The emergence of multicellularity is one of the major transitions in evolution that
happened multiple times independently. During aggregative multicellularity, genetically
potentially unrelated lineages cooperate to form transient multicellular groups. Unlike
clonal multicellularity, aggregative multicellular organisms do not rely on kin selection
instead other mechanisms maintain cooperation against cheater phenotypes that benefit
from cooperators but do not contribute to groups. Spatiality with limited diffusion
can facilitate group selection, as interactions among individuals are restricted to
local neighbourhoods only. Selection for larger size (e.g. avoiding predation) may
facilitate the emergence of aggregation, though it is unknown, whether and how much
role such selection played during the evolution of aggregative multicellularity. We
have investigated the effect of spatiality and the necessity of predation on the stability
of aggregative multicellularity via individual-based modelling on the ecological timescale.
We have examined whether aggregation facilitates the survival of cooperators in a
temporally heterogeneous environment against cheaters, where only a subset of the
population is allowed to periodically colonize a new, resource-rich habitat. Cooperators
constitutively produce adhesive molecules to promote aggregation and propagule-formation
while cheaters spare this expense to grow faster but cannot aggregate on their own,
hence depending on cooperators for long-term survival. We have compared different
population-level reproduction modes with and without individual selection (predation)
to evaluate the different hypotheses. In a temporally homogeneous environment without
propagule-based colonization, cheaters always win. Predation can benefit cooperators,
but it is not enough to maintain the necessary cooperator amount in successive dispersals,
either randomly or by fragmentation. Aggregation-based propagation however can ensure
the adequate ratio of cooperators-to-cheaters in the propagule and is sufficient to
do so even without predation. Spatiality combined with temporal heterogeneity helps
cooperators via group selection, thus facilitating aggregative multicellularity. External
stress selecting for larger size (e.g. predation) may facilitate aggregation, however,
according to our results, it is neither necessary nor sufficient for aggregative multicellularity
to be maintained when there is effective group-selection.
