Choanoflagellates are single-celled organisms that are mentioned in The Design Matrix(pp. 264-265) to illustrate the plausibility of front-loading evolution. While summarizing a 2003 study, I noted, these single-celled organisms contained a whole toolkit of genes that allowed the researchers to conclude choanoflagellates have “key proteins required for animal development” that probably “evolved before the origin of animals.”

Recently, the choanoflagellate genome has been sequenced. These protists have abouit 9200 genes, far less than the 25,000 seen in flies or humans. Yet the genome shows an abundance of data that further strengthen the plausibility of front-loading.

I have not had the chance to look over the research article yet, but consider some excerpts from the ScienceDaily review:

One finding confirmed by the sequencing is that choanoflagellates have many genes that, in animals, produce proteins essential to cell-to-cell signaling and in determining which cells stick to one another. Since Monosiga does not form colonies as do some other choanoflagellates, these proteins’ roles are a mystery, King said.

“In animals, some of these proteins, called cadherins, evolved for linking cells together; they are the glue that prevents clumps of cells from falling apart,” King said. “Choanoflagellates show no hint of multicellularity, but they have 23 genes for cadherin proteins, about the same as the fruit fly or the mouse.”

In other words, these single-celled organisms are loaded with genes that had previously thought to be unique to multicellular organisms because they function in an extracellular context. But it gets better:

The choanoflagellate genome, like the genomes of many seemingly simple organisms sequenced in recent years, shows a surprising degree of complexity, King said. Many genes involved in the central nervous system of higher organisms, for example, have been found in simple organisms that lack a centralized nervous system.

Likewise, choanoflagellates have five immunoglobulin domains, though they have no immune system; collagen, integrin and cadherin domains, though they have no skeleton or matrix binding cells together; and proteins called tyrosine kinases that are a key part of signaling between cells, even though Monosiga is not known to communicate, or at least does not form colonies.

It is now clear that many genes, once thought to exist only to service the needs of multicellular life can exist in a unicellular context for hundreds of millions of years. The hypothesis of front-loading evolution continues to become more attractive.
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* Some background.

* And for my own notes, an unrelated echo of front-loading (?): The authors write: “The surprising similarities in the patterns of neural stem and intermediate progenitor cell division in Drosophila and mammals, suggest that amplification of brain neurogenesis in both groups of animals may rely on evolutionarily conserved cellular and molecular mechanisms.”