NSF Proposal - 8. Evolutionary Implications

The efforts described above are expected to yield a well-supported phylogeny of green plants. This will be used, alone and in combination with other phylogenetic information, to make a variety of evolutionary inferences. Specifically we will focus on the evolution of a set of morphological characters, on patterns of molecular evolution, and on rates of diversification. Meaningful answers to these questions require large taxon sampling. Tree size is a critical factor with respect to the ability to distinguish between competing evolutionary models, such as symmetrical versus asymmetrical probabilities of character change (e.g., [138, 139]) and correlated character evolution (e.g., [140]). As a result of the concatenation analyses discussed in the previous section, we anticipate the assembly of phylogenetic hypotheses large and well-supported enough to ensure coverage of the groups that are necessary to address specific evolutionary hypothesis, and to allow statistically meaningful comparative analyses.

Our studies of character evolution will focus initially on issues of broad evolutionary significance from the standpoint of the entire Tree of Life. Specifically, basal green plants will allow analyses of (1) the origins of multicellularity, (2) transitions to life on land, and (3) the evolution of an exceptionally wide variety of life cycles. (1) Multicellularity appears to have evolved repeatedly within green plants. In the streptophyte clade, multicellularity preceded the evolution of embryophyte land plants. In chlorophytes there have been multiple paths to multicellularity, including colonial forms in the volvocine line, multiple origins of filamentous and more complex parenchymatous plant bodies in Chlorophyceae and Ulvophyceae, and siphonous forms culminating in coenocytic thalli within Ulvophyceae. (2) Transitions to life on land also occurred repeatedly early in green plant evolution. The embrophytes represent one such occurrence, but multiple independent events are apparent within Trebouxiophyceae (see Chapman et al., 1998), often entailing symbiotic relationships with fungi and animals. (3) Regarding life cycle evolution, great interest has focused on the transitions that preceded embryophyte evolution, and the subsequent origination of alternation of gametophyte and sporophyte generations. Of equal evolutionary interest are many apparent shifts within chlorophytes, including the evolution of animal-like life cycles (diploid dominance, products of meiosis functioning as gametes) in Codium and other Ulvophyceae.

In each of these cases, and others related to ultrastructural features and genome evolution, we will infer ancestral states and evolutionary sequences using parsimony and maximum likelihood approaches (see [141, 142, 138, 139, 143-145] To test for the correlated evolution of characters we will employ a battery of comparative techniques, again using parsimony, likelihood, and Bayesian methods (e.g., [146- 150]), and performing relevant sensitivity analyses (e.g., [151, 152]).

Finally, we will focus on issues concerning the tempo of green plant evolution. The absolute time of key divergences will be inferred using molecular data, calibrated by the placement of fossils (see letter from A. Knoll). Methods that variously relax the molecular clock assumption will be used, including penalized likelihood and Bayesian methods, in which fossil evidence places minimum and maximum bounds on the estimates [153-160]. We will also use a variety of methods to assess if and when shifts in diversification rate occurred in basal green phylogeny (reviewed in [161-163]), and explore correlations between diversification shifts, character changes, and rates of molecular evolution (e.g., [164]).

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