Sexual reproduction: gametes, zygote and conjugation in Spirogyra
Sexual reproduction needs a male and a female individual, each making gametes. A male gamete and a female gamete meet and fuse. This fusion is fertilization, and it gives a zygote. The zygote divides and grows into an embryo, then a young one, then an adult that combines the features of both parents. The young therefore receive nuclear material from both parents and become a blend, whereas the young of asexual reproduction receive it from one parent and are copies. Sexual reproduction costs more time and energy, and only the females give new individuals, so fewer new individuals appear than in asexual reproduction. But it gives the offspring new combinations of features, and this helps them carry on when the surroundings vary. It rests on meiosis: in an animal, when gametes are made the number of chromosomes is halved, so gametes are haploid (N); at fertilization the two nuclei fuse and the number returns to 2N. One main way of sexual reproduction is conjugation. Some primitive organisms, including some protozoa, algae and fungi, reproduce by mitosis in good conditions but turn to conjugation when they meet drought or a change in the temperature or purity of the water. Spirogyra is an alga of still fresh water. Its filaments are single rows of cells. In scalariform conjugation two filaments lie side by side, a protrusion grows between opposite cells, and the wall between them disappears to make a conjugation tube. The contents of each cell of one filament round up and move across the tube into the opposite cell, and a zygote forms. The zygote gets a thick wall and is called a zygospore. It rests until conditions improve, then its nucleus divides by meiosis into four haploid nuclei. Three degenerate and the fourth divides by mitosis to make a new filament. In lateral conjugation the contents of one cell move into the neighbouring cell of the same filament through an opening in the wall between them. The cells of the filament are haploid (N) and the zygote is diploid (2N).
| Asexual | Sexual | |
|---|---|---|
| Parents | one | two (male and female) |
| Cell division behind it | mitosis | meiosis for gametes, then fertilization |
| Offspring | copies of the parent | a blend of both parents |
| Time and energy | less | more |
Model the steps, with a grown-up. Take two strips of paper to be two filaments and draw six cells on each. Cut a small link between a pair of facing cells (this is paper, not a living thing). Slide a coloured counter from one cell to the opposite cell, and write zygote (2N) on that cell. Then write, in three short lines, what happens to the zygote when conditions improve, and which of the four nuclei survive.