Multicellularity: no big deal after all
One of the great transitions in the history of life on our planet was that from single-celled blob to multicellular… well, probably also blob. But a better blob; a more complex blob, with aspirations of one day having tentacles or growing leaves or inventing the talking thermometer.
Unfortunately, this precise moment in the lineages we care about (ie. animals; or, more specifically, us) has been lost to the sands of time. There is no surviving species we can point to as how we would have looked at that crucial time in our past. Thus, although we can talk all we want as to how it might have happened, we are adrift on a sea of speculation, without the land of empiricism in sight.
However, there exist other systems that we can study to get some idea of the changes involved in moving from unicellular to multicellular life.
Prochnik SE, Umen J, Nedelcu AM, Hallmann A, Miller SM, Nishii I, Ferris P, Kuo A, Mitros T, Fritz-Laylin LK, Hellsten U, Chapman J, Simakov O, Rensing SA, Terry A, Pangilinan J, Kapitonov V, Jurka J, Salamov A, Shapiro H, Schmutz J, Grimwood J, Lindquist E, Lucas S, Grigoriev IV, Schmitt R, Kirk D, & Rokhsar DS (2010). Genomic analysis of organismal complexity in the multicellular green alga Volvox carteri. Science (New York, N.Y.), 329 (5988), 223-6 PMID: 20616280
The Volvocine algae apparently have nothing to do with Swedish cars, but are a morphologically diverse lineage of green algae, containing both unicellular and multicellular members. Not only this, but they exhibit different levels of complexity, from simple sheets of identical cells as in Gonium, to differentiated somatic and reproductive cells as in Volvox.
(The idea of somatic cells is worth thinking about for a second. We humans take it for granted that we have regular body (somatic) cells, and specialised sex cells. But from the point of view of a single-celled organism that’s really weird. Some cells give up their own reproduction to help someone else? What’s in it for them?? So while just two different cell types may seem very basic to us, it’s a big deal for the algae.)
The authors of this paper attempted to characterise the changes from single- to multi-celled in Volvocine algae at the genomic level. The single-celled alga Chlamydomonas reinhardtii already had a genome assembly available; they used whole-genome shotgun sequencing to get the Volvox carteri genome. The idea is to look at the primitive multicellular organism and see what it has that its single-celled relative doesn’t.
So what did they find? Well, the two genomes were very similar sizes: 138 Mb for Volvox compared to 118 Mb for Chlamydomonas. In fact, the difference in size was largely made up of more repetitive DNA (junk DNA) in Volvox; the number of protein-coding genes was almost identical (14,520 and 14,516).
Not a lot to go on there. The difference must be in the type of genes, not the number. They looked in detail at genes involved in pathways they thought would be important to Volvox: protein secretion and trafficking; the cytoskeleton; the extracellular matrix (ECM); and cell-cycle regulation.
They found that, and I quote, “The components of these pathways are nearly identical in Volvox and Chlamydomonas.”
The ECM proteins showed the biggest differences, with 2 families in particular having substantially more members in Volvox. They also found a few extra genes involved in regulating the cell cycle; important if you have different cell types (somatic and reproductive) supposed to be doing different things at the same time.
But that was it! Fundamentally, they found that a few minor changes and duplications of existing genes were enough to let the unicellular ancestor of Volvox make the supposedly giant leap to multicellular co-operative harmony.
Firstly, Chlamydomonas has more genes than most other single celled organisms to start with. Thus the gains in gene number required to “go multi” may have mostly been already met.
Secondly, this paper is not saying, “oh, this is how it happened for all multicellular life.” Not at all. This is one possible pathway. There have almost certainly be multiple independent lineages that made the leap at some stage; probably many more than have survived to let us know.
Thirdly, and perhaps most importantly, the authors focused almost exclusively on the protein-coding content. As we now know, there’s a lot more to genetics than proteins. There might be regulatory RNAs running the whole show in there; we have no idea. Not only that, but they were looking at known protein content. If it couldn’t be identified or a function deduced from identified functional domains (that’s where you look at individual bits of a protein to figure out what it does), they couldn’t do much with it.
But that said: the expectation is that you find a whole lot of functional innovation in the genome that allow Volvox to be multicellular while Chlamydomonas is not. And they didn’t. Even if there were important genes in the pool that they couldn’t identify, there were very few genes that didn’t have a counterpart in Chlamydomonas.
The more we learn about genetics, the more we find that what we think of as big changes needing brand new genes is just an illusion. It’s easy to look at something as complex as a hand or a flower or an insect’s wing and think there’s no way that could have just grown. But they do, all around us, every day. The bacterial flagellum is just the product of a protein-trafficking gene gone a bit wrong. The human eye is only a sheet of light-receptive cells with accessories tacked on a bit at a time. And as it turns out, multicellular organisms are just single cells that stuck together.