Somatic differentiation evolves rapidly and repeatedly through the modification of developmental plasticity

Plasticity can potentially lead to the evolution of new types of organisms, but it is uncertain whether it can initiate a transition to an entirely novel individual. In a study, researchers examined the experimental evolution of Eudorina, a species known for its predominantly undifferentiated state that produces plastic somatic cells under cold stress conditions.

The team analyzed 2 laboratory lines and 46 undergraduate research lines subjected to multiple cold shocks. Surprisingly, 44% of these lines (21 out of 48) exhibited increased somatic differentiation under baseline conditions. Furthermore, the researchers identified a lineage with obligate somatic differentiation, marking the first documented instance of this phenomenon through experimental evolution.

Eleven lines, including the obligately differentiated one, also showed variations in the proportion of somatic cells within differentiated colonies, with their proportions converging towards those of Pleodorina, a genus of polyphyletic soma-differentiated volvocine algae. Genome sequencing uncovered that 93% of the mutated genes in the obligately differentiated line, absent in the control group, possessed at least one homolog with soma-specific expression in either Obligate Volvox carteri or Astrephomene gubernaculifera, or cold-induced expression in unicellular Chlamydomonas reinhardtii.

Additionally, 72% of these mutated genes shared both soma-specific and cold-induced expression. All these genes have regulatory mutations and are situated at the intersection of cold shock and somatic differentiation regulatory modules. The findings offer the initial empirical evidence that altering an organism's plastic responses to environmental changes can catalyze the evolution of somatic differentiation, a pivotal step in the emergence of a novel evolutionary individual.