Telomerase maintained in self-renewing tissues during serial regeneration of the urochordate Botryllus schlosseri.

Telomerase is critical for the protection of germ line and stem cell chromosomes from fatal shortening during replication. In most organisms, telomerase activity is suppressed in progressively committed cells and falls to basal rates in terminally differentiated lineages. The colonial ascidian Botryllus schlosseri propagates asexually and sexually, presumably from pools of stem cells that self-renew throughout the 2- to 5-year colony life span. Asexual budding takes place continuously from the parental body wall. When the colony reaches a critical size, sexual reproduction commences with the generation of gonads. Here, we establish the existence of 6-15 kb telomeres on the ends of Botryllus chromosomes. We develop a real-time quantitative PCR telomeric repeat amplification protocol (TRAP) assay that reliably detects 0.2-100 TPG units in cells and tissues. We find highest levels of enzymatic activity in the gonads, developing embryos, and tissues containing the earliest asexual buds. Telomerase activity appears to be suppressed in later buds during organogenesis and falls to basal rates in mature zooids. We postulate that this pattern reflects maximum telomere restoration in somatic stem cells of early buds and suppression of telomerase activity in progenitors and terminally differentiated cells, indicative of an alternate role for stem cells as repeated body regenerators in colonial life histories.