Experimental evolution of increased size and complexity in <italic>Anabaena variabilis

Abstract

The evolution of multicellularity has occurred over 25 times in the history of life. Previously, we have shown the evolution of multicellular traits can readily be observed in laboratory populations across model unicellular organisms like yeast, chlamydomonas, and <italic>E. coli.</italic> Cyanobacteria are the oldest multicellular organisms, dating back 3.5 billion years. Many species appear morphologically unchanged, suggesting they have remained primitively multicellular. Are they incapable of evolving increased complexity? Model prokaryote <italic>Anabaena</italic> is a filamentous cyanobacteria, predating fossil records, existing as single strands or loose mats with three distinct cells types. Rapid settling was used to select for increased size advantage. Response to selection resulted in dramatic size increase; microscopic strands became inseparable macroscopic aggregates. <italic>Anabaena</italic> also became more complex; growth rate increased, two distinguishable morphologies developed, and growth and reproduction patterns changed. This shows that <italic>Anabaena,</italic> although primitively multicellular for billions of years, rapidly evolves increased size and complexity.