The fertilization in the plant world is extremely diverse and often very complicated, but essentially boils down to the confluence of the male and fe

Fertilization in algae [lower plants, Thallophyta]

The fertilization in the plant world is extremely diverse and often very complicated, but essentially boils down to the confluence of the male and female germ cells. It is inherent in all higher plants and in all multicellular and certain unicellular algae.

In almost all cases, the gametes are formed in the sex organs, which are called gametangia (from Greek “gamete” and “angenion” — “jar”). The only exception is single-celled organisms.

In Chlamydomonas algae, as well as in any other single-celled organism, the real sex organs cannot be formed. So it becomes gametangia by itself. At the same time the cell loses flagella, and in it a series of successive mitosis occur, leading to the formation of two to eight flagellated gametes. Externally, they are exactly the same, but are divided into two groups, differing with physiological properties. One group consists of gametes, called “-” gametes, and in another — gametes, called “+” gametes. They come out of the mother cell and copulate with each other. Only cells with opposite properties can merge. As a result of copulation, forms a zygote without flagella in which meiosis occurs, the consequence of which is the appearance of four new young Chlamydomonas. Taken from http://worldofschool.org

In more highly organized multicellular algae gametes are produced in special cells, which can already be considered as a sex organs. Large fixed, devoid of flagella gamete are a female. They are produced in the female organ — oogonia (from Greek “oon” and “gonia” — “descendant”; it should not be confused with oogonia — cells, the precursors of egg cells in animals). The small mobile gametes are a male. They develop in the male reproductive organs — antheridia (from Greek “anteros” — “blossoming”). Normally fertilization occurs in algae in the water, but in some species, such as in Volvox, the egg remains in oogonia, where gets fertilized by the male gamete.

Questions:
  • How does the fertilization occur in unicellular organisms?

  • Why the fertilization hasn’t appeared in amoebas?