A survey of seed and seedling characters in 1744 Australian dicotyledon species: cross-species trait correlations and correlated trait-shifts within evolutionary lineages

Abstract

Abstract Seedling traits have been described across 1744 species of Australian plants. Six traits were coded as binary alternatives: (1) phanerocotyly vs cryptocotyly; (2) first leaf scale-like vs leaf-like; (3) first leaf single vs paired; (4) cotyledons hairy vs. glabrous; (5) hypocotyl hairy vs glabrous, and (6) embryo green vs non-green at maturity. Seed volume was calculated from measurements of seed dimensions. Three approaches were used in analysing the data: (1) the taxonomic distribution of binary character states was described; (2) the strength of bivariate character associations was quantified at species level («cross-species» correlations and regressions); (3) the data were arrayed on a phylogenetic tree in order to analyse by «phylogenetic regression» for correlated evolutionary shifts in trait pairs. All the traits appeared evolutionarily malleable. For example, while cryptocotyly was the minority condition (22% of species), it occurred in many different taxa (124 genera, 40 families, 24 orders) with high levels of polymorphy (16%, 25% and 42% for genera, families and orders). Similarly, the less common attribute states for first leaf type, hypocotyl texture, cotyledon texture and embryo colour occurred in species from right across the dicotyledon phylogeny, as did independent evolutionary divergences in each of these characters. These patterns indicate that debate over which conditions are primitive and which are advanced will not have any general answer, only an answer for a specific branch-step in the phylogenetic tree. In nearly all cases, correlated-divergence analyses showed the same patterns as cross-species analyses. The strongest associations were between seed volume and cryptocotyly, seed volume and scale-like first leaf, and seed volume and presence of green embryo. In addition, cryptocotyly and scale-like first leaf, and cotyledon and hypocotyl type, were strongly associated in both correlated-divergence and cross-species analyses. Interpretation of results was mostly presented with respect to seed size, a trait which we consider to be pivotal in a species» seedling establishment strategy. All possible pairwise combinations of binary seedling traits were found in our study species. Taken together, the various lines of evidence presented here suggest that the traits have assorted more or less independently of each other and provide no evidence of functional groups based on these attributes. Thus, the several existing seedling typologies should be regarded as classifications of convenience rather than as reflecting fundamental types.