A PHYLOGENETIC PERSPECTIVE ON THE EVOLUTION OF TEST MORPHOLOGY IN FORAMINIFERA

Foraminifera is a clade of single-celled eukaryotes that branches within the more inclusive clade Rhizaria as the probable sister group to Polycystinea (polycystine radiolarians). Diagnostic traits of Foraminifera include: granuloreticulopodia, external tests & complex life cycles. Forams are unique among the single-celled rhizarians in that some taxa grow to unusually large cell sizes & form large, complex tests. A recent molecular phylogeny of Foraminifera is used to interpret evolutionary trends in the morphology of the foram test. Basal lineages possess single-chambered tests or are non-testate; however, the vast majority of both living & fossil forams branch within an unnamed clade of predominantly multichambered taxa that includes the subclades Tubothalamea, Xenophyophora & Globothalamea. All testate forams possess anisotropic shapes, as even spherical tests possess at least one aperture. The tests of basal single-chambered taxa grow by expansion, while the tests of more derived taxa with cylindrical or multichambered tests grow by accretion at the apertural end of the test (polarized cell growth). Evolution of multichambered tests (episodic addition of discrete chambers) is suggested to have arisen only once during the evolutionary history of the clade. In the Tubothalamea, basal lineages possess tests with an initial globular chamber followed by an undivided tube (rectilinear or coiled) while tests with discrete chambers are observed in the derived subclades Spirillinida & Miliolida. Xenophyophora, a clade of deep-sea taxa with large, agglutinated tests, possess test morphologies that include: single-chambered forms, hyperbolic shapes, tests formed of interconnected tubes & tests with discrete chambers. In the Globothalamea, basal lineages possess tests with a rectilinear arrangement of discrete chambers; tests with coiled chamber arrangements are observed in the more derived clades. At the cellular level, the large cell sizes of forams are facilitated by endopolyploidy (multiple nuclei or endoreplication in a single nucleus) & an extensive network of granulose reticulopodia. Morphogenesis of the multichambered test is ultimately controlled by the interaction of the different cytoskeletal components (e.g., actin filaments & microtubules) & the extracellular matrix (=test).