New research has used whiptail lizards to investigate whether species that reproduce asexually have more harmful genetic mutations than species that get playful with sexual reproduction.
This finding could explain why sexual reproduction appears to be the dominant way to pass on genetic material, despite asexual reproduction being faster and easier.
“Our study shows that when whiptail lizards transition from sexual reproduction to asexual, it is followed by the accumulation of deleterious mutations in the mitochondrial genome,” says Jose Maldonado, a biologist from the University of Texas at Arlington.
“If asexuals accumulate more deleterious 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 others sexually.
There is also a great abundance of them in 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 that these whiptail lizards use is called: parthenogenesis – it is a form of asexual reproduction in which the embryo can grow and develop without fertilization by sperm. In zoos and other institutions it is called ‘virgin births’. This can also happen in some species that normally reproduce sexually.
The team used full mitochondrial genome data from asexual and sexual whiptail lizards to investigate their prediction that parthenogenetic lines accumulate mutations faster than sexual lines.
The team sampled multiple populations of both asexual and sexual whiptail species in the Southwestern United States and obtained additional tissue samples from collections in US museums.
Their research showed that the transition to asexuality led to relaxed natural selection in parthenogenetic lizards and the build-up of non-synonymous mutations, which alter the protein sequences of a gene and are often subject to natural selection.
“The main finding of our study is that asexual vertebrates, or at least these lizards, accumulate amino acid substitutions, which can potentially be bad for the organism, at a much faster rate than sexual species,” says TJ Firnenoone of the researchers and an evolutionary biologist from the University of Denver.
“This is important because there is a paradox that it is much more expensive to reproduce sexually, but it is the pervasive form of reproduction.”
This supports previous theoretical predictions that “loss of sex should lead to an irreversible accumulation of deleterious mutations due to a reduction in the efficiency of purifying selection, and sex facilitates the removal of deleterious mutations”, They wrote.
This, of course, will not be the end of the matter. Pure asexual reproduction still occurs in many species – including bacteria and other single-celled organisms. Why asexual reproduction has persisted, and the madness of reproduction, in general, is a question for another day.
The research was published in Evolution.
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