Selective factors in the evolution of multicellularity in choanoflagellates

Choanoflagellates, unicellular eukaryotes that can form multicellular colonies by cell division and that share a common ancestor with animals, are used as a model system to study functional consequences of being unicellular versus colonial. This review examines performance differences between unicellular and multicellular choanoflagellates in swimming, feeding, and avoiding predation, to provide insights about possible selective advantages of being multicellular for the protozoan ancestors of animals. Each choanoflagellate cell propels water by beating a single flagellum and captures bacterial prey on a collar of microvilli around the flagellum. Formation of multicellular colonies does not improve the swimming performance, but the flux of prey-bearing water to the collars of some of the cells in colonies of certain configurations can be greater than for single cells. Colony geometry appears to affect whether cells in colonies catch more prey per cell per time than do unicellular choanoflagellates. Although multicellular choanoflagellates show chemokinetic behavior in response to oxygen, only the unicellular dispersal stage (fast swimmers without collars) use pH signals to aggregate in locations where bacterial prey might be abundant. Colonies produce larger hydrodynamic signals than do single cells, and raptorial protozoan predators capture colonies while ignoring single cells. In contrast, ciliate predators entrain both single cells and colonies in their feeding currents, but reject larger colonies, whereas passive heliozoan predators show no preference. Thus, the ability of choanoflagellate cells to differentiate into different morphotypes, including multicellular forms, in response to variable aquatic environments might have provided a selective advantage to the ancestors of animals.

A multigene timescale and diversification dynamics of Ciliophora evolution

Ciliophora is one of the most diverse lineages of unicellular eukaryotes. Nevertheless, a robust timescale including all main lineages and employing properly identified ciliate fossils as primary calibrations is lacking. Here, we inferred a time-calibrated multigene phylogeny of Ciliophora evolution, and we used this timetree to investigate the rates and patterns of lineage diversification through time. We implemented a two-step analytical approach that favored both gene and taxon sampling, reducing the uncertainty of time estimates and yielding narrower credibility intervals on the ribosomal-derived chronogram. We estimate the origin of Ciliophora at 1143 Ma, which is substantially younger than previously proposed ages, and the huge diversity explosion occurred during the Paleozoic. Among the current groups recognized as classes, Spirotrichea diverged earlier, its origin was dated at ca. 850 Ma, and Protocruziea was the younger class, with crown age estimated at 56 Ma. Macroevolutionary analysis detected a significant rate shift in diversification dynamics in the spirotrichean clade Hypotrichia + Oligotrichia + Choreotrichia, which had accelerated speciation rate ca. 570 Ma, during the Ediacaran-Cambrian transition. For all crown lineages investigated, speciation rates declined through time, whereas extinction rates remained low and relatively constant throughout the evolutionary history of ciliates.