Honeybee genetics: how do they work?

Your Name: Farhad
Your Bug Question: Hello. I read in a book that worker bees are clones. But how this can happen? Can you explain?

Tonight’s question comes from Farhad, who emailed us and asked how bees can be clones. It’s important to note that it’s the male drones, and not workers who are the clones. Drones are a type of clone, because they’re produced asexually from unfertilized eggs. Workers are produced sexually from fertilized eggs. This is called haplodiploidy, where males only have half the chromosomes of females.

In any organism, the males and females are going to be different by definition. Most organisms, including humans, have to be capable of producing two different types of organisms from the same set of genes. You can see a lot of these differences pretty easily, and Nancy hit on this in the post Is That Bug a Boy or a Girl.

Tonight, we’re going to go a lot deeper into that topic.

A lot of the stuff in this post is a really hard concept to wrap your head around, and I admit that it took me awhile to get this back when I took genetics. I didn’t fully understand what much of this stuff meant until I started taking upper level biochemistry classes, and actually saw some of this stuff for myself. It’s a story best told in pictures, so this will be a very picture intensive post.

So first off, bee genetics are weird to the point where they’re almost nonsensical. For example, female bees produce male clones and these clones can never have sons. They can have grandsons, though.

So let’s go through what it means to be haplodiploid, and then we’ll talk about why a fully functional organism can be haploid.

Haplo-what?

So first off let’s discuss sex determination, the difference between diploid and haploid, and what that means for our insect friends.

In our cells, our genome exists as chunks of protein and DNA known as chromosomes. These chromosomes live in a huge organelle called the nucleus, in the center of the cell.

A ploidy number is a fancy way of saying that an organism has a certain number of chromosomes. For example, wheat is hexaploid which means that it has six (hexa) copies of each chromosomes (ploid). Humans are diploid, which means that we have two (di) copies of each chromosome (ploid).

You can take pictures of these chromosomes, and this picture is called a karyotype. Here’s what my karyotype probably looks like:

Human male karyotype, courtesy of the Human Genome Project, via Wikipedia.

Human male karyotype, courtesy of the Human Genome Project, via Wikipedia.

For the most part, those pairs of chromosomes are identical. Each pair contains different genes, but each chromosome in a pair contains the same genes.

The sole exception to this rule sticks out like a sore thumb, and it’s located in the lower right corner. These seemingly un-matched partners are called sex chromosomes and they determine whether the organism will be biologically male or female. That dinky little thing on the right is the Y-chromosome, and it’s what makes me a boy. The one on the left is the X chromosome, and Nancy would have two copies of that. You can see an example of a human female karyotype here.

This is called XY sex determination, where females have two identical sex chromosomes (XX) and males have two different sex chromosomes (XY). Our chromosomes exist in identical pairs, except for that last set which is different in males. This is what forms the basis of sexual dimorphism in XY animals like humans.

So let’s look at a bee karyotype.

Honeybee karyotypes

Looking at a karyotype in this case is as easy as counting blobs. If you count the blobs on the left karyotype under the male honeybee, there’s 16 little black blobby chromosomes. On the right, under the female bees, there’s 32 black blobby chromosomes. So males have half the number of chromosomes as females.

Once you see a karyotype, haplodiploidy becomes pretty easy to understand!

How can you get different bodies from the same genes?

This is where we get into some nitty-gritty genetics and biochemistry, which is by far my favorite subject to discuss. Bees are in a unique situation, genetically speaking.

In people, the genes that make *ahem* male characteristics aren’t found in females. However, I do have all the genes which make *ahem* female characteristics. Females are kind of the base-code for the animal kingdom in this way.

Male bees aren’t any different. Male bees are more or less clones of their mothers that have half the number of chromosomes. The mothers have chromosomes which come in pairs, and the genes in these chromosomes can recombine to create new combinations, so it’s not quite that simple…but they come from unfertilized eggs so all their genes are a combination of whatever’s in mom. So they have all the stuff that’s needed to make a working female bee, despite having half the chromosomes.

But look at the queen and worker bees…their bodies are completely different. So is the male’s body, for that matter. So bees are capable of producing at least three sets of unique body types from the same genes. This is called polymorphism, which is a fancy word that means ‘many bodies from one’. To understand this, we need to need to know a little bit about genomics.

A lot of people believe that your set of genes is what makes you unique. That’s not necessarily the case. A mouse and human have similar sets of genes, and you can swap out most yeast genes with human genes and still have living yeast. Instead, it’s the sequence those genes activate and how much of a particular protein your body allows itself to produce. There’s a lot of stuff that goes into this. There’s promotors which only allow a certian amount of genes to be produced, and epigenetic programming which shuts genes off in response to certain things.

