Ponder this: we contain multitudes

Karl Zimmer in the NYTimes is writing about the notion of a minimal genome. What's the fewest number of genes a creature can have and be viable?

The human genome contains around 20,000 protein-coding genes. Many other species get by with a lot less. The gut microbe Escherichia coli, for example, has just 4,100 genes.

How low can you go?

Today, the record-holder is a microbe called Tremblaya princeps, which contains only 120 protein-coding genes.

Have we found the minimal genome at last? The answer, once again, is no. But the reason for that reveals something else intriguing about life.

Tremblaya lives in one particular place: the body of a mealybug. And the mealybug, in turn, depends on Tremblaya for its survival.

You can see where this is going. It turns out that Tremblaya relies on yet another bacteria, Moranella endobia.

The two species split up the work involved in building amino acids and assembling them into proteins. Just as the mealybug cannot live without its microbes, the microbes can’t live without each other.

When you dredge through these genomes it turns out that, in the past, other creatures have been involved:

Six separate species apparently donated genes to the insects. Dr. McCutcheon and his colleagues suspect that the insect uses some of these genes to manage its microbial residents — perhaps using bacteria proteins to extract amino acids from them, for example.

So, just how many creatures do WE contain>