Life History Variation in Mosses: Water Relations, Size and Phylogeny

Abstract

We evaluate the influences of gametophyte size, taxonomic affiliation (as an indicator of phylogenetic relationship), and water relations on variation and covariation of five life history traits among 335 moss species representing the orders Funariales, Polytrichales, and Pottiales. Size effects, though statistically significant, account for a relatively small proportion of the variation in most life history traits. Canonical Correlation Analyses, conducted across all species as well as within orders and larger families, reveal significant relationships between patterns of covariation in life history traits and variation in morpho-anatomical traits associated with water relations. Water relations account for 40-50% of life history variation depending on taxonomic group, but patterns of relationships between the two sets of variables are broadly similar, irrespective of the taxonomic group in which they were examined. The first water relations variate is a gradient describing water uptake/retention capacity. The life history variate associated with this is an axis that arranges species from those that are monoicous, short-lived and produce few large spores to those having the opposite suite of traits. The second water relations variate represents an endo-ectohydric gradient, which correlates with a life history variate describing differential investment in spores as a function of life expectancy. We used Nested Analyses of Variance and Covariance to partition variance in the life history traits into taxonomic components and evaluate whether size and water relations differences among particular groups mediate apparent taxonomic effects. Generic influences on all traits are strong, and independent of size and water relations effects ; together, genus and water relations account for >50% of the variation in each of the life history traits considered. Apparent ordinal effects largely disappear after adjustment for size and water relations, while substantial family level effects exist for spore number and spore size. Variation among species within genera is modest. These results suggest that most of the diversification in moss life histories occurred at the level of genus and that life histories have been relatively conserved in the process of subsequent cladogenesis. The persistent occurrence of water relations effects at several taxonomic levels, and the relatively large proportion of variance in life history traits, independent of taxonomic affiliation, for which these effects account, may imply adaptive coevolution of water relations and life history. These ideas can best be tested in an explicitly phylogenetic framework.