Abstract
Nutritional-based mutualistic associations between microorganisms and insects with unbalanced diets is pervasive, with several insects having developed obligate associations with either bacterial or fungal symbionts. The last common ancestor of Cinara aphids (Hemiptera: Aphididae) harboured two obligate nutritional endosymbionts: Buchnera (the primary obligate endosymbiont of most aphids) and most probably a Serratia symbiotica. The latter shows a very dynamic pattern of replacement by different bacterial taxa, thus offering a special opportunity to study the turnover of obligate mutualistic associations. We have assembled the endosymbionts' genomes from over 60 species of Cinara. We found that most secondary co-obligate endosymbionts belong to taxa commonly associated to aphids as facultative endosymbionts, hinting at the origin of these lineages. Also, we observed common patterns of genome reduction and convergence in metabolic pathway retention. Within a group of Erwinia-associated aphids, horizontal transfer of B-vitamin-biosynthetic genes between endosymbiotic lineages has played an important role in the establishment of the new mutualistic associations. Finally, in the species Cinara strobi, we found that while the species has already acquired a new co-obligate symbiont, it retains the former S. symbiotica symbiont. It keeps a large but highly degenerated genome and has become unable to biosynthetise all B vitamins and essential amino acids, rendering it unable to fulfil its mutualistic role. In brief, the results provided by analysing the genomes of these obligate di- and tri-symbiotic consortia are illuminating the different processes involved in the evolution (from birth to death) of new obligate mutualistic associations between