Experimental evolution in the cyanobacterium Trichormus variabilis: increases in size and morphological diversity

Area of Science:

• Evolutionary biology
• Microbiology
• Developmental biology

Background:

• Cyanobacteria exhibit remarkable evolutionary stasis, with morphology largely unchanged for billions of years.
• Maximum morphological complexity in cyanobacteria was reached around 2 billion years ago, unlike other multicellular lineages like plants.
• Evolutionary conservatism in cyanobacteria is often attributed to developmental constraints, slow evolution rates, or ecological flexibility.

Purpose of the Study:

• To investigate the potential for rapid morphological evolution in filamentous cyanobacteria.
• To explore the evolution of size and multicellularity in cyanobacteria under selective pressure.
• To identify novel morphological phenotypes and assess their heritability in Trichornus variabilis.

Main Methods:

• Experimental evolution of the filamentous cyanobacterium Trichornus variabilis (syn. Anabaena variabilis).
• Selection for increased organismal size over 45 cycles.
• Observation and characterization of emergent morphological phenotypes (Tangle and Cluster).
• Assessment of phenotype heritability through growth from single cells.

Main Results:

• Significant increases in size (over 30-fold) were observed in selected Trichornus variabilis populations.
• Two distinct emergent morphologies, Tangle (long, tangled filaments) and Cluster (clusters of short filaments), were consistently identified.
• The evolved Tangle and Cluster morphologies demonstrated heritability, indicating genetic basis.

Conclusions:

• Morphological diversity can evolve rapidly in filamentous cyanobacteria, challenging the concept of evolutionary stasis.
• Experimental evolution can induce significant changes in size and form, leading to novel multicellular arrangements.
• These findings provide a model for studying the evolution of higher biological organization within the cyanobacterial lineage.