Fertilisation

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

There are different ways this joining can happen. For example, in plants, pollination brings the male part to the female part, but the actual joining of cells is what we call fertilisation. In some plants, called angiosperms, a special thing happens called double fertilisation, where one cell joins with two others to help make food for the growing baby plant.

Fertilisation is a key part of sexual reproduction, which is how most animals and many plants make new individuals. It is important because it helps create new life and keeps species going. For humans specifically, 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. Much later, in 1784, 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. In 1827, another scientist, Karl Ernst von Baer, was the first to see an egg from a mammal. Then, in 1876, Oscar Hertwig in Germany watched how tiny parts from a father and mother come together in sea urchins.

Evolution

The way fertilisation developed is connected to how cells divide and 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 (male) and egg (female) cells come together to form a new plant. In some plants, this happens inside a special structure, while in others, a tiny tube grows to carry the sperm to the egg.

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

Fertilisation in animals

See also: Sexual reproduction in animals and Animal sexual behaviour

Scientists have studied how fertilisation works in sea urchins and mice. This helps us understand how sperm and eggs meet and how just one sperm enters the egg. There are three main steps in fertilisation that make sure the right species can combine:

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

Internal vs. external

Animals can use two different 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. This means the sperm reaches the egg without having to go through the thick shell. Other animals, like some fish, use external fertilisation. The eggs are fertilised outside the body, which can help reduce disease and allow for more genetic mixing.

Sea urchins

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

Mammals

In mammals, sperm are released into the vagina and travel through the reproductive system to meet the egg. This journey involves changes in the sperm that help them reach and fertilise 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 unique 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 allows them to fertilise eggs even after mating has occurred, sometimes many days or even years later.

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, making a special cell with many centers. This cell can then grow and change. The second step is when the centers join to make a new cell that can grow into something new.

In chytrid fungi, this joining happens all at once, like in animals and plants.

Fertilisation in protists

Fertilisation in protozoa

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

• gametogamy
• autogamy
• gamontogamy

Fertilisation in algae

Algae, like some plants on land, 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 organisms reproduce, they go through a process called meiosis, which mixes up 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 a huge number of 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

After a sperm joins with an egg, two tiny parts from the sperm form 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 the parts get closer, the center point splits into two, helping guide everything as it begins to grow into a new organism.

Parthenogenesis

Main article: Parthenogenesis

Some plants and animals can make new babies without needing to join their cells with 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 might be just like the mother, or it might have some differences, but it will always get some of its DNA from her. Scientists have even made this happen in mice by using special tricks on the eggs.

Allogamy and autogamy

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

Autogamy, or self-fertilisation, happens in some plants and flatworms. In this case, two gametes 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 avoid problems that can happen when similar relatives have babies together. Charles Darwin studied this in plants and found that babies from two different parents grow better than babies from the same parent. This helps all living things stay strong and healthy. It also helps them change and adapt to new situations, making it more likely they will survive in the future.