Researchers at the University of Oregon have discovered a 480-million-year-old sex chromosome in the California two-spot octopus – before the evolutionary split between octopuses and nautiluses.
Researchers at the University of Oregon have discovered a sex chromosome in the California two-spot octopus that is 480 million years old—before the evolutionary split between octopuses and nautiluses. This finding makes it one of the oldest known sex chromosomes. It also confirms that mollusks like octopuses use chromosomes to determine sex. The chromosome was detected in female octopuses but not in males, and another study found it in other mollusks as well, indicating an ancient and stable system.
How is sex determined in animals?
In humans and many mammals, sex is determined primarily by chromosomes. However, there is considerable variation among animals. For example, in turtles, sex is determined by the temperature at which the eggs are kept during embryonic development. In some fish, sex is determined by a single gene rather than an entire chromosome. Even in humans, the X/Y sex chromosome system is not absolute, and there are different genetic conditions that produce gender differences.
Because mollusks, like octopuses, are not standard laboratory animals like mice or fruit flies, not many genetic studies have been conducted on them. Researchers at the University of Oregon, who recently sequenced the DNA of a female two-spot octopus, found a chromosome that contained only half the genetic material found in other chromosomes. This chromosome was not found in previously tested males, leading the researchers to conclude that it was a sex chromosome.
Tracing the evolutionary history of the chromosome
To confirm their findings, the researchers examined existing genome data from other octopus species. They found that a similar chromosome was also found in other octopus species and even in squid, which split off from octopuses about 248 to 455 million years ago. After further study, they found evidence of the chromosome in the nautilus, a mollusk that split off from octopuses about 480 million years ago.
The presence of the chromosome in such diverse species suggests that their sex-determination system has been conserved over hundreds of millions of years. This finding is surprising, as sex chromosomes tend to undergo rapid changes due to evolutionary selection pressures. However, octopuses seem to have found a system that works well for them and have stuck to it over the generations.
Ancient sex chromosomes have also been found in plant groups such as mosses, which were among the first plants to evolve. Insect sex chromosomes are also about 450 million years old, but they have changed greatly over time.
At first, researchers thought that the sex determination system of octopuses was similar to that of birds and butterflies, where males have a ZZ chromosome and females have a ZW chromosome. However, so far they have not been able to find a W chromosome in octopuses. It is possible that their sex determination system is based only on the Z chromosome, with males having two copies and females only one. This is a topic for future research, as the octopus still keeps some of its secrets.
More of the topic in Hayadan:
