Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

It's a Girl!

February 17, 2012

Consider for a moment a bacterium.
Actually, people rarely consider a bacterium. Rather, they think of a whole lot of bacteria. When they talk about bacteria growing, they do not talk about one bacterium getting bigger, instead they mean that the bacteria are multiplying - that the number of bacteria is increasing. For a baterium to be considered a success, it must create offspring. Then it’s offspring must create more offspring An uninterrupted line of bacteria must be passed from one host to another. Only in this way does the life of a bacterium amount to anything.

Consider for a moment that there are some bacteria that can only live in the cytoplasm* of it’s hosts’ cells. Like all bacteria, to be considered a success, it must be passed from one host to another, one generation to the next. Taking into account the fact that it can only live inside the cells of its host, how can it be passed from one host to another? Try to think of some methods that this might happen.

In fact, before reading on, give this some thought and write down your ideas.

OK. Here is what I have come up with. It is possible for an animal infected with some of these bacteria to be eaten by another animal. This would transfer the bacteria to the new host while they remain inside the original host cells. It might also be possible for an infected animal to bite another animal, delivering some infected cells in the saliva of the biting animal into the body of the bitten animal. Can’t think of too many other ways that an infected cell can enter an uninfected animal.

A group of bacteria called Wolbachia has come up with a unique way to infect new hosts. Their hosts typically are invertebrates: insects, spiders, sowbugs and nematodes (roundworms). Among the cells they invade are the egg cells of their hosts. Not the sperm cells - just the egg cells. The sperm cells are too small to house a Wolbachia bacterium. Wolbachia
Since the Wolbachia bacteria live in the egg cells of the hosts, whenever the host animal lays eggs, the newly hatched offspring are pre-infected with the Wolbachia. In this way, the Wolbachia is passed from host to host, from generation to generation. Whenever an infected female lays eggs, all of her progeny are infected before birth. Then when they grow up and reproduce, all of their progeny will be infected as well, and so on for each successive generation. This is a great way to spread bacteria that need to live inside a cell. There is only one drawback to this system. Maybe you have spotted it.

Let’s assume that half of the eggs an infected animal lays are female and half of them are male. All of these eggs will house the Wolbachia bacteria. Therefore, when the eggs hatch, both the females and males of this second generation will be infected with Wolbachia. But it is only the Wolbachia in the females that will get passed on to the third generation, since only the females will lay eggs. The Wolbachia in the males of this second generation will come to a dead end. They will not get passed on to another generation since Wolbachia doesn’t live in the sperm cells. You might say that these Wolbachia have failed in their mission as bacteria.

One might think that a 50% success rate is not bad for a bacterium. However, no one can argue that a 100% success rate would be twice as good. If there was only some way for the Wolbachia to infect only egg-laying females…

The two-spotted ladybug is a common insect in both Europe and North America. It is easily recognized by the two spots on its back - hence the name Two Spotted Ladybug. Ladybugs are beloved insects because of their gentle nature (to us humans) and their hefty appetite for aphids and other insects we consider pests. Few people, however, realize that ladybugs are not bugs but beetles. Also, few people realize that there are boy ladybugs as well as girl ladybugs. It is believed that the ladybug is named after Our Lady - Mother Mary who is often shown wearing a red cloak, the color of many ladybugs wing covers.
The two-spotted ladybug is one of the insects that can be infected with the Wolbachia bacteria. When an infected two-spotted ladybug lays eggs, all of her eggs will be infected - the eggs that are destined to be males as well as the eggs destined to be female. If there was just some way that the bacteria could only infect the eggs destined to become female, then its’ success rate would be 100%. Unfortunately, it is not possible for the bacteria to infect only the female eggs. However, once inside the eggs, Wolbachia does something just as effective. The bacteria in the male eggs end up killing those eggs. You might say that these bacteria sacrifice themselves for the good of their brethren. Because now, only female eggs will hatch into new ladybugs. All of these new ladybugs are infected with Wolbachia and being female, can pass the bacteria on to the next generation when they lay eggs. This means that the success rate of the bacteria will now be 100%.
So what happens to the eggs that were destined to become males and were killed by the Wolbachia? In nature nothing goes to waste. These eggs are eaten by the newly hatched female ladybugs, thus providing an important first meal that may help insure their survival. Of course, helping insure the survival of these female ladybugs also helps insure the survival of the Wolbachia.
Even though ladybugs were not named ladybugs because they are all female, in some populations of ladybugs that have a high incidence of being infected with Wolbachia, most of them are in fact female because of the Wolbachia. Of course, it is hard to tell when this is the case since it is near impossible for us to tell a girl ladybug from a boy.

There are a number of different strains of Wolbachia and they have different strategies for insuring that females predominate in the population. Here are the strategies that scientists have identified so far:

Male Killing
Male eggs are killed by the Wolbachia. Example - ladybugs

Cytoplasmic Incompatibility
Wolbachia-infected males can not successfully mate with non-infected females or with females infected with a different strain of Wolbachia. Example - mosquitoes

Infected males develop into females. Example - pillbugs

Infected females can reproduce without males. Example - Trichogramma wasp

Scientists have discovered that the Wolbachia bacterium infects over 20% of the world’s invertebrates. This number may actually be much higher, but since there are so many invertebrates in the world, it takes time to test them all for Wolbachia. So the scientists are asking for help. If you are a high school biology teacher and would like to have your class help scientists in this investigation, check out Discover the Microbes Within! The Wolbachia Project, a citizen science project.

*cytoplasm - the jelly-like substance inside a cell that holds its internal parts in place.