Molecular anatomy of tunicate senescence: reversible function of mitochondrial and nuclear genes associated with budding cycles

Zooids of the asexual strain of Polyandrocarpa misakiensis have a lifespan of 4-5 months; before dying, they produce many buds, enabling continuation of the strain. This study was designed to investigate the nature of gene inactivation and reactivation during this continuous process of senescence and budding. During senescence, the zooidal epidermis showed acid β-galactosidase activity, lost proliferating cell nuclear antigen immunoreactivity and became ultrastructurally worn, indicating that the epidermis is a major tissue affected by the ageing process. Semi-quantitative PCR analysis showed that the genes encoding mitochondrial respiratory chains (MRCs) engaged in decreased transcriptional activity in senescent adults compared with younger adults. The results of in situ hybridization showed that the epidermis dramatically attenuates MRC expression during ageing but restores gene activity when budding commences. During budding and ageing, the nuclear gene Eed (a polycomb group component) was activated and inactivated in a pattern similar to that observed in MRCs. In buds, RNA interference (RNAi) of Eed attenuated Eed transcripts but did not affect the gene expression of pre-activated MRCs. A tunicate humoral factor, TC14-3, could induce Eed, accompanying the reactivation of MRC in adult zooids. When RNAi of Eed and Eed induction were performed simultaneously, zooidal cells and tissues failed to engage in MRC reactivation, indicating the involvement of Eed in MRC activation. Results of this study provide evidence that the mitochondrial gene activities of Polyandrocarpa can be reversed during senescence and budding, suggesting that they are regulated by nuclear polycomb group genes.