Hermaphrodite - Hermaphrodite

In biology, this refers to a case in which both male and female characteristics are present in the same individual. Opposite of dioecy. In plants, it is also called monoecious. In animals, there are cases where ovaries and testes are present at the same time, such as in earthworms, and cases where both sexes are present in a single gonad, such as in snails. Even in hermaphrodites, fertilization (self-fertilization) between reproductive cells of both sexes within the same individual is rare. Slugs lay self-fertilized eggs only when there is no partner to mate with. In the ascidian, the layer of cells that surrounds the eggs makes it difficult for the sperm of the egg to pass through. On the other hand, in oysters and black porgy, the maturation of the male and female gonads is different at different times, so self-fertilization does not occur. In the tropical fish swordtail, sex change occurs easily and the sexes can alternate. Among frogs, there are some in which all individuals have ovaries when young, and some of them later change sex to become males, and some in which vestigial organs attached to the male testes develop to perform the functions of ovaries. In hermaphrodite animals, the number of chromosomes in eggs and sperm is generally the same, and it is thought that there is no distinction between sex chromosomes.

[Takeo Machida]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

生物学用語で、同一の個体に雌雄両性の形質がみられる場合をいう。雌雄異体の対語。植物では雌雄同株ともいう。動物では、ミミズのように卵巣と精巣とを同時に備えている場合と、カタツムリのように一つの生殖巣中に卵と精子とをもつ両性腺(せん)を備えている場合とがある。雌雄同体の動物でも、同一の個体内の両性の生殖細胞の間で受精（自家受精）することはほとんどない。ナメクジは、交尾する相手のいないときに初めて自家受精卵を産む。ユウレイボヤでは卵を包む被覆細胞層が自家の精子を通過させにくい。一方、カキやクロダイでは雌雄の生殖巣の成熟期が時間的にずれているため自家受精はおこらない。熱帯魚のソードテールでは性の転換が容易におこり、雌雄を交互に繰り返すことがある。カエルのなかには、幼時にはすべての個体が卵巣をもっており、やがて一部が性転換して雄になるものや、雄の精巣に付随する痕跡(こんせき)器官が卵巣の機能を果たすように発達するものなどがある。雌雄同体の動物では、一般に卵と精子の染色体数は同じで、性染色体の区別もないと考えられる。

［町田武生］

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