Unraveling the Power and Evolution of Supergenes in Genetics
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The definition of a supergene is rather technical, and scientists still argue about its finer points. But at its simplest level a supergene is a group of genes that are inherited together as a unit, often with a lot of other noncoding DNA. You can continue to produce two distinct traits with multiple genes and not worry about them becoming jumbled up. That jumbling often occurs during the production of egg cells and sperm. In that process, the maternal and paternal copies of chromosomes line up and randomly swap segments of DNA in a ballet called recombination. Recombination hedges nature’s bets about the value of different permutations of genes; it boosts genetic diversity and helps weed out harmful mutations.
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The superpower of supergenes is that they block this. Typically, supergenes contain DNA deletions, insertions or inversions (sequences that were cut out and spliced in backward). As a result, those parts of the chromosomal DNA don’t align with a partner and are far less likely to recombine.
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In the 1970s, researchers showed that this same mechanism – with misalignments in chromosomes blocking recombination in segments of chromosomes that then continue to lose genes – led to the evolution of Y sex chromosomes from X chromosomes in mammals. Both supergenes and sex chromosomes exist because there’s sometimes a benefit to having some sets of genes inherited together. In those cases, it would be ideal to not have any recombination but to have the things that go well together stuck together for good.
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To understand why that might be advantageous, think about doing laundry. Say you have a basket of white towels and a basket of red towels. Recombination does the equivalent of tossing both loads into the same drum, flipping on the hot water, and pressing start. What results is a bunch of pink towels…
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Many researchers have been probing how supergenes arise and what the consequences for species might be as their supergenes continue to evolve. In one recent effort, Katie Lotterhos, an evolutionary marine biologist at Northeastern University, built a computer model to study the first tentative steps taken on the path from inversion to supergene. Her model showed that the larger the initial DNA flip-flop, the more likely a supergene was to evolve. The reason was simple: A larger inverted fragment of DNA was more likely to capture multiple genes and lock them together as a single entity. Any beneficial mutations arising within the inversion could then promote its spread as a supergene.
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But the more important insight from Lotterhos’ model was that inversions themselves do not necessarily provide an evolutionary advantage. If a suite of genes is already well adapted to its surroundings, locking it into an inversion will not suddenly allow it to take off as a supergene. That fact may help to explain why complex vital traits aren’t routinely secured as supergenes: Ordinary selection pressures are often sufficient to preserve the traits. The question of whether an adaptation precedes an inversion or vice versa, Lotterhos realized, might never be answerable. “What comes first, the inversion or the adaptation?” she said. “It’s probably a little bit of both.”
