Though their bacterial origins are now well established, many questions remain about the evolution of these organelles. The now ubiquitous mitochondria, for example, evolved just once, and scientists are only able to see the result of the event and not its origin. Tremblaya might illuminate the process that led to mitochondria. “Because it only happened once, it’s hard to know what happened,” said McCutcheon. “Studying endosymbionts gives some insight into that.”
Tremblaya shares certain attributes with organelles — its genome size is similar to that of some mitochondria and chloroplasts, it is missing a number of vital genes, and its biology is deeply intertwined with that of its host. One obvious difference, however, is that organelles are found in almost every cell in an organism, while endosymbionts, because their main role is to provide nutrients for their hosts, are found only in certain cells. Tremblaya, for example, is found in specialized cells called bacteriomes.
One of the key questions surrounding Tremblaya biology is how the tiny organisms survive. One theory proposes that, like organelles, they saddled their insect hosts with some of their genes, which would help explain their small size and put them on a similar evolutionary path as mitochondria. McCutcheon’s team saw no evidence for this in theirCellpaper, but what they found is even more complicated.
The mealybug genome harbors 22 genes from bacteria with ancestors unrelated to Tremblaya and Moranella, but these genes, which code for proteins involved in the production of essential nutrients and the synthesis of the cellular wall enclosing the bacteria, “fit with things missing in the symbionts,” said McCutcheon. “These [organisms] are not getting small by transferring genes to the host,” he said. “They are getting small by coopting bacterial genes in the host, a level of complexity that we would not have predicted.”
The findings provide a more detailed understanding of the symbionts’ differences and similarities to organelles. Tremblayahasn’t transferred genes to its host, a defining property of organelles. But, likemitochondria, McCutcheon’s findings suggest, Tremblaya coopts some host-derived proteins that originally came from other types of bacteria. “This is a murky gray area; the host encodes genes the symbiont needs to survive, which suggests the hosts target proteins to the organism,” said Patrick Keeling, a biologist at the University of British Columbia, who was not involved in the Cellstudy. “That’s something organelles do but not usually endosymbionts.”
Not everyone agrees that understandingTremblaya may help illuminate the evolution of organelles. William Martin, a biologist at the University of Düsseldorf in Germany, wrote in an email that Tremblaya is instead “a beautiful contrast to organelles.” He noted, for example, that organelles import the vast majority of proteins from the host. Tremblaya also seems to import some proteins, but “it’s a far cry from the protein import apparatus of chloroplasts and mitochondria,” he wrote.
Even if Tremblaya doesn’t meet all the requirements of an organelle, the bacterium does seem to be integrated with its host in a way that is similar to organelles.
“They appear to be more integrated at more levels than any other endosymbiont and they share that characteristic with organelles,” said Keeling, who added that the question of what to call Tremblaya and other tiny intracellular dwellers can “get some people’s blood boiling.” (He said he personally doesn’t care what Tremblaya is called.)