ORCID
0000-0002-2950-1664
Date of Award
3-27-2025
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
The evolution of multicellular organization represents a key innovation in the history of life, yet fundamental questions remain about the evolutionary mechanisms that allowed some lineages to achieve remarkable complexity, while constrained others in their elaboration. The widely accepted single-cell bottleneck model provides a compelling explanation for the success of clonal multicellularity by reducing internal conflict and facilitating developmental integration. However, this perspective does not fully account for the stability and repeated evolution of aggregative multicellularity, where cooperation emerges among previously independent cells that may not be genetic relatives. Aggregative multicellularity demonstrates that the evolution of multicellularity is not a singular process and challenges our understanding of how multicellular complexity evolves and persists. These systems provide unique opportunities to examine how evolutionary and developmental processes intersect to enable major transitions in biological organization. My research focuses on an integrative analysis of aggregative multicellularity combining social evolution theory, experimental manipulation, comparative phylogenomics and transcriptomics. Using the social amoeba Dictyostelium discoideum as a model system, I examine three key aspects of aggregative multicellularity: mechanisms maintaining group integrity, developmental coordination between cells, and evolutionary constraints on developmental innovation. I demonstrate how collective behavior can emerge and persist despite potential genetic conflicts in the evolution of aggregative systems. First, through a comprehensive synthesis of empirical evidence from D. discoideum and the bacterium Myxococcus xanthus, I demonstrate how aggregative organisms employ multiple complementary mechanisms to maintain cooperation, including population structure, kin discrimination, and pleiotropic constraints on cheating. This analysis reveals that while aggregative multicellularity faces unique challenges compared to clonal development, robust solutions have evolved to maintain group cohesion. Second, I experimentally tested whether mixing genetically distinct cells impairs developmental coordination independent of social conflicts. Using experimentally evolved lines of D. discoideum that were not selected for social traits, I found that while basic coordination of slug migration remained intact in chimeric mixtures, there was a significant failure in coordinating migration with spore production. This provides the first direct evidence that developmental incompatibilities, separate from social conflicts, can emerge between divergent cell lines. Third, through phylogenomic and transcriptomic analyses spanning the developmental and evolutionary history of all protein coding genes in D. discoideum, I showed that early developmental stages are dominated by ancient genes while novel genes are predominantly expressed later in development. This pattern supports von Baer's law of early developmental conservation and reveals how evolutionary innovations become integrated into aggregative development. Together, these findings demonstrate that aggregative multicellularity represents a distinct but viable evolutionary strategy that has solved fundamental challenges of collective organization through multiple mechanisms operating at different scales. This work advances our understanding of major evolutionary transitions and highlights how different routes to multicellularity navigate universal constraints while finding unique solutions.
Language
English (en)
Chair and Committee
David Queller
Committee Members
Joan Strassmann; Anya Plutynski; Jonathan Myers; Liz Mallot
Recommended Citation
Jahan, Israt, "Barriers and Breakthroughs in the Evolution of Aggregative Multicellularity" (2025). Arts & Sciences Electronic Theses and Dissertations. 3457.
https://openscholarship.wustl.edu/art_sci_etds/3457