Evidence for a Programmed Life Span in a Colonial Protochordate

The variety of theories that have attempted to define the mechanisms of aging and life span can be broadly divided into two alternative but nonexclusive viewpoints. The first stipulates that random changes of cellular and molecular structures lead to death following progressive "wear and tear." The second argues that life span is, at least in part, genetically programmed, and therefore aging may also result from time-dependent intrinsic processes. Here we demonstrate that ramets (clonal replicates) experimentally separated from colonies of the ascidian protochordate Botryllus schlosseri died months after their separation, almost simultaneously with their parent colony and sibling ramets. In addition, in experimentally joined chimeras between ramets of senescent and nonsenescent colonies, elements from different parent colonies displayed parent-colony-specific timing of mortality. Thus, the senescent phenotype was simultaneously expressed both in chimeras and in unfused ramets of the parent colony that was undergoing senescence, whereas control ramets from the other partner survived. These findings provide experimental evidence for a heritable basis underlying mortality in protochordates, unlinked to reproductive effort and other life history traits of this species.