Insects like grasshoppers do not have the Y chromosome, so they have XO males and XX females. Birds and moths have females with ZW and ZO chromosomes. Additionally, insects such as ants and honey bees exhibit haplodiploidy. Furthermore, sex determination in certain species is influenced by temperature.
What determines whether an organism develops into a male or female? While it is true that XX represents females and XY represents males, this system is only predominant in a few species, such as humans.
Not all animals rely on the Y chromosome and the SRY gene from the male parent to determine the sex of the embryo. In fact, there are various sex determination systems in nature. Different organisms have evolved mechanisms that work best for them. Let’s explore some of the most common methods.
What Is Sex Determination?
Sex determination is a mechanism that determines the sexual fate of an organism. This mechanism can be influenced by genetic factors, such as genes on sex chromosomes, or by environmental factors, such as temperature. In either case, the main change induced by genetics or the environment is in the hormone levels, particularly estrogen and testosterone. These hormones then influence the sex of the embryo. Estrogen plays a major role in the development of female sex characteristics, while testosterone is crucial for the development of male sex characteristics. The SRY gene in humans is an example of genetic influence in sex determination, but there are other genes that influence sex differentiation in different species.
Birds
In birds, the ZW-ZZ system is in place. The gene DMRT1 on the Z chromosome acts as the master switch. When a single copy of the gene is present (ZW), the embryo develops as a female, but when two copies are present (ZZ), male characteristics develop. A female’s egg carries either a Z or W chromosome, while a sperm always has a Z chromosome. Therefore, the presence of a Z/W chromosome in the egg determines the primary sex.
Grasshoppers
In some grasshopper species, the XX-XO system is in operation. Males do not have a Y chromosome, and the “O” represents the absence of the X pair. Females, on the other hand, have two sex chromosomes (XX).
Male grasshoppers have only one X chromosome, so they produce two distinct gametes – one with an X chromosome and one without. Female gametes, on the other hand, are all of the X type. Therefore, the fertilization of the egg by a male gamete with an X or no sex chromosome determines the sex of the embryo.
Moths
In certain animals, it is not always the male that carries the different pair of chromosomes. This is the case in some moths, where the females are heterogametic and produce different gametes.
Moths have a ZO-ZZ system, where male moths have ZZ sex chromosomes and produce a single type of gamete (Z), while females have ZO allosomes (sex chromosomes).
During gamete formation, the Z chromosome behaves as a univalent, as it has no partner to pair with. It simply moves to one of the two gametes formed. The other gamete does not receive a sex chromosome at all. Therefore, the distribution of the Z chromosome from the female parent determines the sex of the embryo.
Honeybees And Ants
Insects like ants and bees have haplodiploidy, where the sex is determined by the CSD (complementary sex determiner) gene, rather than X or Y chromosomes.
Female bees have heterozygous CSD genes, while males have only a single copy. This is similar to the XO/ZO system, but with a twist. Unlike XO/ZO systems, where autosomes are received from both parents, male honeybees and ants only receive chromosomes from the female parent (the Queen). This means they develop from unfertilized eggs and do not have a father. Fertilized eggs produce female progeny.
The Queen bee is the largest female and the egg-layer of the colony. When the egg is fertilized by sperm from a drone, it develops into a diploid worker (female). Unfertilized eggs develop into haploid drones (males). (Photo Credit: -kanyanat wongsa/Shutterstock)
In rare cases, if a diploid male honeybee does develop, the worker bees (female) kill them.
Temperature-dependent Sex Determination
Some animals do not rely on genetic factors for sex determination, but rather on environmental factors.
The temperature stimulates hormones and enzymes that aid in sexual differentiation, thereby controlling the sex determination.
There are three main patterns of environmental effects in nature. The first pattern is seen in many turtle species, where females are more common when temperatures are above a certain critical temperature, while males develop when temperatures are below the critical temperature. The second pattern is observed in reptiles like Rainbow lizards, where males are found at higher temperatures and females develop at lower temperatures. The third pattern is seen in crocodiles, where females are dominant at extreme high and low temperatures, while males grow at intermediate temperatures.
In the case of Rainbow lizards, the dominant male has distinct breeding colors, while the females and submissive males have different colors. The eggs of these lizards develop into females at lower temperatures and males at higher temperatures.
In conclusion, sex determination is not solely determined by the X and Y chromosomes. Other factors, such as female heterogamety, haplodiploidy, and the environment, can also play a role. It is important to note that an individual’s genetic makeup cannot always be determined based on their phenotype. For example, there are cases of XO females (Turner syndrome) and XXY males (Klinefelter syndrome) in humans. This flexibility in sex determination is not limited to humans, as other organisms like birds also have the potential to change their primary sex. Overall, the science behind the sex of organisms is more complex and adaptable than we may have previously thought!