The same process that led to the evolution of complex life may be happening all over again in insects, according to a new study in the journal Cell.
About 900 million years ago, the Earth was covered in vast oceans containing giant mats of bacteria. Single-celled organisms with little more than a nucleus topped the food chain. At some point, they engulfed photosynthetic organisms called cyanobacteria, incorporating them into their cells and forming the world’s first proto-plants and algae, according to recent research in the journal Proceedings of the National Academy of Science.
From there, the tree of life blossomed.
This process of one organism Pac-Manning another is called endosymbiosis. The two organisms in the relationship are completely dependent on each other, and eventually, the smaller organism sometimes loses its ability to survive on its own and becomes a component in its host’s cell. Reach back to your high school biology — remember the chloroplast and mitochondria? They originated as free-living bacteria before being swallowed up by something bigger and becoming organelles.
Scientists have now discovered a case of endosymbiosis run amok — a bacterium living within another bacterium living within an insect. It kind of reminds you of those Russian nesting dolls.
The insect, Planococcus citri, is a species of mealybug that sucks sap from plants. Living within its cells is the bacterium Tremblaya princeps and living in its cells is another bacterium, Moranella endobia.
John McCutcheon of the University of Montana had previously sequenced the genome of T. princeps and was stunned to find the smallest recorded bacterial genome ever — only 140,000 base pairs. (For the sake of comparison, humans have 3.16 billion base pairs.)
“Why is it so degenerate?” McCutcheon wondered.
He immediately suspected that the shrunken genome was due to its clingy relationship with its insect host, the mealybug. In many cases of endoymbiosis, the bacterium transfers most of its genes to its host, keeping only the essentials. Perhaps T. princeps had simply outsourced its genetic code to the mealybug.
And if so, there was the possibility that T. princeps was well on its way to becoming an organelle, the same fate experienced by the bacterial precursors of chloroplasts and mitochondria.
“I look at this like molecular archaeology,” McCutcheon said.
To test these ideas, McCutcheon and his team first sequenced the bug’s RNA to see which genes were turned on in the bug. They found 22 expressed genes, which they then compared with the genome of T. princeps and a database of other bacterial sequences. If the bug’s 22 genes were closely related to T. princeps, they could infer that the bug had acquired them from the bacterium sometime in the past; if not, perhaps the bug inherited those genes from some other bacteria.
It turned out the genes came not from T. princeps, but from three different groups of other bacteria. The mealybug genome was in essence a patchwork quilt of past bacterial infections. Those bacteria had donated genes to the bug sometime in the past; now those genes ensure its survival.
So perhaps T. princeps, despite its much-reduced genome, is not on an inevitable path to becoming an organelle? After all, unlike many other endosymbionts, it had not donated its genes to its host.
Maybe, maybe not. It all comes down to how you define an organelle.
“The word organelle is just something we made up, so where do you draw the line?” said organelle expert Patrick Keeling of the University of British Columbia who was not involved in the mealybug study.
“There’s no tablet from the mountains here,” he said.
Keeling pointed out that many structures that we call organelles are dependent on the genes encoded by their host, but those genes did necessarily originate from the organelle itself. Just like T. princeps and its mealybug host.
Organelle or not, T. princeps is recapitulating the history of life on Earth. Inside of a bug and home to another, it’s one of the few organisms that can both eat and be eaten.
A summary of the study is available at the Cell website.
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