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SCIENCE OBSERVER

Attack of the Pseudo-Clones

Rebecca Sloan Slotnick

Parasitic Cape honeybees . . .Click to Enlarge Image

In bee societies, the majority gains when everyone concentrates on raising the queen's young. Of the approximately 30,000 workers in a hive, only three or so have functioning ovaries. Worker policing further reduces the number of surviving worker-laid eggs. According to kin-selection theory, workers are kept in line by the mathematics of their relatedness: They are more related to the sons of their queen than to the sons of their half-sisters, and thus the colony's interests support the queen and her reproductive success.

The Cape honeybee (Apis mellifera capensis), however, flies in the face of those rules. It has evolved its own system—clever parasitism. When a Cape worker invades a hive, it successfully evades worker policing by mimicking the host queen's pheromones, which allows its eggs to hatch unimpeded. If that isn't enough, capensis workers lay diploid eggs—each contains a full set of genetic material, from which hatch another generation of parasitic workers, earning them the moniker "pseudo-clones" from the scientists who study them. The pseudo-clones reproduce by a process called thelytoky: When the worker bee lays an unfertilized egg, it develops into a new, genetically identical (barring the occasional mutation) female bee. In contrast, in other honeybee subspecies, unfertilized eggs develop by arrhenotoky into male drones, which are incapable of reproduction.

The pseudo-clones are obligate parasites and don't forage; they rely on host African honeybees (A. m. scutellata), or "scuts" as the scientists call them, to gather their food. However, as the number of invaders surpasses the number of scuts, the amount of incoming food dwindles and, eventually, the host colony collapses. The entire process takes little more than twelve weeks, says Per Kryger, a scientist at the University of Pretoria in South Africa who has studied the bees for years and is currently working to conserve the wild honeybee populations found in South Africa. "The pseudo-clones see themselves as queens, or princesses maybe," Kryger says.

Black Cape honeybees . . .Click to Enlarge Image

Before the colony collapses, the Cape bees must catch a ride—perhaps on the truck of an unwitting beekeeper—to their next host. Unbelievably, it may take only a single worker to propagate the invasion. Using genetic analysis, Kryger, along with Annelize Lubbe of the Plant Protection Research Institute and Michel Solignac at Universitý Paris-Sud, has traced the billions of current invaders to a single worker that was alive in 1990. The scientists used more than 300 DNA microsatellites, or lengths of repeated nucleotide sequences that identify relatedness between individuals, to confirm that the pseudo-clones were daughters, or rather great-granddaughters, of a single worker, Kryger says.

There have been two other recorded invasions of Cape bees in South Africa, the first in 1928 and the second in 1977. Both subsided without much ado, possibly because the invading bees didn't have the range of traits that current pseudo-clones possess, explains Theresa Wossler, a zoologist with Stellenbosch University in South Africa. The Cape honeybee is "the end product of rapid selection of the most virulent line of capensis," she says. The pseudo-clones have just the right combination of traits, including an increased predisposition to wandering, to keep the invasion spreading.

The current invasion of highly effective intruders is causing quite a disturbance in northern South Africa. Wossler estimates the value of crops dependent on honeybee pollination to be 20 billion rand, or about 2 billion U.S. dollars—and the bees' contribution to South African wild flora cannot be overestimated, she says.

Honeybees are the most significant pollinator species in South Africa, she adds. Scientists are wondering what effect such a prolonged invasion will have on endemic honeybee populations. So far, Kryger says, beekeepers have been able to catch enough swarms in the wild every year to make up for those that have collapsed, but this temporary solution only spreads the invasion by introducing new, wild colonies to the pseudo-clone.

The real solution is to stop mixing healthy, new swarms with old infected colonies. Although it may sound simple, Kryger says, translating scientific principles into business practice is quite difficult.—Rebecca Sloan Slotnick


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