Researchers uncover the molecular dice game behind bee sex determination

A recent breakthrough in bee research conducted by a team from Heinrich Heine University Düsseldorf has shed light on the long-standing mystery of bee sex determination. Published in the journal Science Advances, their findings reveal a fascinating mechanism akin to a dice game.

In the natural world, an organism’s biological sex carries significant implications for its appearance and behavior, often determined at the beginning of its life. For instance, in humans, the presence or absence of the “Y chromosome” decides whether a person is male or female.

Back in 1845, the Silesian priest Johann Dzierzon explored the mechanisms governing the sex of honey bees (Apis mellifera) and made notable discoveries, including the asexual reproduction of male bees, known as “drones.”

Unlike humans, bees don’t rely on a single sex-determining chromosome. Professor Dr. Martin Beye and his research team at the Institute of Evolutionary Genetics, HHU, have revealed that bee sex hinges on a single gene called “Csd” (Complementary sex determiner) and operates through a unique mechanism.

The Csd gene can exhibit over 100 variations, known as alleles, similar to how different alleles of a gene can influence the color of flower petals.

During sexual fertilization, the egg and sperm chromosomes combine to form a diploid chromosome set. Consequently, two variants of the Csd gene are present in sexually fertilized bees.

The breakthrough insight from the Düsseldorf bee researchers is this: When the two alleles of the Csd gene differ, a female bee develops. Conversely, if the gene alleles are identical on both chromosomes, a male bee takes shape. However, worker bees avoid raising these male eggs to prevent inbreeding.

The mystery, however, lay in how this sex determination occurred at the molecular level. Dr. Marianne Otte, the lead author, explains that each distinct allele of the Csd gene produces a slightly different Csd protein variant. Only when different Csd proteins interact can they activate a molecular switch signaling the development of a female bee. When the proteins match, the switch remains inactive, resulting in a male bee that is not nurtured.

Professor Beye, the study’s last author, aptly likens this process to a molecular dice game, where the outcome of rolling two different numbers is necessary to produce a new female bee.

In contrast, drones emerge from unfertilized eggs, containing a simple chromosome set with identical Csd proteins, as the queen bee intentionally avoids adding sperm during the egg-laying process.

Dr. Otte remarks, “We’ve unraveled a genetic puzzle spanning over a century by tracing it back to the Csd protein’s role as a switch.”

Professor Beye hints at future inquiries, stating, “The method worker bees employ to identify whether the fertilized egg possesses two distinct Csd proteins and is thus destined to become ‘female’ remains a mystery. Since hives are dark, there might be an olfactory cue at play.”

These groundbreaking findings are poised to advance bee breeding practices.

Source: Heinrich-Heine University Duesseldorf

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