Fertilisation

Fertilisation is the way new living things are made when special cells from parents join together. In animals and plants, this happens when two types of cells, called gametes, come together to make one cell. This new cell is called a zygote. It then grows and develops into a baby animal or plant.

There are different ways this joining can happen. In plants, pollination carries the male part to the female part, but it is the joining of cells that is called fertilisation. In some plants, called angiosperms, there is a process named double fertilisation, where one cell joins with two others to help make food for the growing baby plant.

Fertilisation is an important part of sexual reproduction. This is how most animals and many plants make new individuals. It helps create new life and keeps species going. For humans, you can read more at Human fertilization.

Discovery

Long ago, a thinker named Aristotle thought that new living things came from a mix of fluids from both parents. Later, a scientist named Spallanzani showed that a tiny part from a mother and a tiny part from a father need to join to start a new life in frogs. After that, Karl Ernst von Baer was the first to see an egg from a mammal. Finally, Oscar Hertwig watched how tiny parts from a father and mother come together in sea urchins.

Evolution

The way fertilisation developed is linked to how cells divide to create new life. Both fertilisation and a special way cells split are parts of how animals and plants make new individuals. One idea is that this cell splitting began from a simpler way cells divide.

Fertilisation in plants

In plants, the sperm and egg come together to make a new plant. In some plants, this happens inside a special part, and in others, a tiny tube carries the sperm to the egg.

In flowering plants, two sperm cells help. One sperm joins with the egg to start a new plant, and the other helps make food for the growing seed.

Fertilisation in animals

See also: Sexual reproduction in animals and Animal sexual behaviour

Scientists study how fertilisation works in sea urchins and mice. This helps us learn how sperm and eggs meet. There are three main steps in fertilisation:

1. Chemotaxis
2. Sperm activation/acrosomal reaction
3. Sperm/egg adhesion

Internal vs. external

Animals can use two ways of fertilisation: internal or external. This depends on how they lay their eggs. Animals like chickens, which lay eggs with thick shells, use internal fertilisation. Other animals, like some fish, use external fertilisation. The eggs are fertilised outside the body.

Sea urchins

Sperm find eggs using a process called chemotaxis. After finding the egg, the sperm goes through a process to break through the egg’s outer layer. This lets the sperm bind to the egg and start forming a new organism.

Mammals

In mammals, sperm are released into the vagina and travel to meet the egg. Once fertilised, the egg becomes a zygote, which then develops and implants in the uterus to start pregnancy.

Humans

Main article: Human fertilisation

In humans, fertilisation happens when a sperm meets an egg in the fallopian tube, creating a zygote. This is the first step in developing a new individual. Scientists first learned about human fertilisation in the nineteenth century.

Insects

Some insects, like dragonflies and bees, can store sperm for later use. This lets them fertilise eggs even after mating has occurred.

Fertilisation in fungi

Main article: Mating in fungi

In many fungi (except chytrids), joining together happens in two steps. First, the inner parts of two cells come together. The second step is when the centers join to make a new cell that can grow.

In chytrid fungi, this joining happens all at once.

Fertilisation in protists

Fertilisation in protozoa

Protozoa have three ways to join their tiny cells to make new life:

• gametogamy
• autogamy
• gamontogamy

Fertilisation in algae

Algae, like some plants, have a special way of growing. They switch between two forms. Some algae look the same in both forms. When they make new life, the tiny cells from the mother and father look different. One is a big egg that doesn’t move, and the other can move. When they join together, they make a new cell that starts the cycle again.

Fertilisation and genetic recombination

When animals and plants have babies, they go through a process called meiosis. This mixes the genes from each parent. This makes each gamete, or reproductive cell, genetically unique. When fertilisation happens, the chromosomes from both parents come together.

In humans, this mixing can create many different zygotes, or new cells that start the development of a baby. The exact number depends on whether certain genetic events, called crossover, happen. Mitochondrial DNA, which is a special kind of DNA, comes only from the mother.

The sperm aster and zygote centrosomes

When a sperm joins with an egg, two tiny parts from the sperm make the first center point and star-shaped structures in the embryo. These structures help move the parts of the egg and sperm closer together. As they get closer, the center point splits into two, helping guide everything as it starts to grow into a new organism.

Parthenogenesis

Main article: Parthenogenesis

Some plants and animals can make new babies without needing another animal. This is called parthenogenesis. In this way, an egg from a mother can grow into a new baby all by itself. The new baby will be like the mother, but it might look a little different.

Allogamy and autogamy

Allogamy, also called cross-fertilisation, is when an egg from one individual joins with a sperm cell from another individual.

Autogamy, or self-fertilisation, happens in some plants and flatworms. In this case, two sperm cells from the same individual come together.

Other variants of bisexual reproduction

There are some special ways that animals can reproduce. In gynogenesis, a tiny part from a male helps an egg start growing, but the egg does not join with the male part fully. In hybridogenesis, one part of the genetic material is removed to make eggs. In canina meiosis, some genetic material is passed down normally, while other parts are copied exactly without changing.

Benefits of cross-fertilisation

Main articles: Allogamy and Heterosis

Cross-fertilisation helps plants and animals stay strong and healthy. When two different parents have babies, the babies often grow better than when the same parent has babies together. This idea was studied by Charles Darwin in plants. It helps living things change and adapt, making it more likely they will survive in the future.