In the transition to multicellularity what function becomes specialized first?

Question

It's my understanding that in the transition from single celled to multi-celled organism a lineage will transition between stages where existing in a colony of cells is optional to one where it is obligate. Integral to this process is the parts of the colony taking on specialized roles in the colony's survival. First is tissues, and eventually full blown organs.

With that as background (please correct any misconceptions), which job tends to be taken over by specialized cells first in the transition? The three main ones in every multicellular organism I can think of are: digestion, structure (including skin and any rigidity), and reproduction (please suggest roles if I missed any, or over-included). Is there a single answer to this, or does it vary between lineages?

Answer 1

Lyons & Kolter (2015) On the evolution of bacterial multicellularity. Curr Opin Microbiol 24: 21-28; emphasis mine:

Multicellularity is one of the most prevalent evolutionary innovations and nowhere is this more apparent than in the bacterial world, which contains many examples of multicellular organisms in a surprising array of forms.

cell-cell adhesion to form a new evolutionary unit, and intercellular communication leading to coordinated activity (...) are the minimal requirements to redefine a group of cells rather than any single cell as an "individual."

[M]ulticellular magnetotactic prokaryotes (MMPs) [are the only known bacteria] without a unicellular phase in their lifecycle.

[The key features of multicellularity are: cell differentiation, intercellular signaling, extracellular matrix, tight cell-cell junctions, coordinated movement, division of entire structure (Table 1).]