The ‘Science’ of the birds and the Combees Part 1: Are you a boy or a girl?

combee
Vespiquen and Combee

Every so often in the Pokemon world, we come across a Pokemon that has a special evolution based on the biological sex of the animal. Some examples of this are Gallade, Salazzle and Vespiquen. The latter example, Vespiquen, will only evolve from a Combee if it is female. Vespiquen, unsurprisingly, is based on the queen bee of a bee hive and is said to look after all the Combee in her colony. The bottom of her body looks like the hexagonal walls of wax that hold eggs and larvae and she also secretes pheromones to control the larvae she raises.

Vespiquen.
Vespiquen

Obviously, Vespiquen are all female because they are based on the female queen bee, but the thing that makes this Pokemon really interesting is how the thing it is based on, female bees, came to be female. It is all based on chromosomes.

In humans, sex determination is due to a certain combination of chromosomes, which group together in pairs. Chromosomes are structures that hold highly compact DNA and genes. Different genes are on each chromosome, so the specific chromosomes can determine various features. Human sex chromosomes we have two variations; the X chromosome and the smaller Y chromosome. The egg produced by females in the ovary only has an unpaired chromosome and it is always an X. When that egg meets a sperm, the other sex chromosome pairs with the X in the egg. The sperm can either carry an X or a Y.  An XX combination will lead to a female, and an XY will lead to a male. We get one chromosome from each of our parents and that determines our biological sex. Having a pair of chromosomes like this is called diploidy (di refers to two, and ploid refers to chromosomes)

human chromosomes
Sex determination in humans is caused by combinations of X and/or Y chromosomes

That is all straight forward in humans and most animals, but bees like to buck the system. First of all, not all Bees are diploid; Some are haploid and only contain a set of single chromosomes, instead of a pair. What really makes this interesting is that it is not the type of chromosome that determines sex but whether the offspring are haploid or diploid, which all depends on the queen.

The bee colony is made up of the mother queen, the female workers, and a few male drones. The queen bee in a colony is responsible for giving birth and will have given birth to most of the colony. In humans, a baby can only be made with both a mother and a father, and queen bee will also produce eggs that have been fertilised by a male, and result in a diploid larva. The queen only needs to mate once, with 10 or so males, and she stores up the sperm from that event so she can use it for her entire life (1-5 years). She gets to decide which eggs get fertilised and when. These diploid larvae will ALWAYS be female.

But this is where it gets weird. The queen doesn’t actually need another male bee to reproduce. If she wants, she can lay a viable egg all by herself. Of course, like the example in humans above, this egg will only have “half” a chromosome pair and will therefore only have a single chromosome. In humans that would be unviable, but in bees, this will lead to a male.

Bee chromosomes
When a queen breeds with a drone, a female is born. When a queen lays an egg by herself, a male is born

So all that weirdness culminates in this; Female bees are produced from fertilised eggs and male bees are produced through unfertilised eggs. Females are diploid, and males are haploid. It is determined by the amount of chromosomes, not the type.

But bee breeding doesn’t stop there. Why, exactly, is it evolutionarily beneficial for a male bee to be haploid? When a male bee breeds with a queen he contributes 100% of his DNA to the offspring and the queen contributes 50% of her DNA. Sisters from this union will therefore share 50% maternal DNA and 100% paternal DNA between them, and leads to a very strange occurrence when all of the females produced from that breeding pair are 75% related, instead of 50% related like offspring from two diploid parents. This method of breeding is a way of keeping the bees genetically related and loyal to each other, as well as attempting to preserve good genes (of course, some are only half sisters with different fathers, but the principle above is sufficient that it doesn’t matter). Having only the single chromosomes prevents males from growing stingers, and they make awful workers, so they don’t bother with that. They are built for breeding, and that is about it.

Bee relatedness.png
Bee relatedness is denoted by a blue arrow. The queen is 100% related to her sons, the daughter is 100% related to her father, 50% to her mother and 75% related to her sister. Female Combee have a red triangle on the bottom head

It is quite possible that female Combee are the only Combee that can evolve because they have a whole lot more chromosomes. The male Combee are just held back by have a set of single chromosomes instead of a pair. In real bees, haploidy prevents stinger growth and workability, but in Combee, it prevents evolution. This method of sex determination may or may not also apply to Beedrill, but Metapod isn’t inhibited from evolving into a Beedrill due to its chromosome count, but it seems like Vespiquen is all about the chromosomes!

First image was made by Haychel and is found at https://haychel.deviantart.com/art/Vespiquen-Speedpaint-397814977

More stuff to read

Gempe, T. & Beye, M. (2009) Sex determination in honeybees. Nature Education 2(2):1

Foster, K. R., et al. (2006). Kin selection is the key to altruism. Trends in Ecology & Evolution 21(2): 57-60.

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