Scientists have long wondered how much life can be stripped down and still remain alive. The answer seems to be that the true essence of life is not some handful of genes, but coexistence.
There’s a tail on the frog on the bump on the branch on the log in the hole in the bottom of the sea…
This is a fascinating article looking at the very unusual life style of a mealybug through the lens of genomic information.
This mealybug lives on the sap of particular trees. But, like us, it relies on bacteria it carries to digest this food in ways to provide important nutrients. This bacteria – Tremblaya princeps – has the smallest genome of any living organism – only 120 genes in about 139 kb of DNA.
It is missing many essential genes., especially several required for protein production. How does it live?
It turns out that this bacteria is actually infected with another bacteria – Moranella endobia – which, unbelievably, has 406 genes. Yes, Tremblaya carries within it a bacteria which is more complex than it is. This second bacteria provides many of the gene functions Tremblaya lacks.
These two bacteria share duties, each providing some of the biological processes needed to survive. But not all of them. A recent paper in Cell dissects much of this.
It turns out that the mealybug itself provides several of the gene products required for these two bacteria to live; bacteria the mealybug needs itself in order to live. This was found by looking at another species of mealybug that hosts a Tremblaya species that does not itself contain Moranella. Thus they could see what genes were lost and taken over by the bacteria’s partner.
When they did this, they were able to determine the full complement of essential genes contained in both the bacteria was incomplete. There were still some missing.
It turns out that the mealybug itself contains bacterial genes, genes it picked up over time through a process known as horizontal gene transfer.
The transfer of genes across species has been observed several times. In fact, our mitochondria – which started life as endosymbiotes of our own cells, much as Tremblaya contains Moranella – transferred several of their genes to our own nuclear DNA in an act of horizontal gene transfer.
So, is Tremblaya on the same path as our mitochondria? It actually does not look like it. Because none of the transferred bacterial genes in the mealy bug actually come from Tremblaya.
Nope. The essential bacterial genes – at least 22 of them – that the mealybug carries, genes that Tremblaya needs in order to survive, actually appear to come from OTHER bacteria species. In addition, some of these genes are needed for Moranella to survive, permitting the insect to control the levels of both bacteria simply by controlling its own production of the needed gene products.
Examination of the mealybug’s genome reveals that at least 3 different bacteria species contributed the genes found in the mealybug’s DNA and needed by Tremblaya for survival. These acts of horizontal gene transfer actually predate the symbiosis with Tremblaya.
So Tremblaya is not acting like the mitochondrion because it has so far not moved any of its genes to another location. In fact, it appears that it has simply lost the genes it carried that could be dealt with by genes elsewhere. That is why it has such a small genome.
Like the song, first impressions do not tell the whole story. There is more than just a hole in the bottom of this sea. In fact, this mealy bug can only survive through the genetic efforts of 6 different organisms – its own DNA, Tremblaya, Moranella and the 3 bacteria species that contributed 22 genes.