Endosymbiont change as a key innovation in the adaptive radiation of Soritida (Foraminifera)

A phylogeny of 54 Recent and fossil species of Soritacea (Foraminifera) was used to test the hypothesis that endosymbiosis has driven the evolution of the clade. Endosymbiosis with photosynthetic eukaryotes is the plesiomorph ic condition for the entire clade Soritacea. Living species dwell in tropic al-subtropical, shallow-water habitats and are characterized by the possess ion of rhodophyte, chlorophyte, or dinophyte photosymbionts. Two distinct c hanges in endosymbiont type are recognized when endosymbiont type is mapped in the cladogram of Soritacea: (1) a change from rhodophyte to chlorophyte endosymbionts occurred in the stem lineage of the least inclusive clade co ntaining New clade B, Orbiculinida. and Soritida; and (2) a change from chl orophyte to dinophyte endosymbionts occurred in the stem lineage of the lea st inclusive clade containing New clade G. New clade H, New clade I, Sorite s, Amphisorus, and Orbitolites. When habitat and ontogeny are optimized on the cladogram of Soritida, the acquisition of dinophyte endosymbionts appea rs as a key innovation that facilitated a switch in habitat from free-livin g to attached living on non-phytal and phytal substrata. A subsequent chang e in the attached habitat from nonphytal to predominantly phytal (seagrasse s and macroalgae) substrata is accompanied by a peramorphic trend in the me galospheric tests. The diversification (adaptive radiation) of the crown So ritida subclade resulted from the interplay between the acquisition of a ke y innovation (dinophyte endosymbionts) and the subsequent change in the eco logy of the group (radiation to phytal substrates).