A genome is more like a grocery list, and the final organism is more like a specific recipe made from certain ingredients. To make it simple, let’s consider two recipes I recently made. For the Fourth of July, I made this Sauerkraut ribs recipe from NPR. I’m also making fish tacoes, which involve some similar ingredients.

Food genomics

The genome is more like a grocery list, because it just contains a bunch of potential ingredients. After you figure out what stuff to bring out of the fridge (or transcribe to mRNA, in the case of genomics), there’s a lot of processing which goes on. Some stuff is combined together in new ways, whereas other things might be left out of the recipe completely if they’re not needed.

The body preferentially uses certain genes to make different parts. In some cases, the parts the body uses may be the same. Some genes may be used all the time, like people who put black pepper on everything. Other genes may only be used for certain recipes, like the Tilapia and ribs above. There are also genes which are used in all body plans, but are prepared differently depending on which body plan is used. This is analogous to the cabbage and sauerkraut in the recipes above.

Okay…so the genome is a Swiss-army knife. How are workers, drones, and queens different?

Food preparation and genetics do have some superficial similarities…but what does this process look like in a real animal?

Queen VS Worker

Workers are produced from fertilized eggs, but are fed a different diet during development. Specifically, this is a high protein diet called royal jelly which is produced by the workers. The effects of this diet aren’t well understood, but it’s known that feeding the bees the high protein diet causes them to mark their genomes differently. These marks, called methylation, prevent some genes from being made. Preventing these genes from being made turns larvae into queens.

Male VS Female

Whether a honeybee is a boy or a girl depends on a gene called complimentary sex determiner, usually called CSD in the literature. The bee needs two different copies to make a girl, and only one to make a boy.

If there are two different copies, CSD is made as a working protein which helps make a second protein, feminizer. Feminizer, as it’s name suggests, is largely responsible for making girl parts on bees (and other insects). This gene helps make a third protein, doublesex, which is a really important protein that tells the body what to do. Doublesex directs the production of all the girl genes, if it’s made with the help of feminizer.

So what if there’s only one copy of CSD?

The male still makes feminizer, but it makes a copy that doesn’t work. Since feminizer is broken in boys, a male-specific doublesex protein is made. This copy of doublesex, only made in boy bees, tells the bees to make boy proteins.

I understand pathway stuff best when I draw it out, so here’s a diagram:

Editor’s note: WordPress is having some issues with picture uploads at the moment. We’ll get the diagram inserted into the post as soon as we can. In the meantime, it can be found at the Twitter link.

It’s important to note that not all bees, wasps, and ants work like this. There are a lot which lack the CSD gene altogether, so there are a lot of different ways to make boys and girls from the same sets of genes.

The Bottom Line

Bee genetics are weird, complicated, and wonderful. Males come from unfertilized eggs, which means they only have half the chromosome count of females. Boys get a male-specific version of a gene called doublesex which turns on all the boy genes. Those boy genes make the male body including those big, dreamy drone eyes.

Females come from fertilized eggs, which means they have a standard diploid chromosome count. They get female-specific version of doublesex which is made by a working feminizer gene. Female bees can either become queens or workers, depending on what their diet looked like as larvae.

Again, I want to stress that this is a simplified version of events here. After bees emerge, it’s possible for some females to start producing eggs destined to become males if the queen becomes weak or dies. This is prevented by a complicated mix of pheromones…which have their own genetic story.

Works Cited:

Gempe, Tanja, and Martin Beye. “Function and evolution of sex determination mechanisms, genes and pathways in insects.” Bioessays 33.1 (2011): 52-60.

Brito, Rute M., and Benjamin P. Oldroyd. “A scientific note on a simple method for karyotyping honey bee (Apis mellifera) eggs.” Apidologie 41.2 (2010): 178-180.

Heimpel, George E., and Jetske G. de Boer. “Sex determination in the Hymenoptera.” Annu. Rev. Entomol. 53 (2008): 209-230.

Honeybee Genome Sequencing Consortium. “Insights into social insects from the genome of the honeybee Apis mellifera.” Nature 443.7114 (2006): 931.

This entry was posted in Chemistry, Developmental Biology, Evolution, Physiology, Pollination and tagged , , , . Bookmark the permalink.

2 Responses to Honeybee genetics: how do they work?

  1. rB says:

    Thank you, thank you, THANK YOU!!

    Like

  2. Pingback: How do butterflies make boys and girls? | Ask an Entomologist

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