Why do organisms sometimes have duplicated or triplicated genes? What can this teach us about evolution and our biology? Cockroaches have up to three copies of the same gene in their genome. Do these copies serve the same function? Why have they retained them?
Female of Blattella germanica with ootheca. Credit to Cristina Olivella.
Over time, living organisms pass on their genes to their descendants, generation after generation, through reproduction. During this process, mutations can occur, including gene duplications. These are common and can range from duplications of a few DNA bases to the duplication of an entire genome. Such random errors can either disappear or persist in a species' genome over successive generations.
This phenomenon has occurred in insects. Until recently, it was known that all winged insects possess two copies of the insulin receptor gene (InR) due to a duplication that occurred approximately 400 million years ago. However, recent studies have revealed that around 350 million years ago, a second duplication of this gene occurred in the common ancestor of cockroaches, termites, mantises and stick insects. Thus, these insects have not two but three copies of the InR gene. Why have they retained all three copies of the same gene?
Now, a study led by the Institute of Evolutionary Biology (IBE), a joint centre of the Spanish National Research Council (CSIC) and Pompeu Fabra University (UPF), provides an answer.
350 million years ago, a second duplication of this gene occurred in the common ancestor of cockroaches, termites, mantises and stick insects
The research reveals that the two most recent copies of the InR gene might strengthen the function of the ancestral insulin receptor, which is essential for the survival of cockroaches and related groups. The team suggests that this genetic "error" could have conferred an evolutionary advantage to these insects and sheds light on the mechanisms of evolutionary innovation in animals.
To disappear or to persist
“Mutations occur constantly and randomly in the genome. When they affect a gene of vital importance, the individual often does not survive, and the mutation is not passed on. However, if the mutation benefits the individual, enabling them to have more offspring than others of their species, the mutation—transmitted to their descendants—will likely become fixed,” explains José Luis Maestro, lead author of the study and principal investigator of the Nutritional Signals in Insects group at IBE.
In the case of the InR gene, it plays an essential role in animals as it is part of “a cellular signalling pathway that regulates key processes such as growth and cell proliferation, longevity, sugar levels, and reproduction,” explains David Pujal, a PhD student at the IBE and co-first author of the study.
“The insulin receptor is essential in animals. That’s why we believe having an extra copy of this gene may have reinforced its function, which was key to the persistence of the new copies,” notes Maestro.
“Gene redundancy, where multiple genes perform the same functions, can provide stability in critical processes,” notes Rosa Fernández, principal investigator of the Metazoa Phylogenomics Lab at the IBE and a contributor to the study.
“The insulin receptor is essential in animals. That’s why we believe having an extra copy of this gene may have reinforced its function, which was key to the persistence of the new copies"
Developing a new function or reinforcing an existing one
Generally, mutations accumulated in a duplicated gene gradually erode its original function until it becomes non-functional. However, in some cases, these changes offer an evolutionary advantage to the individual. The gene may either develop a new function (neofunctionalisation) or enhance a function vital for survival (subfunctionalisation).
With this premise, the IBE team sought to determine why cockroaches and their closest relatives have retained three copies of the same gene for over 350 million years.
To do so, they conducted functional studies on the reproduction of the cockroach Blattella germanica. The results showed no signs of neofunctionalisation in any of the copies.
To identify the functions of each copy of the InR gene, the team reduced the activity of each one in adult individuals of Blattella germanica. They used RNA interference, a molecule capable of specifically binding to the various messenger RNA copies of the insulin receptor and deactivating them.
Through these experiments, they discovered that reducing the levels of InR1 and InR3 had no significant effects on Blattella. However, blocking InR2 caused a major inhibition of the insulin receptor pathway. They found that InR2 is the most active of the three copies, as it has the highest expression levels across all the tissues studied.
“InR2 is the copy that maintains the primary activity of the gene, while the other two copies act as reinforcement. This is known as genetic redundancy. The InR2 copy would be like the lead singer of a musical group, with two backup singers who can support them and carry on the song when needed,” explains Xavier Bellés, principal investigator of the Insect Metamorphosis Evolution group at the IBE and a contributor to the study.
The study thus determined that the three copies of the InR receptor have been conserved across generations to reinforce the vital function of insulin in cockroaches. This represents a process of subfunctionalisation. However, the team does not rule out the possibility of finding specific functions for the different copies of the InR gene in other processes or other species with this gene triplication.
“We have conducted some functional studies on Blattella germanica, but there are many aspects of its physiology, as well as that of other related insect groups, that remain to be explored and that could reveal a new function for one of the InR gene copies,” concludes Maestro.
Applications for Pest Control
“In our group, we study the physiology of cockroaches for several reasons. One reason, and the most applied part of our research, is the development of new methodologies for pest control,” explains José Luis Maestro. “Cockroaches are a particularly troublesome urban pest that also poses health risks.”
These insects can carry diseases and, most significantly, can trigger respiratory allergies that may lead to asthma, the researcher clarifies. At the IBE, scientists have study the biology of cockroaches for many years, and their findings include the crucial role of certain proteins in the development and metamorphosis of these insects. This opens the door to new pest control strategies.
“Understanding how an organism works provides us with tools to identify ways to interfere with its growth, reproduction, or other vital processes,” Maestro adds.
This research can also shed light on the biomedical field. “The insulin signalling pathway functions in exactly the same way in humans as it does in insects"
Also in the Biomedical Field
This line of research can also shed light on the biomedical field, explains José Luis Maestro. “The insulin signalling pathway works exactly the same way in humans as it does in insects, and indeed in all metazoans [animals composed of numerous cells that form tissues, organs, systems, and structures].”
“The pathway is activated by the binding of insulin or similar peptides to the insulin receptor, which in all cases are similar proteins. The intracellular signalling, involving various protein phosphorylations, is also similar. Many other processes are also similar.”
Additionally, the small size of insects, their short life cycle, and the experimental possibilities they offer make them ideal models for biomedical studies. The way these processes work, including the signalling of the insulin receptor pathway, “is very similar in insects and humans, meaning that findings from insect research—easier to obtain than in vertebrates—can often be directly extrapolated.”
Evolutionary studies, concludes the expert, “like the one in this study, also reveal how nature operates as a whole, including the evolutionary processes that have affected or continue to affect us.”