Some stick insects have reproduced without sex for around one million years, yet they still carry a fully functioning genetic system associated with sexual reproduction, according to surprising new research.
The discovery challenges a common assumption about evolution: that biological systems that are no longer useful will gradually break down or disappear.
The research, led by Dr. Darren Parker, a lecturer in evolutionary biology at Bangor University, examined several species of Timema stick insects.
The findings were published in the journal Proceedings of the National Academy of Sciences (PNAS).
Timema are small, wingless stick insects native to the western United States. While some species reproduce sexually, others have abandoned sex completely.
Instead, females reproduce through a process called parthenogenesis, sometimes described as “virgin birth.” Their eggs can develop into offspring without being fertilized by males.
Some Timema species have been reproducing this way for about one million years, making them the longest-known asexually reproducing insect lineages.
This gave scientists an unusual opportunity to investigate what happens to genetic systems that apparently have not been needed for an extremely long time.
The researchers focused on a process known as dosage compensation.
In sexually reproducing animals with sex chromosomes, males and females can have different numbers of X chromosomes. In the system examined by the researchers, females have two X chromosomes while males have only one. Without some form of adjustment, this difference could cause genes on the X chromosome to be expressed at very different levels between the sexes.
Evolution has therefore produced mechanisms that balance this gene activity. Collectively, these mechanisms are known as dosage compensation.
But asexual Timema presented scientists with an intriguing question. If these insects have been reproducing without males for a million years, why would they still need a system designed to balance gene expression between males and females?
The researchers expected the unused machinery to have gradually deteriorated because there would be little evolutionary pressure to maintain it.
To investigate, they examined gene activity in extremely rare males that occasionally occur in these otherwise asexual stick insect populations.
To their surprise, the dosage compensation system still worked.
Rather than showing signs that this biological machinery had disappeared or become badly damaged, the researchers found that it remained fully functional—even after roughly one million years without being regularly needed.
The discovery provides an interesting example of how evolution does not always work like a ruthless cleaning process that quickly eliminates anything unnecessary.
Traits can sometimes remain intact for extremely long periods, even when the conditions that originally made them useful have disappeared. Exactly why the dosage compensation system has survived for so long remains an important evolutionary question.
The findings could therefore help scientists better understand what happens to biological traits when natural selection stops actively maintaining them.
For these unusual stick insects, a million years without sex apparently has not been enough to erase an old piece of their evolutionary past.
Source: KSR.
