When two filaments contact each other, a protrusion grows inwards between opposite pairs of cells.

The walls inbetween disappear forming a conjugation tube.

The protoplasm of each cell of one filament rolls up into a sphere and moves across the conjugation tube to fuse with the protoplasm of the cells of the opposite filament forming a zygote (2N).

Zygote becomes coated by a thick wall that protects it along unsuitable conditions and is then called zygospore (2N).

The zygospore remains dormant till the surrounding conditions improve then it germinates to a new filament.

B. Lateral conjugation:

This conjugation occurs between the adjacent cells of the same filament. The protoplasm of one cell moves to the adjacent cell through an opening in the wall inbetween them.

N.B.:

Cells of the algal filament are haploid (N), and after conjugation, the zygote (2N) is formed, then the zygospore (2N) forms. It divides by meiosis before germination of the new filament leading to the haploidity of the algal cells once more.

2. Reproduction by sexual gametes:

Higher plants and animals reproduce by means of sexual gametes that are differentiated into 2 kinds: male gametes and female gametes. Both are derived from meiosis that takes place in gonads.

a. Characteristics of male gametes:

Have the ability for locomotion.

Are adapted to locomotion, as the lose much of its cytoplasm.

Their bodies become pointed and provided with a locomotary tail or flagellum to help transport the genetic material to the female gamete during fertilization.

4. Since some of them are subjected to loss, 4 male gametes are produced from each original cell.

b. Characteristics of female gametes:

They are formed in the female ovary, and usually remain stationary within the female body till fertilization.

They are spherical.

3. They are enriched with food, where they are produced in few numbers.

Means of transportation of male gametes to meet female gametes:

Male gametes may be transported to the female via water, as in aquatic animals like bony fishes and toads where both the male and the female shed their gametes in water where external fertilization occurs and the embryo develops in water.

2. In terrestrial animals, fertilization takes place internally. Sperms should be introduced to the eggs (ova) that remain within the female body till fertilization.

N.B.:

Fertilization:

Fertilization is the fusion of the male gamete nucleus (N) with that of the female (N) to form the zygote (2N) that resumes its diploid nature, and passes towards embryonic development by means of mitosis.

Alternation of Generations:

Some plant and animal species can breed both Asexually and Sexually in an alternation of generations during their life cycles. They gain from both methods their advantages of rapid production and genetic diversity. These enable them to disperse widely and to conform to the environmental fluctuations. This may associated with variation in chromosome number of these generations. This phenomenon can be shown by the following examples:

1. Life Cycle of Plasmodium (Malaria Parasite):

This is a sporozoan parasite that belongs to Protozoa that infects both Man and female of Anopheles mosquito. Its life cycle starts when the infected mosquito bites the Human skin.

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