PHYLOGENETIC TREES IN COCCOLITHOPHORES: TELLTALES OF DIFFERENTIAL EVOLVABILITY AMONG CLADES

Coccolithophores, a group of tiny, planktonic, marine, unicellular calcifiers, secrete an exoskeleton (coccosphere) composed of composite, structurally complex components (coccoliths). Sustained since the Late Triassic (~221 Ma) this physiological activity has resulted in massive amounts of coccolith-rich sediments that record in great detail the phylogenetic history of these haptophytes. Although they exhibit a remarkable morphologic diversity, coccoliths belong to three basic categories which differ by the type of modified rhombohedra of which they are composed and their arrangement into “structural units”. Differences between these two associated characters support classification in ten orders rooted in the Mesozoic and extending into the Cenozoic, half of which became extinct during the last 50 Myr. Comparison between the phylogenetic histories of these orders shows little differences in their overall longevity, but strong differences in the longevity and diversity of their clades (genera, families). These differences are associated with the number and location of the structural units and the shapes of rhombohedra. For instance, the potential for diversification is much higher when structural units are in superposition rather than in concentric arrangement. The former facilitates morphologic innovation while the latter considerably restricts differentiation. As a result, the superposition of structural units leads to adaptive radiations, whereas their concentric arrangement leads to convergence. Macroevolutionary patterns in the coccolithophores are thus directly related to a few primary characters that determine phenotypic evolvability, itself a mirror of the evolvability of the genes responsible for calcification in the group. Reconstructions of coccospheres enhance the contrast between clades with high potential for innovation and clades with obligate convergence, i.e., differential evolvability between clades. Interestingly, convergence is widespread in the coccolithophores, probably because most orders diverged from an ancestral form with concentric structure, demonstrating once again the role of contingency in evolutionary history.