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Two supposedly asexual species of stick insects may engage in occasional bouts of sex, helping to widen their gene pool and avoid harmful mutations.

A handful of animals reproduce asexually, primarily through a process known as parthenogenesis, which involves creating embryos from unfertilised eggs. Species that do this include some insects, reptiles and fish.

鈥淎ll members of a parthenogenetic population can produce offspring, so they have this huge demographic advantage,鈥� says at Bangor University in the UK. This is because asexual reproduction leads to much faster population growth than sexual reproduction.

But parthenogenesis has its disadvantages. A lack of genetic diversity makes it harder to adapt to environmental changes. Species can also accrue harmful mutations over time 鈥� if they accumulate enough of these, they undergo 鈥渕utational meltdown鈥� and die out, says at the University of Lausanne, Switzerland.

A few stick insect species in the genus Timema are parthenogenetic, and while the majority of individuals are female, some males pop up too. 鈥淲e wanted to check if these males were just errors, or if they were actually participating in the population and reproducing,鈥� says Freitas.

Parker, Freitas and their colleagues analysed the genes of four parthenogenetic species of Timema (Timema genevievae, Timema shepardi, Timema monikense and Timema douglasi), looking for two features that indicate sexual reproduction.

The first is the distribution of certain alleles 鈥� different forms of the same gene 鈥� across the genome. In parthenogenesis, some alleles that sit closely together usually stay close when passed down to offspring. In sexual reproduction, however, these close alleles can sometimes get split up due to the mixing of different genes.

Another marker is heterozygosity, the extent to which an individual has different alleles of its genes. In species that reproduce through parthenogenesis, the offspring receives its alleles from one parent. In sexual reproduction, it has a chance of receiving different alleles from two parents. The more heterozygous an individual, the more varied their alleles.

The team found that T. douglasi and T. monikense have a much higher heterozygosity and more widely distributed alleles compared with the other parthenogenetic species they studied. The finding suggests that the males and females must have mated with each other at some point.

鈥淭here鈥檚 definitely a little bit of sex going on,鈥� says Parker. 鈥淚t鈥檚 actually a good solution for a lot of the problems that parthenogenetic species face, by widening the gene pool.鈥�

鈥淭his is a nice demonstration that sexual and asexual reproduction are rarely absolute,鈥� says at the University of New South Wales, Australia. 鈥淢any species that we think of as sexual are also capable of reproducing asexually on occasion, as reported recently in crocodiles and condors. Similarly, species that are thought to be asexual can engage in occasional sex.鈥