Science1 publisher3 min readPublished
Beetles with five potential mates purged harmful mutations over 156 generations
In University of East Anglia beetle lines where females had five males to choose among, genomes ended up carrying fewer damaging mutations at similar diversity. Mutation load, more than the mating arrangement, predicted which populations died out under inbreeding.
The Scientist · Science desk
What happened
- A University of East Anglia team ran red flour beetle populations for 156 generations, some in a competitive multi-male mating environment and some with each female paired to a single male.
- Pointer said the mutation load turned out to be a better predictor of extinction risk than the mating system a population had been kept under.
- The work is published in the Proceedings of the National Academy of Sciences and rests on a 15-year experiment involving thousands of beetles.
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Why it matters
- decision Programs that pair one male to one female by studbook now have a genomic result to argue against, with Richardson saying captive and wild programs should think carefully about restoring natural levels of sexual selection.
- constraint The purging mechanism works across many generations, and Richardson says hundreds of generations cannot be run in larger animals, so a manager of a long-lived vertebrate cannot expect this to act inside a planning horizon.
- capability Because load fell without a measurable cost in genome-wide diversity, the standing reason to keep sexual selection weak in managed populations loses its empirical support, at least in this beetle.
- precedent If sequenced mutation load outranks mating arrangement as a predictor, genetic monitoring in breeding programs has a candidate metric to report alongside pedigree and founder statistics.
Fifteen years of work and 156 generations comes to roughly ten beetle generations a year [15]. That turnover is the reason the experiment could be run. David Richardson, the senior author, said "Experimental beetle populations allow us to test fundamental evolutionary principles over hundreds of generations, something that would be impossible in larger animals" [13].
The manipulation was blunt on purpose. Michael Pointer, the lead researcher at UEA's School of Biological Sciences, said "In one type of population, females could mate with up to five males in each generation. In the other type, the opportunity for sexual selection was removed by pairing each female with just one male each generation" [4]. Because the regime was imposed by the experimenters and held for 156 generations [3], the genome difference at the end has a cause with a known direction.
Then the sequencing. "We then sequenced the genomes of multiple beetles from each population and identified the different mutations within those genomes, including those predicted to be harmful. We found that when the females had more males to potentially mate with, those populations had fewer harmful mutations, but similar overall genetic diversity," Pointer said [5]. The populations were then inbred, and those carrying higher levels of harmful mutations "were far more likely to become extinct when subjected to inbreeding," according to Pointer [6].
"Indeed, this mutation load proved a better predictor of extinction risk than the mating system itself," Pointer said [7]. Anyone running a breeding program should sit with that ranking: polyandry is a route to a lower load, and the load is what did the predicting.
The report describes the result as "substantially fewer harmful genetic mutations" and the extinction difference as "far more likely", and gives no number for either [16]. It does not say how many replicate populations were run, or how much inbreeding it took to drive them extinct. No vertebrate was tested, and Richardson said the aim "was not to understand the beetles themselves" but to test a broader question about sexual reproduction [14].
One old objection does get an answer. The concern had been that strong sexual selection shrinks the effective breeding population and costs a line its genetic variation; here the high-selection beetles held genome-wide diversity comparable to the low-selection ones [9]. "This means that populations may be able to rid themselves of harmful mutations while still preserving the genetic variation needed to adapt to future challenges," Pointer said [10].
Richardson's reading for practice is careful, and worth quoting as he put it: "Conservation programs, whether in the wild or in captivity, should think carefully about restoring natural levels of sexual selection" [12]. He added that allowing females multiple males "could help populations naturally weed out damaging genetic variants and improve their long-term chances of survival" [17].
What to watch
- The PNAS paper's replicate counts, effect sizes and inbreeding protocol, none of which appear in the release.