Disrupting a single gene causes genetically female embryos to develop as males.
There’s a lot more to honey bee sex determination than most of us think. When we were new honey bee enthusiasts, what makes a bee male or female was probably one of the first bee biology facts that we learned: Eggs with one set of chromosomes turn into males, and eggs with two sets turn into females, says the ubiquitous lore. If your bee mentor was particularly knowledgeable, they may have even told you that it all comes down to a specific part of the DNA – the complimentary sex determiner (csd) gene – and that in rare cases, it is possible to have males with double the number of chromosomes they’re supposed to. But it is even more complicated than that.
Researchers at the Heinrich-Heine University in Germany have discovered that disrupting a single gene (feminizer) in the honey bee is enough to fully switch female bees to males. Annika Roth and Dr. Martin Beye, the lead researchers of the study (which was recently published in PLOS Biology)1, used the CRISPR gene editing system to modify the feminizer gene and flick the sexual switch. They went even further to show that editing another gene, doublesex, caused the females to grow organs that were more like testes than ovaries, while keeping a feminine face and figure.here’s a lot more to honey bee sex determination than most of us think. When we were new honey bee enthusiasts, what makes a bee male or female was probably one of the first bee biology facts that we learned: Eggs with one set of chromosomes turn into males, and eggs with two sets turn into females, says the ubiquitous lore. If your bee mentor was particularly knowledgeable, they may have even told you that it all comes down to a specific part of the DNA – the complimentary sex determiner (csd) gene – and that in rare cases, it is possible to have males with double the number of chromosomes they’re supposed to. But it is even more complicated than that.
“Human sex is determined via X and Y chromosomes, but in honey bees it is normally determined by if an egg is fertilized or not,” says Roth. “The doublesex mutants develop intersex reproductive organs that are neither male nor female.” Roth’s research focuses on sex genes because she is interested in figuring out how sex gene expression is linked to the food a larva eats, and whether a bee develops as a worker or queen, on top of being generically female. “We took a large step towards the understanding how the external food signal is integrated with the internal genetic signal.”
The idea of switching a bee’s sex by disrupting a single gene may sound alarming, but in fact, these results aren’t exactly surprising. The data emulate previous research from over a decade ago, which achieved the same outcome using a different technique (RNA interference) to disrupt some of the same genes.2 The feminizer and doublesex genes are part of the sex determination cascade: Each gene in the cascade acts like a switch, and when all the switches are flicked in one direction, a female bee is born. In the other, a male.
It all starts with the csd gene (Figure 1). Like you may have learned early in your beekeeping career, having two different copies of this gene (the normal outcome of fertilization) is what genetically programs an egg to be female. Genes contain the instructions to make messenger RNA (mRNA), which are in turn translated into proteins – the miniature machines that do most of the jobs for our cells. We still don’t know why, but the proteins produced by the csd gene are not functional when there is only one copy, or when both the copies are the same. But when the two copies are different – that is, they have a slightly different sequence – they work together to flick the next switch in the sex determination cascade (the feminizer gene). When feminizer is switched on, it rearranges the mRNA of the next gene (doublesex) into a new sequence that specifies development of female sex organs. Otherwise, the default doublesex sequence remains, which specifies development of male sex organs.
This cascade may sound like a convoluted goose chase. But if the system is a series of switches, the first switch is whether the CSD proteins are functional (female) or not (male), the second switch is whether feminizer is functional (female) or not (male), and the third switch is whether doublesex takes on the female- or male-specific sequence. Using genetic engineering, Roth, Beye, and their colleagues have shown that the first switch can be circumvented; that is, despite being genetically female, knocking out the feminizer gene is sufficient to fully switch a female bee to male. But knocking out doublesex only changes the sex organs (not the body type), and only some of the time, making it more like a rheostat than a binary switch.
Genetic engineering is an immensely powerful research tool. Scientists have been using it since the 1970s, when the first genetically engineered mouse was created by Rudolf Jaenisch and Beatrice Mintz3 (Figure 2). Today, genetically engineered mice are integral to medical research, since they can be engineered to have ailments that mimic human diseases and allow us to explore new treatment methods. With the click of a button and a credit card, researchers can order genetically engineered mice from the company Jackson Laboratories, which specializes in producing mice for biomedical research, straight to their laboratory door. Genetically engineered fruit flies, too, are commercially available to research labs and have unlocked a wealth of information about development, from embryonic growth to metamorphosis. And genetically engineered nematodes, with their simple nervous systems, have unearthed nuances of neural circuitry. All of these organisms have contributed substantially to our understanding of biological processes underlying human diseases. And the field is continuing to expand.
Today, genetic engineering is expanding beyond the bounds of the usual model organisms, reaching less-studied species like honey bees, mainly due to having more advanced biochemical tools. In their work, Roth and Beye used a technique called CRISPR – a relatively new, highly efficient gene editing tool. CRISPR is a molecular system derived from ….