The evolutionary-developmental origins of multicellularity.

Area of Science:

• Evolutionary Biology
• Developmental Biology
• Genomics

Background:

• Multicellularity has evolved independently multiple times across eukaryotic and prokaryotic lineages.
• Standard multilevel selection (MLS) models propose key requirements for multicellular evolution: cell adhesion, communication, cooperation, and specialization.
• These requirements necessitate alignment-of-fitness and export-of-fitness phases to resolve cell-cell conflict and establish reproductive integration.

Purpose of the Study:

• To review the evolutionary developmental motifs underlying multicellularity across diverse taxa.
• To analyze how these motifs and MLS models explain observed body plan transformations.
• To explore alternative evolutionary pathways to multicellularity.

Main Methods:

• Review of existing literature on multicellularity evolution.
• Analysis of generalized evolutionary developmental motifs.
• Mapping of motifs and MLS models onto a theoretical body plan morphospace.
• Comparison of theoretical transformation series with empirical observations in algae, plants, fungi, and animals.

Main Results:

• Identified a "unicellular ⇒ colonial ⇒ multicellular" transformation series, consistent with trends in algae, plants, fungi, and animals.
• Proposed an alternative "siphonous ⇒ multicellular" transformation series, potentially explaining aspects of fungal and algal evolution.
• Demonstrated that natural selection often acts on functional traits rather than underlying mechanisms.
• Highlighted that genome sequence homology does not always equate to morphological homology.

Conclusions:

• Multicellularity can arise through multiple distinct evolutionary routes.
• Evolutionary developmental motifs provide a framework for understanding the diverse mechanisms of multicellularity.
• Functional traits, rather than specific genetic mechanisms, are primary targets of natural selection in the evolution of multicellularity.