Are You My Father?
In the wonderful children’s book by P.D. Eastman called Are You My Mother? a baby bird hatches while his mother is off the nest. The baby bird, too impatient to wait for its mother to return, sets off to ask “Are You My Mother?” of everyone and everything it meets. Sorry to spoil the ending, but eventually, the baby bird is happily reunited with its mother.
Life has no such happy ending for a baby spider mite looking for his father, however. He would be looking forever. But oddly enough, if he looked hard enough, he might find his grandfather.
Male spider mites can’t ever find their fathers because they don’t have any.
There are, of course, animals that manage without any males and therefore any fathers. These “parthenogenic” reproducers have only daughters, who don’t need to mate in order to have daughters of their own. So no males, no fathers.
But in spider mites, there are males, but still no fathers. How does that happen?
Spider mites are those tiny 8-legged, often bright red, arachnids that are the bane of farmers and house plant lovers alike. (Click here for more information at Curious Nature’s companion site, AskaNaturalist.com) If you look at the chromosomes of a spider mite under a high-powered microscope, you’ll very quickly notice that female spider mites have twice as many chromosomes as males. It turns out that female spider mites have two complete sets of chromosomes (just as people do), but male spider mites only have a single set. When both sexes have two sets, as with people and other mammals and most of the rest of the animal and plant species, it’s called diplodiploid. When males have only one and females have two, as with spider mites, it’s called haplodiploid.
In spider mites, an egg that is fertilized with sperm from a male has two sets of chromosomes, one from Mom, and one from Dad. And this diploid egg always develops into a female. So where do the haploid males come from? Whenever an egg is not fertilized, it develops using the single set of chromosomes it got from its mother. And it becomes a male. So fertilized eggs become diploid females, unfertilized eggs become haploid males.
What this means is that a male spider mite has a mother – she provided the egg he developed from. But he has no father. If he did, he’d have two sets of chromosomes and he’d be a female spider mite. His mother, however, like all females, developed from a fertilized egg. So she has a father. Which means our spider mite boy has a grandfather … but he can ask “Are you my Father?” till his short life is spent, and he’ll never find peace.
Haplo comes from the Greek word haplous, meaning single, and diplo comes from the Greek word diplous, meaning two. The first term refers to males, the second to females. So haplodiploid means one set of chromosomes in males, two in females. And diplodiploid means two in both sexes.
That’s the haplodiploid system. It’s the rule in most mites; in insects in the large ant, wasp and bee order, and in some species of bark beetles; as well as in certain nematode worms; and some rotifers. Genetic evidence shows that haplodiploidy has evolved at least 20 times and each time from a diplodiploid species. So the question is “If diplodiploidy works for the overwhelming number of animals, what’s going on with spider mites, and bees, and bark beetles?”
One clue is that in all of these groups, there is a fair amount of inbreeding. Inbreeding is when animals that are closely related mate. We’re primed to think inbreeding is a bad thing, but in nature that’s not always true. For one thing, if it’s very hard to find a mate because you are a small organism that can’t move far and your food sources are spaced very far apart compared to your level of mobility, and your life is very short – all of which pretty much describes the life of spider mites – being able to mate with the relatives in your colony is better than dying with no offspring. Inbreeding can lead to extreme genetic similarity between mates. This is a bad thing when it concentrates genes for genetic diseases or defects, but it’s a good thing if it standardizes a very successful set of genes in a particular habitat.
The other interesting thing about inbreeding is that if you’re stuck with an inbreeding lifestyle, you don’t want to have too many sons.
Imagine a female spider mite in a colony of spider mites on the underside of a leaf. The colony is a couple of weeks old, and the leaf it’s feeding on is dying. So when a female spider mite completes her four-stage metamorphosis and is fertilized by a male, she can either stay where the food is being depleted, or she can disperse to find another leaf. If she disperses, by walking to another leaf, or being carried away by wind (some spider mite females actually wave their legs in air to help the wind catch them), she may well land where there aren’t any other spider mites. If she lays her eggs there, and they hatch into sons and daughters, it’s likely that the siblings will end up mating.
If our dispersing female had 10 female offspring and 10 male offspring, the male offspring, which are capable of mating with multiple females, will end up competing with each other. So what? Well, let’s assume all the female offspring end up being mated, and each lays 20 eggs of their own. So now there are 200 eggs to hatch into the next generation.
But what if our original disperser laid 20 eggs and 15 of them were female and only 5 were male. If we assume the 5 males could each mate with three females, then we would end up with 15 mated females, each laying 20 eggs, for a total of 300 eggs for the next generation – one and a half times as many as with an even 50-50 sex ratio.
It’s not hard to see why any system that favored daughters over sons under this kind of inbreeding would very quickly win out. And any female who had genes that led her to produce more daughters than sons would pass on more of those genes to the next generation than a female whose genes led her to have equal numbers of sons and daughters.
Now this isn’t usually true in the animal world. In fact, theory suggests that under conditions other than inbreeding, it’s almost always better to have 50% daughters and 50% sons. And a sex-determination systems like ours, where if you get an X chromosome from your father, you develop into a girl, and if you get a Y chromosome from your father you develop into a boy, tends to lead to a 1:1 sex ratio. (For an explanation of the 1:1 theory of sex ratios, click here.)
But if you’re a spider mite, stuck with inbreeding, how can you skew the sex-ratio of your offspring so you get significantly more daughters than sons? Well, what if you had a system of sex determination that was under the control of the mother? What if, for example, eggs that were fertilized became daughters, and eggs that weren’t fertilized became sons? In other words, what if you were haplodiploid? Maybe you could control the number of eggs that become fertilized.
That, in fact, is exactly what spider mite females do. It has even been shown that spider mite females can modify the sex ratio of the eggs they lay. Females of many arthropods like mites and insects have the capability to store sperm internally until it's needed to fertilize eggs. And female spider mites seem to be able to manipulate sperm to fertilize some eggs and not others. It’s not that she is consciously doing this, but if a female spider mite is on a leaf alone, and therefore, her offspring are very likely to end up mating with each other, she will lay mostly fertilized female eggs and a few unfertilized male eggs. If she’s on a leaf with hundreds of other spider mites, she’ll lay a more even mix of fertilized female eggs and unfertilized male eggs. This is in close agreement with theoretical models of how females should optimize the sex ratio of their inbred offspring.
Across the animal world, nearly all the haplodiploid groups have significant inbreeding, or probably did in the past. But now that we know that haplodiploidy and inbreeding seem to fit together, the question becomes, which comes first. Does haplodiploidy make it possible for some organisms to develop a successful inbreeding lifestyle? Or does inbreeding set the stage for the evolution of haplodiploidy?
Does a spider mite boy have no father because his family is inbred, or is his family inbred because he has no father?
For now, that’s a question without a clear-cut answer. We can save the spider mite baby boy some heartache and tell him to stop saying, “Are you my father?” because unlike P.D. James' baby bird, the spider mite baby’s search is futile. But we can’t yet tell him why he was born to such a sad fate.