The workers of a South African subspecies of honeybee can clone themselves, with one individual having done so many millions of times over the past 30 years. Some of the clones can even develop into queens that can take over the hive.
Asexual reproduction – parthenogenesis – isn’t uncommon in the insect world, but having offspring that are genetically identical to the parent is. That’s because, during the reproductive process, genetic material gets mixed up in a process called recombination. As a result, even if there is only one parent its offspring end up with a slightly different genetic makeup.
However, the female workers of the Cape honeybee (Apis mellifera capensis), native to southern South Africa, have developed the unusual ability to clone themselves while effectively avoiding recombination during reproduction, says Benjamin Oldroyd at the University of Sydney.
Doing so carries important benefits. Normally, asexual reproduction can be lethal in honeybees because about a third of the genes become inbred, and the larvae don’t survive, Oldroyd says. But because Cape honeybee worker bees perfectly clone themselves, each clone remains as genetically healthy as her mother. “It’s quite remarkable,” he says.
One line of clones has been re-cloning since 1990, with the same individual copied hundreds of millions of times, says Oldroyd.
The clones can prove problematic for the health of the colony, however. Generally speaking, the queen bee is the only individual that reproduces, while other bees carry out their own duties to keep the colony healthy. If the workers begin cloning themselves – which can happen if the hive is disturbed in some way – this delicate balance is destroyed. In some cases, one of the clones can even develop into a queen of a dysfunctional hive.
“Eventually the workers just sort of hang around laying eggs not doing any work,” says Oldroyd. “The colony dies, and [the cloning workers] spread to the next colony.”
Even after these workers have invaded a new colony they continue to lay eggs, disrupting and potentially taking over that colony too.
“They kill about 10 per cent of South African colonies every year. It’s like a transmissible social cancer,” says Oldroyd.
Curious about how the worker clones maintain such strong genetic integrity without inbreeding, Oldroyd and his colleagues compared the genomes of Cape worker bees, Cape virgin queens and their offspring. Cape queens generally reproduce sexually, so to force them to reproduce asexually, the scientists fitted them with a strip of surgical tape glued with nail varnish to prevent them mating. But because they could still interact with males during mating flights, they laid eggs.
The researchers genotyped one queen and 25 of her larvae, and four workers and 63 of their larvae.
They found that the asexually produced offspring of the queen showed levels of genetic recombination that were 100 times greater than seen in the cloned offspring of the worker bees. In fact, the worker bees’ offspring were essentially perfect copies of their mothers, Oldroyd says.
The results suggest that the Cape worker bees have evolved a mutation that effectively prevents genetic recombination during reproduction, says Oldroyd.
The findings are “surprising”, says Kip Lacy at The Rockefeller University in New York, who has studied similar cloning behaviour in the tropical fire ant (Solenopsis geminata).
In S. geminata, the queen can choose to clone herself a little like the Cape worker bees do, says Lacy, but the workers cannot. But even in the ant queens, there is genetic recombination that eventually leads to health problems like sterility, he says.
For the Cape honeybees, the cloning is perfectly in keeping with evolutionary theory, says Laurent Kellerat the University of Lausanne in Switzerland. “Evolution is just selecting what’s doing well at a given time,” he says. “And these honeybee clones are doing quite well as they are.”
Journal reference: Proceedings of the Royal Society B, DOI: https://doi.org/10.1098/rspb.2021.0729