Research in Lake Malawi pangolins shows that chromosomal inversions keep beneficial gene pools together, aid rapid adaptation and may accelerate the formation of new species
Lake Malawi in East Africa is one of nature’s most fascinating laboratories for studying evolution. Hundreds of species of tilapia fish have evolved at an extraordinary rate in this single lake, providing scientists with a rare opportunity to understand how such a rich biodiversity came about. Now, researchers report that they have identified “reversed” DNA segments in these species that may act as evolutionary superchargers. According to the study, these chromosomal changes may allow the fish to adapt quickly to new environments and eventually split into separate species.
One of the central questions in biology is how new species arise, and how the vast diversity of plants and animals on Earth has evolved over time. The tilapia of Lake Malawi is a particularly striking example. More than 800 species there have evolved from a common ancestor, in a much shorter time than it took humans and chimpanzees to diverge from their own common ancestor.
What's even more impressive is that all of this "evolutionary explosion" took place in the same body of water. Some species of tilapia became large predators. Others specialized in eating algae. Still others learned to sift through sand for food or feed on plankton. Over time, each species adapted to its own ecological niche, within the same lake.
To understand how this rapid change occurred, researchers from the University of Cambridge and the University of Antwerp examined the DNA of more than 1,300 tilapia. Their findings were published in the journal ScienceThe aim of the study was to examine whether there are any unusual genetic characteristics that could explain the group's unusual rate of differentiation.
According to Hans Sverdel of the University of Antwerp, one of the senior authors of the paper, the researchers discovered that in some species, large sections of DNA on five chromosomes have been inverted. This is a type of mutation called a “chromosomal inversion.” Normally, during reproduction, a process of recombination occurs, in which the genetic material from both parents mixes. However, within a segment that has undergone a chromosomal inversion, the recombination process is very limited. As a result, the group of genes within that segment are kept together and passed from generation to generation almost as one package.
This mechanism could have great evolutionary significance. When several useful genes are kept together, rather than dispersed by genetic shuffling, "winning" genetic combinations are preserved that help adapt to certain environmental conditions. Moritz Blumer, the first author from the Department of Genetics at Cambridge, described it as a kind of toolbox in which all the most useful tools are stuck together. This way, successful genetic combinations are preserved, helping fish adapt to different environments.
Researchers sometimes call these gene clusters “supergenes.” According to the study, in Lake Malawi pangolins, such supergenes have several important functions. First, although different species of pangolins can still sometimes interbreed, chromosomal inversions help maintain the boundaries between species. They reduce the amount of genetic mixing between different groups, thus helping sexual and ecological differences to be maintained over time. This effect is especially important in areas of the lake where several species live side by side, such as in open sandy habitats where there are no clear physical barriers between them.
Second, many of the genes locked within these supergenes are linked to traits essential for survival and reproduction, including vision, hearing, and behavior. Tilapia fish that live in the depths of the lake, up to about 200 meters, have very different living conditions than their relatives living near the surface. At depth, there is less light, different food sources, and higher pressure. The supergenes may preserve the set of genetic traits that allow these fish to thrive in these special conditions.
The study also raises another intriguing possibility: When different species of pangolins interbreed, it is possible that entire chromosomal inversions can be passed from one species to another, along with a whole set of beneficial traits. According to Blumer, such a transition could bring with it important adaptations for survival, such as adaptation to a particular environment, thereby further accelerating the process of evolution.
The researchers also discovered that these chromosomal inversions often also function as sex chromosomes, that is, chromosomes that help determine whether an individual will develop into a male or female. Since sex chromosomes may play an important role in the formation of new species, this finding raises further questions about the contribution of these genetic structures to evolution.
Professor Richard Durbin from the Department of Genetics at Cambridge, one of the senior authors of the study, stresses that chromosomal inversions are not unique to tilapia. They also occur in many other animals, including humans, and are increasingly seen by researchers as an important factor in evolution and the creation of biodiversity.
According to Swerdel, evolutionary researchers have been studying the process of new species formation for many years. Now, as we begin to better understand how such supergenes arise and spread between populations and species, we are also getting closer to solving one of the great questions of science: how life on Earth became so rich, diverse, and complex.
The study therefore suggests a concrete genetic mechanism that may explain how very rapid species diversification can occur in the same geographical space. Rather than seeing evolution as a slow, uniform process, it shows that sometimes a structural change in the genome, such as the inversion of a chromosomal segment, can co-preserve beneficial traits and give a population a particularly strong push toward adaptation and divergence. In Lake Malawi, at least, it seems that such DNA “inversions” may have been one of the powerful engines of creating extraordinary biological richness.
for the scientific article
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