Kawamura K, Fujiwara S
Zoolog Sci. 2000 Apr 1;17(3):281-91. doi: 10.2108/jsz.17.281.
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.
人们一直认为，通过无性繁殖进行的克隆繁殖对长期生存具有严重的不利之处，因为无性繁殖似乎无法消除有害突变，似乎无法产生进化所依赖的遗传变异，而且似乎无法重置细胞衰老。在本文中，我们通过回顾海鞘Polyandrocarpa misakiensis的无性繁殖来重新审视这些观点。利用形态学和嵌合表型对芽形成和生长进行的追踪实验表明，围绕芽原基的亲代上皮组织不会进入生长中的芽。因此，可以假设出芽涉及清除大量亲代体细胞和组织。与有性生殖不同，无性生殖不会进行减数分裂重组和基因重排，而这是遗传变异的两个主要来源，但我们可以证明，在Polyandrocarpa misakiensis中，即使在克隆群体中，至少有两个基因也具有显著的冗余性和遗传变异。在生殖细胞中表达的端粒酶被认为可以重置由端粒缩短执行的分子时钟。在我们的Polyandrocarpa cDNA项目中，在大约2000个检测的cDNA中，有4个与作为端粒酶催化亚基的逆转录病毒逆转录酶相匹配，这表明端粒酶可能在无性繁殖中起作用。在Polyandrocarpa misakiensis中，去分化系统被用于产生新的无性世代。TC14凝集素在维持形成组织的多能但已分化状态中起重要作用。它受到海鞘丝氨酸蛋白酶（TRAMP）的拮抗，TRAMP对多能细胞具有显著的促有丝分裂和诱导去分化活性。这个系统将有助于延缓体细胞衰老。总之，关于无性繁殖不利于长期生存这一经验性观点，对于Polyandrocarpa misakiensis的出芽现象似乎并不成立。
