A unique three-tiered symbiotic relationship is now being studied in order to better understand how organisms transfer and share genes in mutualistic interactions. Surprisingly, this gene transfer is not analogous to how mitochondria and chloroplasts have evolved with their host genome.
In exchange for the bacteria's help with digesting plant sap, the Mealybugs host the first bacterium, Tremblaya princeps, who host the second bacterium, Moranelle endobia. The Tremblaya have a remarkably small genome since the Moranelle sp. is able to provide the Tremblaya with many genes they have in common. Instead of both organisms expending energy to maintain large genomes, the Tremblaya have been able to transfer many essential genes to their endosymbiont. This is similar to the gene transfer between organelles and their host organisms, but in reverse. Interestingly, the Tremblaya have not transferred any genes to their host, the Mealybugs. While the Mealybugs do possess many genes that Tremblaya and Moranelle require for survival, it appears as if the genes were gained from other bacteria.