Phasmids hailing from New Zealand become asexual after arriving in the UK

A New Zealand stick insect that migrated to the UK more than seven decades ago has given up having sex and become asexual, prompting biologists to wonder about the use of sex at all – especially in Britain.

The Clitarchus hookeri is native to New Zealand but migrated to the UK some time between 1910 and 1935, catching a ride on shiploads of New Zealand plants that were transported to the subtropical Tresco Abbey Garden on the Scilly Isles islands off the coast of Cornwall.

New Zealand female-only stick insect produces 'rogue' male in UK Read more

Biologists from Massey University in New Zealand’s North Island have discovered that some time in the last 100 years the Scilly isles population of Clitarchus hookeri gave up having sex and start to reproduce asexually. The local population of Scilly Isles stick insects is now entirely female.

“It may be that no males were every transferred to the islands, or it may be that during the range expansion the males kind of didn’t keep up,” says Prof Mary Morgan-Richards, an expert in wildlife evolution at Massey.

“One hundred generations is quite short for a multicellular organism to be evolving into asexuality.”

Massey scientists traced the Scilly population to ancestors in Taranaki on the west coast of New Zealand. Despite living in a similar habitat with similar food sources, the New Zealand Clitarchus hookeri have continued having sex and producing equal numbers of male and females.

Recently, specimens of the Scilly Clitarchus hookeri were brought to New Zealand and kept in containment to be studied by Massey scientists. Local males were introduced to the visitors and the two mated without issue – but the eggs produced from the Scilly Isles females were 98% female – showing a barrier to sexual reproduction remained.

Male Clitarchus hookeri may have occasionally been produced on Scilly, Morgan-Richards said, but they would be considered mutations.

Morgan-Richards said the research presented several avenues for further study – including why sex remained useful for species at all, when from an evolutionary perspective females save time and energy by reproducing alone.

“The New Zealand males found them and mated with them; they were receptive, they didn’t kick them off. It is a very recent switch to asexuality so the females haven’t developed any behaviour associated with asexuality yet.”

Populations of stick insects in New Zealand have also been found to switch from asexual to sexual partnerships.

Last year scientists at Massey were “disbelieving” when colleagues in the UK reported they had found a male of the Acanthoxyla inermis stick insect, as the species has only ever been known to produce females. At the time scientists were keen to see if the mutant male would be viable to initiate a sexual relationship – but the specimen died before scientists were able to assess his sexual potential.

“It is a difficult road to get back into sexuality, but the fact it is almost universal shows that there must be an evolutionary advantage to having males and females produce offspring,” said Prof Steve Trewick of Massey.







