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by James VanLeuven, PhD student in John McCutcheon’s lab (Cellular, Molecular, and Microbial Biology), at the University of Montana
Thousands of insect species feed solely on plant sap by piercing the protective layers of a plant with their proboscis and sucking the sap out like a kid with a milkshake. Unfortunately for the insects, sap is a poor food choice because it contains insufficient amounts of certain amino acids (the basic subunits of proteins). However, sap-feeding insects succeed in life because of highly obligate mutualism they have established with bacteria. These bacteria are called nutritional endosymbionts. They exist only in insect cells, and are metabolically integrated with their hosts, and sometimes with other co-symbionts. That is to say, the insect provides the bacteria with the essential components of life—water, sugar, and a suitable habitat—and the bacteria synthesize the amino acids missing in the insect’s diet. The strictly intracellular lifestyle of endosymbionts imparts selective pressures on the endosymbionts that drive their genomes to shrink in size and total number of genes.
The cicada endosymbiont Candidatus Hodgkinia cicadicola DSEM exemplifies the outcome of a strictly endosymbiotic life history, with only 169 genes in its entire genome. Surprisingly, we find two Hodgkinia species coexisting in basal-lineage cicada individuals of species Tettigades undata. These two bacteria are about 5-25 million years diverged from one another, but are genomically complementary. In each, many genes are inactivated by mutations, but together they retain the full set of genes present in their ancestor and in Hodgkinia DSEM. Using fluorescently labeled probes (Figure 2) uniquely targeting each genome, we show that the genomes of these bacteria are cytologically distinct: they are indeed separate bacteria. Therefore, we propose that genetic drift caused the ancestral Hodgkinia lineage to split into two species that remain inseparably linked by genetic complementarity. Their dependency upon one another necessarily includes processes inherent to any cell, like transcription, translation, and replication.
How exactly this dependency arises from the simpler single-species ancestor is unknown, but we imagine that the unusual nature of Hodgkinia’s cell membrane allows for the exchange of gene products between the two bacterial species. These results highlight the possible importance of genetic drift in leading to genomic complexity (not genome reduction) and suggest that even genes needed for the core processes of life can be shared, their gene products somehow made available to neighboring bacteria.
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