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Scientists Find Genes That Let These Bees Reproduce Without Males
The female Cape bee is a renegade. She breaks all kinds of rules and disregards orders. In this isolated subspecies of honey bees from South Africa, female worker bees can escape their queen’s control, take over other colonies and reproduce asexually — with no need for males. Scientists identified the genes most likely to have instigated this unusually powerful worker bee behavior, according to a study published Thursday in PLOS Genetics.
The typical story of reproduction is that males and females of an animal species do it sexually. Generally, that’s what honeybees do, too. Sperm from a male drone fertilizes a queen’s eggs, and she sends out a chemical signal, or pheromone, that renders worker bees, which are all female, sterile when they detect it.
But the Cape honeybee, a subspecies that lives in the Fynbos ecoregion, a unique area of incredible diversity along the southwestern tip of South Africa, evolved a workaround where, in some cases, female workers can become something like a queen and produce offspring of their own.
Like all honeybees, some Cape bee colonies also have male drones. But female workers can start laying their own eggs in their home colony when a queen dies. These females will also invade colonies of other honeybee subspecies and lay eggs in some cases, and they can enter undetected by bees that would normally kick them out.
“The Cape bees will take over the foreign colonies and start eating up all the honey,” said Matthew Webster, a geneticist at Uppsala University in Sweden, who led the study. This behavior is called social parasitism.
To understand what was driving this behavior, researchers compared the whole genomes of 100 honeybee subspecies with those of 10 Cape honeybees. Unsurprisingly, the genomes were very similar: The bees look and act the same in every way except for the egg-laying quirk. But a few select areas of the genome were unique on the Cape honeybee genome.
“Normally that doesn’t cause really big differences,” said Dr. Webster. But in this particular bee, the workers lay eggs that self-fertilize and become female workers in their home colonies or the hives they invade.
Genetic differences likely made social parasitism possible by selecting for bees that could develop ovaries to a greater extent than other worker bees, lay eggs prepackaged with two sets of chromosomes, and possibly emit a chemical signal to mask their presence while laying eggs, said Dr. Webster.
This asexual tendency may sound weird, but it’s not unheard-of in biology. A variety of species of ants, wasps and bees can switch between sexual and asexual reproduction. And scientists have documented virgin births in turkeys, chickens, sharks and reptiles.
During a process called thelytoky, two of the Cape bee’s daughter cells fuse together to make a single cell with both sets of chromosomes — just like Thelma the snake, a reticulated python known for her virgin births. Normally, honeybee eggs split during meiosis into four daughter cells with just one set of chromosomes. Those turn into male drones without a father to contribute the other set to make them female.
What scientists haven’t sorted out is why there might be an evolutionary advantage for a female being able to reproduce without a male. In extreme situations with no males, it could mean the survival of her species. But then again, self-fertilization, the epitome of inbreeding, could leave her offspring more vulnerable to disease and other threats.
Dr. Webster hopes to elucidate why this adaptation on the Cape honeybee genome survived.
“Why doesn’t it take over the whole world, and why doesn’t it die out?” wondered Dr. Webster. “There’s no really good answer to that.”
An earlier version of this article misstated the type of cell division involved in the formation of honeybee eggs. It is meiosis, not mitosis.
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