RESOLVING THE LEDGER
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Add as preferred source Scientists have made a surprising finding about how an asexually reproducing stick insect has evolved. Research led by Dr. Darren Parker, lecturer in evolutionary biology at Bangor University, studied the genes of several species of Timema (a type of stick insect), which have not engaged in sexual reproduction for 1 million years—the longest known asexual period for any insect.
The wingless stick insects are native to the far western United States and reproduce by a form of virgin birth in which an embryo develops from an unfertilized egg, bypassing the need for male fertilization. This phenomenon, known as parthenogenesis, occurs naturally in some animal species.
Because some species of Timema have reproduced without sex for so long, the scientists behind the study, published in PNAS , expected that the system that evolved to balance gene expression on the sex chromosomes would have eroded over time. But that was not the case.
Parker explained, "Males and females have a different number of X chromosomes; males have one (XY), and females have two (XX). As a result, gene expression needs to be balanced equally between them. Species have evolved a wide range of mechanisms known as 'dosage compensation' to do just this. But what happens when these mechanisms for equally sharing male and female genes are no longer needed? That's the question we set out to answer.
"This is the precise situation found in asexually reproducing stick insects. Because the females reproduce without fertilization from males, there is no need to equalize gene expression. By investigating gene expression in rare males, our study found that rather than decaying away as we expected, dosage compensation mechanisms remained fully functional despite not being needed for over a million years."
The findings challenge long-standing assumptions about how quickly biological systems decay when they are no longer under strong evolutionary pressure and offer a reminder that evolution does not always discard what it no longer needs.
Darren J. Parker et al, Dosage compensation and meiotic sex chromosome inactivation are maintained under relaxed selection, Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2531501123
Journal information: Proceedings of the National Academy of Sciences
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