Throughout the lifespan of multicellular species, somatic genetic variation continually emerges through the accumulation of somatic mutations (SM) that arise in cells and tissues, eventually making each individual a genetic mosaic. In many species, this mutational load is reset by a single-cell bottleneck through germline sequestration and sexual reproduction. Due to the separation between disposable soma and heritable germline, known as Weismann’s barrier, somatic mutations have historically been considered evolutionarily irrelevant. However, recent empirical studies and theoretical works strongly suggest that somatic genetic variations play an underestimated and even fundamental role in the evolution of long-lived modular organisms, including the majority of plants, fungi, and many animal species. Nearly half of the metazoan phyla contain species that propagate asexually via agametic reproduction, often forming colonies of genetically identical modules. Colonial growth and modular organization have important physiological and ecological consequences, as seen in many marine invertebrates such as sponges, Cnidarians, Lophophorates, Pterobranchs and Tunicates. Despite the abundance and adaptive success of modular species, evolutionary theories mainly rely on studies conducted in unitary, strictly sexually reproducing organisms. The evolutionary consequences of somatic variation are still widely unexplored, mainly due to the difficulty of tracking the typically low-frequency somatic mutations and reconstructing the sexual and asexual reproductive history of the colonial organism at issue. This project utilizes the modular chordate Botryllus schlosseri (Tunicata) as a laboratory model to investigate the interplay between somatically and meiotically generated genetic variation and to explore different levels of selection within and among modules. B. schlosseri forms colonies through asexual budding of ramets (modules). Each colony belongs to one genet, which originates sexually from a single zygote. The propagation by budding occurs continuously on a weekly basis, corresponding to the succession of many asexual generations. Sexual reproduction is also fast, taking about a week. The hosting laboratory has the ability to control both the asexual and sexual cycles precisely, and genetically identifiable Botryllus colonies are available. The aim is to describe the extent, nature, and dynamics of genome-wide SM across several asexual generations, both intra- and inter-ramet, using deep-genome duplex sequencing. The goal is to obtain a spatial and temporal portrayal of SM propagative dynamics. Secondly, we will test whether selection or somatic genetic drift can increase the frequency of SM in the ramets and colony, and gather information on the potential adaptive value of these SM. Additionally, we will investigate whether SM can be sexually inherited by taking advantage of late germline segregation and plastic sexual fertility, thus breaking the Weissmann's Barrier. The potential transfer of somatic genetic variation to the sexual cycle suggests that SM can recombine into new genetic landscapes, increasing diversity and facilitating adaptation. Additionally, we will characterize the cellular and molecular dynamics of the somatic cell bottleneck associated with the budding onset. This will eventually help to infer intra-ramet mechanisms of cell selection. Together, these approaches will allow to explore, under laboratory conditions, the dynamics of multi-level selections in a modular chordate. Importantly, by providing empirical data appropriately collected based on theoretical expectations, we will be able, for the first time, to follow the fate of SM during an organism's growth and evaluate their role in evolution.