Sexual reproduction - is a reproduction by organisms of the similar organisms using gametes, accompanied by the sexual process. Sexual reproduction

Sexual reproduction [propagation]

Sexual reproduction — is a reproduction by organisms of the similar organisms using gametes, accompanied by the sexual process. Sexual reproduction occurs only in multicellular organisms that have two generations — haploid and diploid.

Sexual reproduction is inherent in all multicellular organisms, in which there is an alternation of haploid and diploid generations. This is a process of duplication of organisms due to special sex cells — gametes (from Greek “gamete” — “wife”). Unlike typical diploid or haploid unicellular spores of plants and fungi from which new organisms develop, gametes are haploid. To give a birth to new diploid organisms, they must at first merge with each other. This fusion of gametes is called a sexual process or fertilization.

Small mobile gamete — the spermatozoon (from Greek “spermatos” — “seeds”, “zon” — “life”) penetrates the huge immobile ovum (human ovum is bigger than the sperm in millions of times). The result is a zygote (from Greek “zigotos” — “twice”) from which a new organism develops. The massive size of eggs (diagram 4) is due to the fact that in their cytoplasm contains a large amount of yolk — all kinds of nutrients needed as a source of energy in the first stages of development of the zygote. Thus, the key process of sexual reproduction is the fertilization — the fusion of two sex cells, when the nuclei of gametes merge to form one common nucleus of the zygote.

Diagram 4. Male gametes — the sperm are on their way to the female gamete — the egg

In all multicellular animals, higher plants, and many fungi gametes are markedly different from each other. They can easily be divided into immobile female eggs and small, often mobile male sperm. But in seed plants male gametes — the sperm — don’t have flagella and are delivered to the egg via the pollen tube. However, the in most primitive organisms that reproduce sexually, for example, some algae, sex cells apparently do not differ from each other, and they may be divided on men's and women’s only conditionally.

Organisms that produce male gametes — sperm are considered to be a male individuals (in animals they are indicated by ♂), and organisms that produce eggs — indicated by ♀. Sex cells develop in specific organs. In animals, the male sex organs are testes, female — ovary. Plants in the process of evolution have lost their sex organs. Therefore sperm of angiosperms form in the male gametophyte — pollen grains, and eggs — in the female gametophyte — embryo sacs. Taken from http://worldofschool.org

Diagram 5. Hermaphroditic animals: a — Diphyllobothrium latum (tapeworm); b — ordinary earthworm; c — a freshwater clam — great pond snail; d — ground mollusk — Burgundy snail

Most interesting is that dioecism — property not of all organisms. Mushrooms and most species of higher plants — are bisexual beings, and therefore they have no division into male and female: the same organism produces female and male gametes. In animals bisexual individuals (diagram 5) are called hermaphrodites (from Greek “Hermaphrodite” — “the son of Hermes and Aphrodite”). And in flatworms (planarians, flukes, tapeworms) male and female gametes are formed simultaneously in the ovaries and testes. Gastropods during life change their sexual orientation. Great pond snail at first is a male, and then both male and female. At the same time his male and female gametes are produced by the same androgynous sex gland. Bisexual in higher animals is not peculiar. In insects and vertebrate specimens-hermaphrodites are extremely rare; they cannot produce offspring.

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Questions:
  • What is sexual reproduction?

  • What are the features of hermaphroditism of flatworms and gastropods?

  • Why are most primitive multicellular organisms, gametes not divided into male and female?

  • Why in animals via the process of meiotic divisions form gametes and in plants — spores?