# Metrics of Genomic Complexity in the Evolution of Bacterial Endosymbiosis

## TL;DR

This paper explores how genomic complexity changes during bacterial endosymbiosis using two metrics, finding that endosymbionts have lower complexity than free-living bacteria.

## Contribution

The study validates GS and BB as metrics for genomic complexity in the context of bacterial endosymbiosis.

## Key findings

- Endosymbiont bacteria have lower Genomic Signature (GS) and Biobit (BB) values than their free-living relatives.
- Genomic erosion in endosymbionts correlates with reduced genomic complexity metrics.
- GS and BB metrics reflect biological complexity in a well-known evolutionary scenario.

## Abstract

The increasing biological complexity in evolution poses a challenge in evolutionary biology due to the difficulty of defining and measuring such organism complexity. This can be approximated in several ways, for example, by determining the number of parts that compose it, by the degree of organization of its hierarchical levels of organization, and by assessing the informational content of its genome. In this work, we approached organism complexity via the latter approach using two metrics: Genomic Signature (GS) and Biobit (BB). We determined the values of these metrics in endosymbiont bacteria versus phylogenetically related free-living bacteria. Endosymbiont bacteria suffer from a process of genetic erosion and the degradation of their genome that would define them, a priori, as less complex than their free-living relatives. We verified that the GS and BB metrics show lower values in endosymbionts than in their free-living relatives, giving support to the thesis that these metrics reflect the genomic (and biological) complexity of the organisms.

Endosymbiosis can be considered a regressive or degenerative evolutionary process characterized at the genomic level by genome erosion and degeneration due to high mutational pressure toward AT (adenine and thymine) bases. The genomic and biological complexity of endosymbionts must be lower than that of the free-living bacteria from which they evolved. In the present work, we contrasted whether two proposed metrics for measuring genomic complexity in both types of bacteria, GS and BB, reflect their complexity, expecting higher values in free-living bacteria than in endosymbionts. On the other hand, we endeavored to delve into the factors that contribute to the reduction in metric values in endosymbionts, as well as their eventual relationship with six genomic parameters associated with functionality. This study aimed to test the robustness of these proposed metrics in a well-known biological scenario, such as the endosymbiosis process.