Yesterday, I introduced you to MAGI, an important scaffold protein that connects membrane proteins to the cytoskeleton and signaling network to form epithelial tissue. Today, let me introduce you to two more players: opisthokonts and Capsaspora owczarzaki.

The opisthokonts are a clade that includes both metaozoa (animals) and fungi. Recently, it was determined that several unicellular lineages also belong to this clade, including the choanoflagellates (Monosiga). We’ve seen that the single-celled Monosiga has enhanced the plausibility of front-loading by being loaded with genes that are known to be essential for multiple multicellular processes that define metaozoa. This fact alone demonstrates that genes once thought to be specific for mutlicellular function can function quite well in a unicellular context.

Capsaspora owczarzaki is an amoeboid symbiont of a pulmonate snail and represents another independent protozoan lineage among the opisthokonts. This little creature is significant for a couple of reasons.

First, even though Capsaspora’s genome has not been sequenced, the genes for MAGI and tetraspanin have been found. Thus, some key components of the epithelial tight junction are coded for by this unicellular life form. Capsaspora also contains several other genes associated with metazoan multicellarity: laminin A, beta-catenin-interacting protein, ankyrin, and fascin. More on these later.

The second area of significance comes from considering the choanoflagellate, Monosiga. It does not possess MAGI, tetraspanin, or ankyrin. Why is this significant? The choanoflagellates are thought to be more closely related to metazoa, meaning that these genes have probably been lost from the choanoflagellate lineage. This supports a hypothesis I recently put forward:

Yet we need to remember that Monosiga brevicollis is not some primitive protozoan, but is a species that has been evolving alongside animals for hundreds of millions of years. It is thus possible that the last common ancestor of choanoflagellates and animals did possess most of the components to form an adherens junction and that the choanoflagellate lineage has since lost these genes.

It is becoming increasingly plausible that the lineage that led to metazoan life was a population of single-celled organisms that contained all the metazoan-essential genes represented in extant choanoflagellates, Capsaspora, and other single-celled opisthokonts.

The story continues…