Schematic representation of the main types of life cycles with alternation of the diploid and haploid phase. 1 - meiosis (reduction); 2 - mitosis (replication); 3 - sexual process (recombination). A - meiosis precedes the formation of meiospores ( vascular plants ); B - meiosis precedes the formation of gametes ( animals ); C - meiosis immediately follows the formation of zygotes ( fungi ). 2n, double lines - diploid phase; n, single lines - haploid phase.
The life cycle is a regular change of all generations (ontogenesis) characteristic of this type of living organisms. It is necessary to clearly distinguish the life cycle (characteristic of the species) from ontogenesis (development of an individual from the moment of its appearance until the moment of death or division).
Content
Classification
By the nature of changes in ploidy , the following types of life cycles are distinguished:
- Haplophase: the organism has a single set of chromosomes (n, haploid) throughout the cycle; These life cycles are characteristic of many bacteria and protists who do not have the sexual process.
- Haplophase with zygotic reduction: after the formation of a zygote (2n), meiosis occurs , and the remaining stages of the life cycle are haploid (this life cycle is characteristic of many green and other algae , as well as most other groups of protists and fungi).
- Diplophase with gametic reduction: all stages of the life cycle are diploid (2n), only gametes are haploid (n) - the life cycle characteristic of multicellular animals.
- Haplo-diplophase: both at diploid and haploid stages, mitotic divisions occur, leading to reproduction or growth, and diploid and haploid generations are present in the life cycle — life cycle with intermediate (sporic) reduction is characteristic of all higher plants, as well as Foraminifera and some other protists.
By the number of generations (ontogeny) in the life cycle [1] :
- Simple: the cycle includes one generation.
- Complex: the cycle includes two or more generations; such a life cycle is typical, for example, for many cnidarians (alternation of generations of polyps and jellyfish ), for most trematodes (alternation of generations of marit , sporocest and redium ).
Notes
- ↑ A. I. Granovich, A. N. Ostrovsky, A. A. Dobrovolsky. Morphoprocess and life cycles of organisms. // Journal of General Biology. Vol. 71, No. 6, 2010.
Literature
- Zakhvatkin A.A. Comparative embryology of lower invertebrates. - “Soviet science”. - M. , 1949. - p. 138-140. - 395 s.
- South R., Uittik A. Chapter 5. Reproduction and development cycles // Fundamentals of Algology = Introduction to Phycology / Per. Tarasov K. L .. - M .: "Peace", 1990. - 597 p. - 3000 copies - ISBN 5-03-001522-1 .
See also
- Heterogony
- Metagenesis (biology)