Iain Johnston 

Endosymbiotic relationships have shaped eukaryotic life. As endosymbionts coevolve with their host, towards full integration as organelles, their genomes tend to shrink, with genes being completely lost or transferred to the host nucleus. Modern endosymbionts and organelles show diverse patterns of gene retention, and why some genes and not others are retained in these genomes is not fully understood. Recent bioinformatic study has explored hypothesized influences on these evolutionary processes, finding that hydrophobicity and amino acid chemistry predict patterns of gene retention, both in organelles across eukaryotes and in less mature endosymbiotic relationships. The exciting new discovery and elucidation of more endosymbiotic relationships affords an independent set of instances to test this theory. Here we compare the properties of retained genes in the recently reported nitroplast, two related cyanobacterial endosymbionts which form “spheroid bodies” in their host cells, and a range of other endosymbionts, with free-living relatives. We find that in each case, the symbiont’s genome encodes proteins with higher hydrophobicity and lower ammonium pKa than their free-living relative, supporting the data-derived model predicting the retention propensity of genes across endosymbiont and organelle genomes.