Study of asexually reproducing honeybee ponders: But why the mass murder?

The secrets that explain how a swarm of female-only honeybees living in South Africa can carry out deadly invasions on other bee colonies have been unravelled by a team of researchers, according to research published in PLOS (Public Library of Science) Genetics yesterday.

Honeybee colonies have rigid social structures, made up of female worker bees, male drone bees and one all-powerful queen bee. The hive is sustained by worker bees gathering nectar, drone bees mating with the queen, and the queen laying eggs.

A unique sub-species population found in South Africa, however, has no drone bees and no queen either – only worker bees.

"All worker bees are laying eggs so the swarm effectively acts like one massive 'pseudo-queen'," Matthew Webster, a researcher from Uppsala University and co-author or the paper, told The Register.

Cape honeybees (Apis mellifera capensis) have the unique ability to both sexually and asexually reproduce. But only when the worker bee reproduces asexually can she unleash her dangerous streak.

Cape worker honeybees have the ability to invade other swarms secretly, also known as social parasitism. The sneaky bees fly into foreign colonies and slowly build an all-female army by laying eggs discreetly. When they hatch, the offspring uses up all the hive's resources so that the old population slowly dwindles away.

"If they have the opportunity to be social parasites, they will," Webster warned.

According to the paper, the "parasitic egg-laying Cape bee worker" bees masquerade as queens, producing queen pheromones that "allow them to assert reproductive dominance over other workers". They also "have a significantly increased lifespan of three to five months compared to six weeks in non-parasitic workers".

African beekeepers have been warned to prevent mixing the Cape species with their own bees to avoid fighting a losing battle against the lethal imposters.

To understand the peculiar behaviour of the Cape honeybee, researchers employed population-scale genome sequencing.

As the researchers explained, "the genetic basis of social parasitism... is facilitated by a range of different traits related to ovary development, behaviour and abnormal meiosis." They specifically looked at whether the part of the gene responsible for asexual reproduction acted as a “master switch” to enable the suite of characteristics that are behind parasitism – as previous researchers had believed. The Swedish and South African team discovered that it did not and that regions were independent.

"It was believed that only one gene region differed between the Cape honeybees and other African bees, but, in fact, it's several gene regions that are different. This explains how the ovaries in worker bees have developed the ability to reproduce sexually and asexually and lay eggs discreetly," said Webster.

Another surprising find for the researchers was that the Cape honeybee is actually genetically very similar to other African bees, suggesting that the Cape honeybee subspecies formed recently or is interbreeding with other African bees.

Apis mellifera capensis is the only subspecies of honeybee that can asexually reproduce – a characteristic that was first described in 1912. The worker bee lays and fertilises its own egg so that the offspring has both chromosomes from its mother.

Webster and his team hope to pinpoint how the reproductive gene mutated for asexual reproduction, and find out what evolutionary advantages there are for a species that can switch between sexual and asexual reproduction. ®