Advantage or Disadvantage: Is Asexual Reproduction Beneficial to Survival of the Tunicate, Polyandrocarpa misakiensis.
It has been believed that clonal propagation by asexual reproduction has serious disadvantages for long-term survival, because asexual reproduction seems not to remove harmful mutations, it seems not to give rise to genetic variations upon which evolution depends and it seems not to reset cell aging. In this article, we re-consider those arguments, by reviewing asexual reproduction of the tunicate, Polyandrocarpa misakiensis. Tracer experiments of bud formation and growth using morphological and chimeric phenotypes showed that the parental epithelial tissues surrounding the bud primordium do not enter the growing bud. It is possible, therefore, to assume that budding involves the purge of a large number of parental somatic cells and tissues. Unlike sexuals, asexuals do not carry out meiotic recombination nor gene shuffling that are two major sources of genetic variation, but we can show that in P. misakiensis at least two genes have significant redundancy and genetic variation even in a clonal colony. Telomerase expressed in germlines is thought to reset the molecular clock executed by telomere shortening. In our Polyandrocarpa cDNA projects, four out of about 2,000 cDNAs examined were matched with retroviral reverse transcriptase that is the catalytic subunit of telomerase, suggesting that telomerase might work in asexual reproduction. In P. misakiensis, dedifferentiation system is used to make new asexual generations. TC14 lectin plays an important role in the maintenance of multipotent but differentiated state of the formative tissue. It is antagonized by tunicate serine protease (TRAMP) that has striking mitogenic and dedifferentiation-inducing activities on the multipotent cells. This system would serve to delay aging of somatic cells. In conclusion, empirical arguments that asexual reproduction is disadvantageous to long-term life do not appear to be tenable to budding of P. misakiensis.
Kawamura
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Fujiwara
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