A gonad is a specialized organ that contains germ cells. Germ cells are responsible for producing the haploid cells required for sexual reproduction. Therefore, gonads – specifically testes and ovaries in humans – allow for sexual reproduction. Typically, in sexually reproducing animals each organism has a set number of chromosomes. In order to create offspring, these chromosomes must be divided equally in order to create an offspring with the same number of chromosomes.
Consider humans for example. Humans have 23 pairs of chromosomes in their cells. You received 23 chromosomes from your mother and 23 from your father. Because these chromosomes represent the same portions of DNA, the pairs that are alike group into 1 chromosome, containing sister chromatids. Therefore, humans have 46 chromosomes in each cell.
Here is where the gonad comes in. Imagine if two humans reproduced without dividing their chromosomes first. 46 + 46 = 92 chromosomes! While some organisms (like plants) sometimes benefit from extra copies of the same chromosome, this condition is detrimental in humans and many other animals. Therefore, before reproducing, an animal must use the germ cells in a gonad to reduce the DNA to 23 chromosomes that do not have pairs. Cells with half the genetic material are known as haploid cells (or gametes), as opposed to the typical diploid cells.
Function of a Gonad
To make haploid cells in humans, a gonad facilitates a special type of cell division known as meiosis. During meiosis, a diploid cell’s DNA is doubled, creating 92 chromosomes, which are then split into 4 cells. Each cell is unique and contains 23 unpaired chromosomes. These cells will become the eggs and sperm involved in sexual reproduction.
A gonad in another organism functions in the exact same way, although the number of chromosomes in each organism varies. In organisms with complex hormonal systems, the gonads are often involved in the regulation of hormones. This can be seen in humans. Male testes have been found to be involved in the production and regulation of testosterone, while female ovaries produce a variety of hormones involved in ovulation and pregnancy, the main hormone of which is estrogen.
The gonad is often connected to the sexual organ, and aids in the release of gametes. This is seen in species that rely on internal fertilization, or the act of copulating. Other species rely on casting their gametes into the environment. In these species, the gonads themselves release the gametes. This can be seen in flowers. The gonad that produces the pollen is the same organ used to release the pollen into the air and onto bees. The pollen is then transferred to the stigma, or female gonad, of another flower. This is where fertilization occurs.
Examples of Gonads
A gonad can exist in many forms throughout the animal kingdom, but a general pattern of males having testes and females having ovaries can be seen. In animals that are sexually differentiated, each gender contains only one type of gonad (male or female). In some hermaphroditic animals, such as earthworms, each organism contains both male and female gonads. In other organisms
A Typical Gonad
Through a human lens, the “typical” arrangement of gonads is that of two distinct genders. This is known as sexual dimorphism. Many sexually dimorphic creatures are also sexually differentiated. This means that each gender receives 1 type of gonad, and produces 1 type of gamete. In many animals, this can be seen as males and females. Males produce sperm, and females producing eggs.
This pattern of life is used in the smallest of creatures to the largest, and it is by far the most common form of reproduction. In this form, two main types of gonad exist – ovaries and testis. Males use testis to produce sperm. These haploid gametes typically have tails that can power them toward the eggs, increasing the chances of fertilization. Female eggs are always produced in the ovaries, but eggs can take a variety of forms in different species.
In humans, the eggs remain in the ovary until ovulation, when the egg descends towards the uterus and can become fertilized. In birds, the egg is fertilized, protected in a hard shell, and kept warm in a nest while the embryo develops. In many sea creatures, eggs and sperm are simply cast into the environment on a synchronous cue. However, in all these forms, the simple gonad is the organ producing all of the gametes.
While it may seem like the above plan of sexual dimorphism is the most common, there are many organisms that exist with both male and female gonads. A common example is a typical earthworm. A single earthworm contains a male gonad and a female gonad. The male gonad may give gametes to another worm, and the female gonad can take gametes from the same worm.
One theory is that this gives earthworms an added benefit when mating because it allows the passage of genetics to go both ways. This is important to animals that may rarely meet underground, though they might only be inches from each other. This way, not 1, but 2 mating events can occur every time a pair of earthworms meet! Oftentimes it is necessary for organisms that are immobile, have poor senses, or travel individually to develop other means for reproducing successfully. This is often seen in the gonad of the animal becoming specialized in some way. In earthworms, it can be seen as the possession of both types of gonad.
The wrasse is a common coral-reef dwelling fish, of which there are many species. Certain species have a unique form of gonads and are known as sequential hermaphrodites. While all individuals possess both male and female gonads, only one individual has active ovaries. In this individual – the dominant female – the male gonad is suppressed and the female gonad is active. She produces eggs for the whole group, and the smaller fish fight to fertilize them.
The second-largest fish remains a male until the current female dies. At that time, it will be his turn to assume the role of head-female. As such, he will undergo a sexual metamorphosis in which his male gonad is suppressed and his female gonad is activated. The new female often becomes a different color and grows rapidly, in response to the quick change of hormones the fish experiences as the female gonads become active. This sequential hermaphroditism can be seen many species of fish – including clownfish and wrasses, as well as in some reptiles!