifers: Functional divergence and convergence in the morphology of diaspores

We provide an integrative view of the evolution of dispersal strategies in modern conifers, by characterizing and examining the phylogenetic distribution of diaspore functional morphotypes, diaspore structural compositions, seed coat modifications, and dispersal syndromes using the phylogeny of Leslie et al. (2012). We first classified diaspores into nine functional morphotypes, which represent overall dispersal strategies that encompass the multiple phases of dispersal. We mapped these morphotypes, the eight different structural compositions of diaspores, two types of seed coat modifications, and the four recognized dispersal vectors onto the phylogeny and used maximum parsimony and maximum likelihood to infer ancestral states and assess shifts in dispersal characteristics. We found that structural traits (diaspore composition and seed coat modifications) are more conserved than ecological traits (functional morphotype and dispersal vector/syndrome). Almost all diaspore functional morphotypes have multiple independent origins, with several instances of parallelism (using the same structures to generate a morphotype) within families, but generally functional convergence (using different structures to generate a morphotype) between families. Within extant conifer families, shifts in the dispersal syndrome occur most frequently with simultaneous shifts in both diaspore morphotype and composition. Shifts from winged wind-dispersed to fleshy animal-dispersed diaspores are infrequent and occur only in the direction from wind to animal dispersal. Shifts to gravity or water dispersal occur from both wind and animal dispersed diaspores, concurrent with the loss of dispersal structures from the diaspore. Within both wind and animal-dispersed syndromes, shifts between functional morphotypes represent further differentiation of overall dispersal strategies, and occur most frequently without corresponding changes in the structural composition of the diaspore. The recurrent evolution of distinct functional morphologies suggests that there are local adaptive maxima that balance tradeoffs in traits related to both transport and establishment, within developmental limitations. Overall, our results suggest that the ancestral diaspore type for all modern conifers consisted only of a seed. Conifers diversified in their dispersal strategies through seed coat modifications or by the incorporation of various parts of the seed cone into the diaspore, with the modern conifer families independently evolving their characteristic diaspore compositions. Almost all functional morphotypes were present prior to the Cenozoic in at least one lineage, with more recent shifts in morphotypes representing functional convergence or parallel evolution rather than ecological novelties.