Bee Sex Gene Find Helps Solve 150-Year-Old Riddle

John Pickrell in England
for National Geographic News
September 8, 2003
For years, science has been vexed to explain a simple question about bee reproduction: Why do unfertilized bee eggs become male, while fertilized eggs produce female worker or queen offspring?

According to new research, the answer lies in a unique form of genetic sexual determination. Female bees carry two slightly different copies of a sex-determination gene: one from their mother and the other from their father, which may work together to trigger female development. Unfertilized male bee eggs, however, have only a single copy of the gene from their mother, which causes their development to proceed along the male route.

"This is a totally different genetic system to that discovered in other animals [humans, mice, and some worms and flies for example]," said lead researcher Martin Beye, a geneticist at the Martin-Luther University in Halle-Wittenberg, Germany.

150-Year Conundrum

Johann Dzierson, a priest living in 19th-century Poland, was the first to carry out bee breeding experiments in 1845 that showed unmated queen bees produced only male progeny, while fertilized larvae developed into female workers, or if fed a diet of royal jelly (a highly nutritious secretion produced by worker bees), egg-laying queens. The conundrum remained unexplained until now.

Later studies proved that male bees have half as many gene-bearing chromosomes as females. Whilst most animals, including humans, inherit one set of chromosomes from their mother, and a matching set from their father, male bee drones get by on one set alone.

Bees, wasps, and ants from the group of insects known as the hymenopteran order and other invertebrates have males with only half the usual complement of chromosomes. These insects and invertebrates comprise 20 percent of all animals.

Specific mechanisms behind sex determination among hymenopterans had not been properly understood, until recently. To fill in that knowledge gap, Beye and colleagues in Norway and the United States undertook a new genetic analysis.

As they reported in an August issue of the science journal Cell, the team isolated a bee gene called complementary sex determiner (CSD), which comes in 19 different varieties. Female bees always carry two different types of the CSD gene, one on each chromosome. Males, on the other hand, only ever have a single copy.

Put to the Test

To prove two different copies of CSD are necessary for female development, the team showed that something odd happens when fertilized larvae contain the same CSD variety on both sets of chromosomes, a very rare occurrence. While all fertilized eggs should become female, these unusual progeny develop into sterile physiological males. Worker bees can detect these larvae in the hive and kill and eat them.

Further experiments showed that when the gene was knocked out in developing eggs, fertilized females also developed into physiological males. Knocking that gene out had no effect on males.

One possible explanation for this, according to Beye, is that the two different gene varieties form proteins that combine and are able to turn on other genes that control female development.

Boon for Breeding

Breeding experiments had shown that a sex determination gene must exist in ants, bees, and wasps. But no one had found it until now, commented honeybee geneticist Jay Evans of the United States Department of Agriculture Bee Research Lab in Beltsville, Maryland.

"It is certainly an important find for bees and for insect sex determination generally," he said. "[However,] establishing the rest of the pathway will take substantial work in bees, since it looks to be distinct from known mechanisms."

The finding could also be important for bee breeding initiatives, said Beye. Breeding bees for useful traits, such as low aggression or improved immunity, requires the sequential mating of closely related animals. This can lead to fertilized eggs having both copies of the same CSD gene, creating sterile physiological males.

In the most extreme cases, 50 percent of eggs produce sterile males, which are killed by workers. This fatally depletes the workforce and often means that the hive fails to make it through the winter. The discovery of the CSD gene means that breeders may be able to genetically test bee lines before mating to ensure that fertilized eggs will end up with two different types of the CSD gene.

"We've known for some time that inbred queens do poorly and often produce morphological males," said Evans. "Knowing the actual [gene] will allow for smarter matings by bee breeders that avoid inbreeding costs," he said.

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