The phylogeny of recent and fossil Soritacea (Foraminifera): The evolutionary significance of photosymbiosis; and a revision of the foraminiferal life cycle

Recent molecular phylogenies have shown Foraminifera to comprise a distinct clade of basal mitochondrial eukaryotes that diverged relatively early in the evolution of eukaryotes. The significance of the haplo-diploid foraminiferal life cycle is reinterpreted within this phylogenetic context using information compiled from the published literature, field and laboratory observations. A new concept of the foraminiferal life cycle is presented that facilitates comparison to other groups that have independently evolved similar life histories.; The Soritacea comprise a monophyletic clade within the Foraminifera, Recent and fossil species of which inhabit shallow-water, tropical to subtropical reef-associated environments. As a group, living Soritaceans are characterized by their association with a diverse array of photosymbionts (rhodophytes, chlorophytes, dinophytes). A phylogenetic (cladistic) analysis was performed on 54 species of fossil and Recent Soritacea using a data set of 195 binary characters derived exclusively from morphological features of the test, such as gross test morphology, surface ultrastructure and internal partitions. Characters such as endosymbiont type, ecological habitat and ontogenetic transformations were not incorporated into the data matrix. Seven outgroup taxa were included in the analysis and characters were polarized using the outgroup method. An heuristic search strategy conducted in PAUP yielded a single most parsimonious tree of 450 steps, with a CI = 0.433 and a RI = 0.851.; Eleven new species (three fossil and eight Recent) and eleven new clades are described. All higher taxa are defined following the recommendations of the new PhyloCode (http://wwwohio.edu/phylocode) and a phylogenetic classification scheme of Soritacea is presented. Soritacea is shown to be the sister group to Nanicella, a group of fusulinacean foraminiferans. Internal structure appears to have been independently derived within several different subclades of Soritacea. The inclusion of fossil taxa in the analysis is shown to be essential for resolving relationships within the Archaiasinina and Soritida subclades.; Character optimization techniques indicate that photosymbiosis is the plesiomorphic condition for the entire clade Soritacea. Endosymbiont type appears to have changed at least twice during the evolutionary history of the clade. In the Soritida subclade, the shift from chlorophyte to dinophyte endosymbionts is accompanied by a change in habitat from free-living to attached to firm substrata. A subsequent peramorphic trend in development is correlated with a shift in habitat from attached to non-phytal to attached to phytal substrata. It is hypothesized that competition for space on rapidly growing phytal substrata, such as macroalgae and seagrasses, selected for the progressive loss of the early coiling stages and the earlier onset of annular chambers observed in the ontogeny of megalospheric tests.