From left to right, A. Latorre, C. Vargas and A. Heddi.

This doctoral thesis, co-directed by Amparo Latorre and Abdelaziz Heddi, studies the genomes of two insects that established bacterial symbioses in very different times: the cedar aphid (150 million years ago) and the rice weevil (about 30,000 years ago). The thesis was defended on June 21, 2019 and received the grade of Outstanding.

Insects, the most diverse group of animals with a number of species estimated at five million, are capable of taking advantage of practically any food source. However, on multiple occasions they depend on microbial companions to obtain scarce nutrients in their diet. In addition to keeping their endosymbionts under control, insects must be able to cope with numerous infections. Although insects only have an innate immune system, it is capable of protecting insects against most infections.

The main objective of this thesis, entitled “Comparative genomics between two insect symbiotic models: Innate immune system and amino acid biosynthetic pathways of the rice weevil Sitophilus oryzae and the cedar aphid Cinara cedri”, was to identify the differences in the genetic repertoire of insects with endosymbiotic relationships with different ages: the rice weevilS. oryzaeand the cedar aphidC. cedri. This selection was due to the relationship betweenC. cedriand its endosymbiont (Buchnera aphidicola) were established at least 150 million years ago, while the symbiosis between S. oryzae and Sodalis pierantonius is much younger (around 30,000 years old). From the study of the immune system, it is clear that the characteristics of aphids make them prone to adopting certain types of bacteria as symbionts. Regarding metabolic capacities, both models confirm the hypothesis that there is a logic in the symbiotic association that is determined by the composition of the diet, so that bacteria are adopted that complement the essential nutrients that are scarce in it.

Carlos Vargas' doctoral thesis has been carried out in the Evolutionary Genetics group of the I²SysBio, under the supervision of Amparo Latorre (professor of genetics at the UV) and Abdelaziz Heddi (professor and member of the group Biologie Fonctionelle Insectes et Interactions, INSA and Université de Lyon).

The panel, which rated the thesis "Outstanding", was made up of Francisco Silva (professor of genetics at the UV, member of l’I²SysBio), Josefa González Pérez (scientist at the CSIC at the Institut de Biologia Evolutiva, CSIC-UPF, Barcelona) and Anne-Nathalie Volkoff (research professor at INRA, Montpellier).