Nuclear phase alternation

A phenomenon in which cells with haploid (n) nuclei and cells with diploid (2n) nuclei alternate in sexually reproducing organisms. This phenomenon is also called karyophase alternation. Gametes are formed through meiosis and therefore have haploid nuclei, while zygotes, which are produced by the union or fusion of two haploid cells, have diploid nuclei. The zygotes and their developmental individuals eventually produce gametes with haploid nuclei through meiosis. This regular alternation of karyophase is called karyophase alternation. Although karyophase alternation is often confused with alternation of generations, the former focuses only on the change in the number of chromosomes and is different from the latter, which is a regular alternation of organisms with different reproductive methods. Furthermore, karyophase alternation can also occur in organisms in which alternation of generations is not recognized. For example, the zygote of the green alga Spirogyra, which has a diploid nucleus, undergoes meiosis when germinating to become an alga with a haploid nucleus, and the algae then fuse to produce a zygote. Conjugation is sexual reproduction, and algae are the sexual generation, but there is no asexual generation in this cycle, and therefore no alternation of generations. On the other hand, in mosses and ferns, meiosis is performed on the asexual sporophyte with a diploid nucleus, which produces haploid nucleated spores, which germinate to become haploid gametophytes (sexual generation). The gametophytes produce gametes, which combine (fertilize) to produce diploid zygotes, which develop into asexual diploid sporophytes. This is an example of alternation of karyophase and alternation of generations. In plants, alternation of karyophase generally coincides with alternation of generations, with alternation between haploid sexual and diploid asexual generations. However, there is a theory that there is no alternation of generations in seed plants because the sexual generation (pollen and embryo sacs) has degenerated. In vertebrates, individuals are composed of diploid cells, which produce haploid gametes (eggs and sperm) by meiosis. Thus, there is no asexual generation, only a sexual generation.

[Taku Shimada]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

有性生殖生物で染色体数が単相（n）の核をもつ細胞と複相（2n）の核をもつ細胞が交互に現れる現象。核相交番ともいう。配偶子は減数分裂を経て形成されるため単相の核をもち、2個の単相細胞の接合や合体により生ずる接合子は複相の核をもつ。接合子やその発育した個体はやがて減数分裂によって単相核をもつ配偶子をつくる。こうして核相が規則的に交代するのを核相交代という。核相交代はしばしば世代交代と混同されるが、前者は染色体数の変化だけに着目したものであり、生殖法を異にする生物個体が規則的に交代する後者とは異なる。また、核相交代は世代交代の認められない生物でもおこる。たとえば、複相核をもつ緑藻類のアオミドロの接合子は、発芽時に減数分裂して単相核をもつ藻体となり、藻体は接合して接合子をつくる。接合は有性生殖であり、藻体は有性世代であるが、このサイクルに無性世代はなく、したがって世代交代はない。一方、コケ類やシダ類は複相核をもつ無性世代の胞子体から、減数分裂により単相核をもつ胞子を生じ、胞子が発芽して単相核の配偶体（有性世代）となる。配偶体は配偶子を生じ、配偶子の合体（受精）により複相核の接合子ができ、接合子は発達して無性複相の胞子体となる。これは核相交代と世代交代が一致する例である。植物では一般に核相交代が世代交代と一致し、単相核の有性世代と複相核の無性世代が交代する。ただし、種子植物では有性世代（花粉と胚嚢(はいのう)）が退化しているため世代交代がないとする説もある。脊椎(せきつい)動物では個体は複相核の細胞からなるが、減数分裂によって単相核の配偶子（卵と精子）を生ずる。したがって、無性世代はなく有性世代のみである。

［嶋田　拓］

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例