title: Degeneration after sexual differentiation in hydra and its relevance to the evolution of aging.
abstract: Aging occurs in most multicellular animals, yet some primitive animals do not show any sign of aging. This raises the following question: How have metazoans acquired the trait of aging in the course of evolution? Comparative studies of various species have provided a clue to this question by showing that sexually reproducing organisms predominantly undergo aging. The evolutionary theory "pleiotropy" also postulates aging as a price for facilitating the reproduction in the early life stage of an organism. For investigating the association between sexual reproduction and aging, a sexual phase-inducible organism in a laboratory would be suitable. One of such organisms is hydra, a genus of Cnidaria. Asexual hydra has been considered to be immortal, but there is the possibility that hydra undergoes aging after sexual reproduction. To search for signs of aging in hydra, we studied sexually differentiated Hydra oligactis at the individual and cellular levels. As a result, we found a significant decline in the capacities for food capture, contractile movements, and reproduction. More importantly, we discovered an exponential increase in the mortality rate of the population. These observations suggest that the degenerative process in H. oligactis represents the aging process. Furthermore, we found that the number of germ cells increased, whereas the number of somatic cells concomitantly decreased. The observed change of the cell composition is thus consistent with the "pleiotropy" theory of aging.
authors.affiliation: Center for Information Biology and DNA Data Bank of Japan, National Institute of Genetics, Yata 1111, Mishima, Shizuoka 411-8540, Japan.
authors.forename: Kengo
authors.initials: K
authors.lastname: Yoshida
authors.forename: Toshitaka
authors.initials: T
authors.lastname: Fujisawa
authors.forename: Jung Shan
authors.initials: JS
authors.lastname: Hwang
authors.forename: Kazuho
authors.initials: K
authors.lastname: Ikeo
authors.forename: Takashi
authors.initials: T
authors.lastname: Gojobori
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005260:Female; D006829:Hydra / Q000235:genetics* / Q000254:growth & development* / Q000502:physiology; D008297:Male; D008957:Models, Genetic; D012098:Reproduction; D012733:Sex Differentiation / Q000235:genetics*; D013045:Species Specificity; D013997:Time Factors