When I'm lecturing about animal diversity and the origins of the multicellular animals (aka metazoans), I point out the similarity between the single-celled protozoans called choanoflagellates and the choanocytes (or 'feeding cells') of sponges. The textbook interpretation is that choanoflagellates may have shared a common ancestor with metazoans, and there's an increasing amount of genetic analysis that supports this relationship. As usual, PZ Myers has done a wonderful job of telling this story, over on Pharyngula, but I wanted to have a go at it myself 🙂
This is another of those 'forensic genetics' examples, where evidence supporting the hypothesis that choanoflagellates & multicellular animals are related has come from comparisons of both nuclear and mitochondrial genes, together with an examination of the mitochondrial genomes of choanoflagellates, sponges, and more complex metazoans. What's more, all metazoans possess genes that code for proteins involved in cell-to-cell signalling and in adhesion (sticking cells together) – pretty much a prerequisite for the evolution of complex multicellular organisms. And choanoflagellates have homologues of these genes – this suggests that the genes must have evolved prior to the last common ancestor of the two groups. To look more deeply into the evolutionary past, King & her co-workers sequenced the nuclear genome of a choanoflagellate species before comparing it with the genomes of animals (& other eukaryotes such as fungi, plants & protozoa).
They found that their choanoflagellate, Monosiga brevicollis, had around 9,200 genes: similar in size to the genome of free-living single-celled eukaryotes. But surprisingly, the brevicollis genome contained almost as many introns as the human genome. The next step was to compare data on introns in conserved genes (genes which have changed very little over evolutionary history) in brevicollis & a variety of other intron-rich eukaryotes. The team's findings: i) "the last common ancestor of choanoflagellates & metazoans had genes at least as intron-rich as those of modern choanoflagellates;" and ii) the last common ancestor of metazoans had many more introns – suggesting considerable genetic change during the early evolution of multicellular animals.
Then they looked more closely at the reasonably large number of choanoflagellate genes – more specifically, their protein domains – involved in cell adhesion. (A protein domain is a section of a protein that can evolve independently of the rest of the protein.) Some of these were previously known only in multicellular animals, which implied that their DNA sequences were found in the common ancestor of the two groups. However, metazoans have many more variants of some sequences e.g. while brevicollis has 5 immunoglobulin domains, animals have between 150 & 1500 – this sector of the genome must have evolved after the choanoflagellate and metazoan lineages diverged. Similarly, while brevicollis possesses some genes involved in cell-to-cell signalling, metazoans have far more, with more complex interactions between them.
So, way back when, relatively simple uni-cellular organisms already possessed much of the genetic machinery that would support the evolution of more complex multicellular animals. This really was a BIG evolutionary innovation – and molecular genetics has provided the tools to peer back into that exciting past.
N. King et al. (2008) The genome of the choanoflagellate Monosiga brevicollis and the origin of metazoans. Nature 451: 783-788