SAFARI TOUR: Womanizing Wolbachia and wrestling woodlice
Fasten your seatbelts!
How many times as ecology students, have we been amazed by the complex interactions between species? Some of us may even say “everyday”! However, when we try to understand these complex interactions, sometimes our concentration gets lost on all the complicated theory. To help you, dear reader, to keep up with the story that we will present soon, we will make it a less bumpy ride. We would like to take you on a ride to the complex interaction of the endosymbiotic Wolbachia bacteria in the woodlice Armadillidium vulgare. This route began at Proffesor Richard Cordaux lecture and we would like to take you with us, because the processes we will see could be at the basis of the creation of sexchromosomes. We will not ask your full concentration for maths and complex chemistry, we will just ask you to fasten your seatbelt for this genome journey from a parasite to host. We will pass by the consequence of sex ratio change due to a parasite and the fight between feminizing and masculinizing genes, which has lead to the birth of sexchromosomes.
Where it all started…
So let’s start by putting the gear in the reverse and let’s travel back to the beginning of this interaction, somewhere between the ocean and terrestrial habitats. We observe millions of Wolbachia bacteria from different strains infecting different arthropods. While going in direction to land, we see Wolbachia stumbling into wet shiny woodlice, infecting fresh meat from the ocean just like a bacteria would like. Now let’s follow two of this woodlice: a male and female. When we enter the organism of the female, we see Wolbachia has ensured the transmission of the infection in her offspring. Now let’s enter into a male, what we see? We see a “dead end sign for Wolbachia”. No further infection of Wolbachia to his offspring.
It seems that we are in front of a big conflict between the woodlice and the Wolbachia. As relatively peaceful as the endosymbiosis has started, it now escalates to a genome wrestle for increasing fitness. In one corner we see the woodlice genome fighting for an optimal sex ratio: 50% females and 50% males; while in the other corner we see Wolbachia genome fighting for a female biased sex ratio to promote its transmission. Who will win the wrestle? The Wolbachia or the woodlouse? So far, it is 1-0 for Wolbachia due to a fortunate event: a feminizing agent. Let’s go deep into this agent to discover what it does. This feminizing agent induces feminization of males, how? by preventing the develop of androgenic glands in males, thus genetic males become neo-females. And as fantastic as it sounds this neo-females can actually mate with males and produce fertile offspring. Thus males with the same sex chromosomes could have cute little woodlice! Wait…too complicated? Let’s slow down the car and give some dazzling facts on basic genetics. Sex chromosomes in some species like woodlouse, can be represented as ZW for females and males’s sex chromosomes as ZZ. In this way, females are heterozygous and males homozygous (but for humans is the other way around: female: XX and male: XY. So if a neo-female (ZZ+wolbachia) mates with a male ZZ, males with the same sex chromosomes could have offspring. “Incredible , isn’t it?”
If we have a look at the population dynamics of infected woodlice, we see a decreased growth of Woodlice. Males become scarce and the W female sex chromosome vanishes. This can be explained by the following: ZZ+ females and ZZ males have offspring. These offspring are always born as ZZ and so the W chromosome fades. Keep in mind that we are talking about infected populations, thus the W sex chromosome remains in the non-infected populations. But why did not all the male woodlice become neofemales in the infected populations? If we go deeper into Wolbachia transmission, we notice that the Wolbachia transmission to the next generation, is only 90%.
If you are sitting in the car now and feeling confused with the quick tour through the evolution of the interaction: “don’t worry”. We will turn off the engine and you just try to relax on the backseat. We will come to the point after the detailed story! Have a break, eat some chocolate and drink some relaxing tea……………ready? let’s start again.
The poor woodlice
If a genome could have feelings, the woodlice genome would have definitely been unhappy in the light of evolution. Will the woodlice populations eventually reach the 1:1 sex ratio again? Let’s travel through time and space to see how evolution has found a way to solve this wrestle. After a lot of evolutionary time we notice something strange, it seems that we are not the only ones that like to take a trip. Fragments of DNA, genes from Wolbachia travel to the Woodlice genome by accidental insertions. This insertions are an example of the importance of random effects: 30% of the woodlice genome had become Wolbachia insertions! One insertion was not so lucky for the woodlice: the feminzing gene from the Wolbachia was transferred. So it came that uninfected woodlice populations started to show more females.
If the genome had emotions, it would have been even sadder. “Why wasn’t this feminizing gene outcompeted by natural selection?” A bias towards females in the population means less offspring. The reason behind this could be segregation distortion: in the first meiotic cell division, the f-gene might be favored to get into the cell that will form the offspring. “Could this mechanism be true? Would it also happen to other organisms?” The answer is probably “yes”. This observation is in contrast with natural selection and has let researchers to investigate the principle of segregation disorder, but that’s another safari tour!
The wrestle continues…
If you thought that the wrestle had come to an end, there is still a final hook: a masculizing gene. So put in your teeth guards and bet your money!
This masculizing gene caused a male biased offspring for carriers and was dominant over the feminizing gene. 2:1 for the woodlice!!!!! This event seems small, but is huge if you give this masculinizing gene more consideration. What happened here was truly miraculous: the W sex determining chromosome had disappeared earlier from the population and now two new sex determining autosomal chromosomes had appeared. The autosomal chromosome with the masculizing gene acted as a male determining sex chromosome now, whereas the chromosome with the f-gene has become the female determining sex chromosome. “Yes, the woodlice population had again returned to the 1:1 male-female ration with the introduction of the masculizing gene”. Evolution always knows best.
Pfew, this intense ride is over. We would like you to step out of the jeep and think about what this story means on a larger scale. Is this just a regular safari tour to entertain you or could there be a bigger message? The appearance of feminizing genes and masculinizing gene was only some thousands of years ago. We can see what evolution has done in such a short time. Could the origin of human sex chromosomes be a similar story? Does our story also involve endosymbionts?
Next time when you see a woodlice, don’t forget that they are the living evidence of how sexchromosomes can evolve.
See you at our next trip!!!!
driver and tourguide: Maria Ariza and Merijn Moens, IMAE 2015-2017