Viviane Callier writes:

There are few relationships in nature more intimate than those between cells and the symbiotic bacteria, or endosymbionts, that live inside them. In these partnerships, a host cell typically provides protection to its endosymbiont and gives it a way to propagate, while the endosymbiont provides key nutrients to the host. It’s a deeply cooperative arrangement, in which the genomes of the host and the endosymbiont even seem to contribute complementary pieces to each other’s metabolic and biosynthetic pathways.

The revealed intricacy of these partnerships continues to hold surprises. In a new study appearing today in Cell, scientists show that a complex three-way symbiosis between an insect cell and two species of bacteria — one an endosymbiont of the other — deeply intertwines the organisms’ genomes and physiologies. Those results may illuminate how mitochondria and other organelles arose from ancient endosymbionts in the earliest eukaryotic cells.

When cells need to quickly acquire a new metabolic trait to survive, their best option may be to borrow one from other organisms. Horizontal transfers can move a few genes between cells, but the chances of horizontally acquiring the complete suite of genes for a complex metabolic pathway are vanishingly small. So the easiest solution is often for cells with dissimilar abilities and complementary needs to merge, explains John McCutcheon, an endosymbiosis researcher at the University of Montana. These mergers are not uncommon in nature. Secondary and tertiary mergers are even known to have occurred, producing the cellular equivalent of a set of nested Russian dolls. [Continue reading…]