22Myxobacteria and dictyostelids are prokaryotic and eukaryotic multicellular lineages, 23 respectively, that after nutrient depletion aggregate and develop into structures called fruiting 24 bodies. The developmental processes and the resulting morphological outcomes resemble one 25 another to a remarkable extent despite their independent origins, the evolutionary distance 26 between them and the lack of traceable levels of homology in the molecular mechanisms of the 27 groups. We hypothesize that the morphological parallelism between the two lineages arises as 28 the consequence of the interplay, within multicellular aggregates, between generic processes, 29 physical and physicochemical processes operating similarly in living and non-living matter at the 30 mesoscale (~10 -3 -10 -1 m) and agent-like behaviors, unique to living systems, characteristic of 31 the constituent cells. To this effect, we analyze the relative contribution of the generic and 32 agent-like determinants in the main phenomena of myxobacteria and dictyostelid development,
33and their roles in the emergence of their shared traits. We show that as a consequence of 34 aggregation collective cell-cell contacts mediate the emergence of liquid-like properties, making 35 nascent multicellular masses subject to new sets of patterning and morphogenetic processes. In 36 both lineages, this leads to behaviors such as streaming, rippling, and rounding up, similar to 37 effects observed in non-living fluids. Later the aggregates solidify, leading them to exhibit 38 additional generic properties and motifs. We consider evidence that the morphological 39 phenotypes of the multicellular masses deviate from the predictions of generic physics due to 40 the contribution of agent-like behaviors. These include directed migration, quiescence, and 41 oscillatory signal transduction of the cells mediated by responses to external cues acting 42 through species-specific regulatory and signaling mechanisms reflecting the evolutionary 43 histories of the respective organisms. We suggest that the similar developmental trajectories of 44 Myxobacteria and Dictyostelia are more plausibly due to shared generic physical processes in 45 coordination with analogous agent-type behaviors than to convergent evolution under parallel 46 selection regimes. Finally, we discuss the broader implications of the existence and synergy of 47 these two categories of developmental factors for evolutionary theory.48 49 Key words: myxobacteria; dictyostelids; liquid tissues; deformable solids; excitable 50 media 51 of multicellular structures called fruiting bodies, containing up to 10 5 -10 6 cells, where cell 85 differentiation takes place (Whitworth, 2008).
86The basis of cell differentiation in D. discoideum has been explained in two ways. There are 87 pre-aggregation tendencies among amoebae, stochastic in origin, biased by the environment 88 they experienced during the phases of growth and division, or, cell differentiation is a post-89 aggregation phenomenon based on intercellular inte...