Sex is better with two: lizard study finds asexual reproduction leads to harmful genetic mutations

New research has used whiptail lizards to investigate whether species that reproduce asexually have more harmful genetic mutations than those getting frisky with sexual reproduction.

This finding could explain why sexual reproduction seems to be the dominant way to pass on genetic material, despite asexual reproduction being quicker and easier.

“Our study demonstrates when whiptail lizards transition from reproducing sexually to asexually, it is followed by the accumulation of harmful mutations in the mitochondrial genome,” says Jose Maldonado, a University of Texas at Arlington biologist.

“If asexuals accumulate more harmful mutations than their sexual counterparts, as our findings show, this could explain why asexual reproduction is rare in nature and why sex is the dominant form of reproduction in the natural world.”

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The researchers tested this theory by studying Aspidoscelis, a genus of whiptail lizards. This genus is particularly interesting because some reproduce asexually and some sexually.

There’s also a high abundance of them throughout the southwestern United States and northern Mexico, so these reptiles are an excellent model system for researchers to study the consequences of asexuality.

The form of asexuality these whiptail lizards use is called parthenogenesis – it’s a form of asexual reproduction in which the embryo can grow and develop without fertilization by sperm. In zoos and other institutions it’s called ‘virgin births’. This also can happen in some species who normally reproduce sexually.

The team used whole mitochondrial genome data from asexual and sexual whiptail lizards to investigate their prediction that parthenogenetic lineages accumulate mutations faster than sexual lineages.

The team sampled multiple populations of both asexual and sexual whiptail species throughout the southwestern United States and received additional tissue samples from collections at museums around the US.

Their research showed that the transition to asexuality led to relaxed natural selection in parthenogenetic lizards and the build-up of nonsynonymous mutations, which change the protein sequences of a gene and are frequently subjected to natural selection.

“The main finding of our study is that asexual vertebrates, or at least these lizards, accumulate amino acid substitutions, which could be potentially bad for the organism, at a much higher rate than sexual species,” says T.J. Firneno, one of the researchers and a University of Denver evolutionary biologist.

“This is important because there is a paradox that it is much more costly to reproduce sexually, but that it is the pervading form of reproduction.”

This supports previous theoretical predictions that “the loss of sex should lead to an irreversible build-up of deleterious mutations due to a reduction in the efficiency of purifying selection, and sex facilitates the removal of harmful mutations,” they wrote.

Of course, this won’t be the end of the matter. Purely asexual reproduction still occurs in many species – including bacteria and other single celled organisms. Why asexual reproduction has stuck around, and the weirdness of reproduction generally is a question for another day.  

The research has been published in Evolution.