Mealybugs are scale insects that feed on plant juices. And like other animals, including us, they have bacteria living inside them.
One of these species of bacteria, Tremblaya princeps, recently had its genome sequenced, leading to a remarkable discovery. It turns out to have a symbiotic relationship with another species of bacteria, Moranella endobia, that lives inside it, like microscopic matryoshkas.
Their relationship is so symbiotic, that is, they depend on each other so much, that it seems Tremblaya has lost a lot of its genes. In fact, with only 139,000 bases, it has the smallest genome ever seen in a cell.
Perhaps the phenomenon is best described using the medium of poetry:
Big fleas have little fleas,
Upon their backs to bite ’em,
And little fleas have lesser fleas,
and so, ad infinitum.
Except in this case, of course, the bitin’ seems to help.
Reference (the science, not the nursery rhyme): McCutcheon JP & von Dohlen CD 2011, “An interdependent metabolic patchwork in the nested symbiosis of mealybugs”, Current Biology, vol. 21, no. 16, pp. 1366-1372, doi:10.1016/j.cub.2011.06.051