Senescence in modular animals: Botryllid ascidians as a unique ageing system

While multicellular organisms go through predictable ageing pathways, some defy progressive ageing by displaying continued growth and negligible senescence. These organisms share two archetypal life history traits: (1) sessile life mode and (2) colonial structures, conglomerates of repeated basic subunits, the modules (i.e. zooids, polyps, leaves). In many of these organisms, no boundaries exist between germ/somatic cell lines, and while the size and age of each individual basic module are usually constrained, the whole colony size/age may escape intrinsic restriction, revealing colonial entities with an unknown upper life span limit. Furthermore, colonial astogenic processes such as fission, fragmentation, fusion between ramets and partial mortality may dramatically alter actual sizes, blur predictions of age and also reveal the trait for negligible senescence with age. Model organisms, such as botryllid ascidians, are an indispensable tool in ageing research. In Botryllus schlosseri, ageing is marked by independent, sometimes contrasting types of ageing and senescence processes at the basic module (zooid) level, at the ramet level and at the genet level; they also exhibit novel rejuvenilisation processes (at the genet-ramet levels) following acute stress, where the stressed organism becomes younger, slowing down senescence. Colonial organisms may also present spatial and stochastic age-mosaic modules, postponement of senescence by a high regenerative power and replacement of basic modules that do not age according to classical criteria, indicating that ageing in modular organisms is possible but not obligatory. When senescence at the whole-genet level occurs, it may reflect a sharp contrast to senescence in unitary organisms. The offsetting of senescence at the basic module level may develop when vigorous totipotent stem cells exist, and there is no formal separation of soma from the germ line. The Botryllus system thus reveals, within the same colonial entity, constructed senescence/rejuvenilisation phenomena, such as semelparity versus iteroparity, programmed life span versus wear-and-tear senescence, weekly ageing of colonial modules versus whole-ramet-genet survivorship, rejuvenili-sation versus extrinsic ageing and the immortality of germ/somatic cell lines. Though still in its infancy, studying ageing and senescence processes in sessile marine colonial organisms may lead to a better understanding of the evolutionary routes of senescence.