Passing on the good beetle genes

Turns out, males do have a purpose – and yes, it is about sex.

According to a new study, published in Evolution Letters, the male seed beetle helps purge bad mutations and retain good genes through strict competition and sexual selection, increasing the long-term genetic health of a population.

Two beetles with their bums together.
A mating pair of seed beetles attempting to disengage (female left, male right). Credit: Mareike Koppik

The researchers, led by Karl Greishop of the University of Toronto, Canada, studied 16 genetic strains of the seed beetle (Callosobruchus maculatus) in an intensive breeding program to see how deleterious mutations – ones that won’t kill you, but might affect health and ability to reproduce – affected both males and females.

They found that deleterious mutations affected males more than females, but that this actually increased the genetic population as a whole.

“Our study shows that production of males, which may engage in intense competition for the chance to mate, enables faster purging of deleterious mutations from the population, which could thereby enable a healthier set of genes and higher reproductive capacity relative to asexual reproduction,” says David Berger of Uppsala University, Sweden.

Read more: The long-term effects of sexual competition

In essence, male beetles with stronger genes outcompeted the males with bad mutations, which were unable to have as many babies and couldn’t pass on bad beetle genes that well. The same effect wasn’t seen in females, which had just as many babies and were able to pass on deleterious mutations.

“This indicates that although these mutations do have a detrimental effect on females’ reproduction, they are more effectively removed from the population by selection acting on male carriers rather than female carriers,” says Grieshop.

“Previous research from our group and others has succeeded in showing this effect by artificially inducing mutations, but this is the first direct evidence that it ensues for naturally occurring variants of genes.”

That meant that male beetles were responsible for purging bad genes from the mutations, purely because they couldn’t pass them on, and the genetically stronger male beetles ended up impregnating more female on their behalf, without the population diminishing.

Read more: Water beetles mate to an evolutionary standstill

“When deleterious mutations are purged from a population through rigorous selection in males, resulting in fewer males reproducing, the process can take place with little or no effect on population growth,” says Greishop.

“This is because relatively few males suffice to fertilise all the females in a population, hence whether those females are fertilised by few males or many males makes little or no difference to the number of offspring those females can produce, especially in species where the male doesn’t look after its own offspring.

“By contrast, such rigorous selection in females would result in fewer females reproducing, hence fewer offspring produced, which could lead to a massive population decline or even extinction.”

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