Clonal-aggregative multicellularity tuned by salinity in a choanoflagellate

Overview

Multicellularity in eukaryotes has evolved through distinct mechanisms: clonal development via cell division without separation, and aggregative assembly of independent cells. These mechanisms are typically considered mutually exclusive. This study demonstrates that Choanoeca flexa, a choanoflagellate, exhibits clonal-aggregative multicellularity, forming cell monolayers through simultaneous operation of both mechanisms. The research characterizes C. flexa populations inhabiting ephemeral splash pools and establishes that multicellular sheet formation represents a reversible developmental strategy responsive to environmental conditions.

Methods and approach

Field characterization of C. flexa life history was conducted in natural splash pool habitats on Curaçao, documenting transitions between unicellular and multicellular states during evaporation-refilling cycles. Genetic analysis of C. flexa strains from different splash pools was performed to assess strain-level differentiation. Kin recognition assays were conducted to examine aggregative constraints between genetically distinct strains. Morphological and behavioral characterization of cell sheet formation identified the constituent mechanisms (clonal, aggregative, or hybrid) underlying multicellularity in different environmental contexts.

Results

Choanoeca flexa forms motile and contractile monolayers through three distinct pathways: purely clonal formation via unseparated sister cells, purely aggregative assembly of independent cells, and hybrid processes combining both mechanisms. Natural populations undergo reversible transitions between unicellularity and multicellularity correlated with splash pool hydrological cycles. Genetic analysis identified strain-level differentiation among C. flexa populations across splash pools. Kin recognition mechanisms restrict aggregation between genetically distinct strains, suggesting strain-specific recognition systems govern aggregative processes. The relative prevalence of clonal versus aggregative multicellularity varied across environmental contexts within the same organism.

Implications

The demonstration of clonal-aggregative multicellularity in C. flexa challenges prior generalizations positing mutual exclusivity between these developmental modes. This flexibility in multicellular strategy provides C. flexa with enhanced robustness in variable, fast-fluctuating ephemeral environments where reversible transitions between cellular states confer selective advantage. The expanded mechanistic repertoire for multicellular development in choanoflagellates—the sister group to animals—suggests greater developmental complexity in early diverging eukaryotic lineages than previously characterized. These findings broaden the conceptual option space for understanding how multicellularity could have originated and diversified in evolutionary history.