Complexity metrics trend analysis through endosymbiosis evolution

Resumen del póster presentado al XLII Congreso de la Sociedad Española de Genética, celebrado de forma virtual del 14 al 18 de junio de 2021.The existence in biological evolution of a trend towards greater complexity is an open question that has generated much debate. This is mostly due to the existence of multiple definitions of complexity and their measures. Moya et al. (2020), assuming that the genome is a good record of the evolutionary history of the species in their interaction with the physical and biotic environment (Adami, 2002), carried out a study of the evolution of cyanobacteria, measuring in their genomes several metrics and genomic parameters. Through phylogenomic analysis, they estimated the values of these metrics in the internal nodes, detected a phylogenetic signal in all of them and showed that some of them presented a positive trend, that is: the genomes of the most recent species were more complex than the older species. This hypothesis can be tested by the investigation of the evolution of an opposite case, that of endosymbionts, intracellular bacteria of eukaryotes whose genomes have undergone with respect to their free-living ancestors a drastic process of reduction and degeneration, as well as loss of metabolic diversity. These organisms should present metrics of decreasing complexity with respect to the organisms from which they are derived and, therefore, the trend should be negative. In this communication, we study a set of 192 genomes where 102 organisms are endosymbionts that belong to 13 different phyla. All the complexity metrics studied and proposed in the study of cyanobacteria were applied to the endosymbiont genomes. We derived a phylogenomic tree based on a matrix of 31 proteins to reconstruct the ancestral states of the metrics and, subsequently, we carried out the trend analysis. The metrics surprisingly show a positive slope, indicating an unexplained increase in the complexity of recent endosymbiont genomes relative to ancestral free-living bacterial genomes. The only connection point that we found between the positive trend metrics in cyanobacteria and endosymbionts is the greater evolutionary acceleration with respect to their values in the corresponding ancestors. These results highlight the need to work on improving the metrics, or new ones, that be independent of the evolutionary acceleration, in such a way as to find those that really measure the evolution towards greater complexity, if it exists.Peer reviewe