SCIENCE OBSERVER
Attack of the Pseudo-Clones
Rebecca Sloan Slotnick
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.
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