Sex lives of NZ stick insects

Friday, 23 August 2019

A New Zealand insect species reproduces differently in the North Island compared to the South Island and differently again on a tiny island in the United Kingdom.

In the northern region of the North Island, native stick insects (Clitarchus hookeri) mostly have female-male sex. After the evening light falls during their mating season, the females release chemical signals that attract the males. All going well the boys insert their phallus into the girls' genital opening to transfer a spermatophore.

The male might stay around for up to a day to prevent another male from mating – a trick called 'mate guarding'. Fertilised eggs are dropped into the leaf litter and hatch less than a year later, by which time both parents have died.

In the southern region of the North Island and the South Island proper, there are few male stick insects. In those regions, females mostly reproduce asexually. The eggs are not fertilised but they nonetheless hatch and a healthy stick insect grows. These offspring are all daughters and they reproduce the same way. It is called parthenogenesis.

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A green female stick insect from the UK mates with a NZ male. The background has been cleared for clarity.

It's not understood why there are few males in these southern regions. DNA analysis led by Mary Morgan-Richards and colleagues of Massey University suggests that female stick insects moved south as the last ice age ebbed, about 15,000 to 20,000 years ago.

Male-female sex carries some costs, and this may explain why the girls departed without the boys. There's probably a greater chance of being eaten during and after copulation. As well, female stick insects store sperm for a time and this may be wearying. Also, disease may be carried by sperm.

But other things have been happening too, almost in real time, much to the delight of the biologists.

One is that between 2003 and 2016 there was probably an error in genetic reproduction at Otari-Wilton's Bush in Wellington, an area that previously featured only females. Male stick insects were born and reproduced sexually with previously asexual females in the area.

The second was that males have arrived on the Kapiti Coast recently, probably by hitch-hiking with humans. They too have converted the locals to sexual reproduction.

There appears to be a barrier that means lineages of previously asexual females are less successful at sexual reproduction, but it can be overcome.

The brown male northern stick insect is 'mate guarding' the female to prevent her from re-mating.

Overcoming this barrier suggests sex must provide a pretty strong advantage over parthenogenesis – but this is not entirely understood.

Meanwhile, a population of NZ stick insects was established on the Isles of Scilly, 45 kilometres southwest of Cornwall in the United Kingdom, within the last 70-100 years.

There's a NZ plant garden at Tresco Abbey on the islands and it's thought a single stick insect egg, or perhaps a few more, were accidentally transferred there with the plants.

Using DNA techniques, Morgan-Richards and colleagues showed the Scilly insects probably came from Taranaki and had previously reproduced sexually. But there were no boys in the UK, or none survived, so the girls moved to parthenogenesis immediately.

Morgan-Richards and colleagues brought a group of them back to New Zealand and introduced them to stick insect boys in the lab. Some of them mated but the offspring were mostly girls and the result of asexual reproduction.

It seems the UK lineage had developed an even greater barrier to sexual reproduction in less than 100 generations.

The speed of evolution is remarkable – suggesting strong selection in this population. More work is needed to understand the cost of sex for the Scilly females and the benefit of sex in Wellington.