Minimal Genome
We do not like to channel NY Times here, but a column by Carl Zimmer is well written.
One species, Mycoplasma genitalium, turned out to have a mere 475 genes one- fiftieth the number in our own set.
For years, M. genitalium held the record for the smallest genome. (Scientists dont allow viruses into this contest, since viruses cant grow and reproduce on their own.) But in recent years, M. genitalium has lost its minimalist crown. 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.
He continues to discuss a recently published study on horizontal gene transfer, where Tremblaya survives by sharing its work with another microbe and the host mealybug.
The smallest reported bacterial genome belongs to Tremblaya princeps, a symbiont of Planococcus citri mealybugs (PCIT). Tremblaya PCIT not only has a 139 kb genome, but possesses its own bacterial endosymbiont, Moranella endobia. Genome and transcriptome sequencing, including genome sequencing from a Tremblaya lineage lacking intracellular bacteria, reveals that the extreme genomic degeneracy of Tremblaya PCIT likely resulted from acquiring Moranella as an endosymbiont. In addition, at least 22 expressed horizontally transferred genes from multiple diverse bacteria to the mealybug genome likely complement missing symbiont genes. However, none of these horizontally transferred genes are from Tremblaya, showing that genome reduction in this symbiont has not been enabled by gene transfer to the host nucleus. Our results thus indicate that the functioning of this three-way symbiosis is dependent on genes from at least six lineages of organisms and reveal a path to intimate endosymbiosis distinct from that followed by organelles.
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