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The sex lives of scales: scale insects have evolved one bizarre genetic system after another. The author argues that they are caught in a game of cat and mouse with internal, symbiotic bacteria, which has unleashed genetic bedlam
Natural History, Sept, 2004 by Benjamin B. Normark
If you were in the backyard this summer, watering your lilacs or checking your apple trees for pests, you may have noticed that the plants were afflicted with little bumps on the leaves or bark, coming down with what looks like nothing so much as a case of botanical acne. Many people are surprised the first time they find out that each bump is actually an animal: a scale insect. Many scale insects look more like mollusks or turtles than like beetles or cicadas--the bodies aren't obviously segmented into head, thorax, and abdomen, and the six legs and four wings typical of most insects are nowhere to be seen. Yet those little bumps are indeed insects, related to aphids, whiteflies, and jumping plant lice.
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All scale insects are parasites of plants, and the insects' habit of sucking the sap out of plants makes them generally disliked by farmers and gardeners. In a sense, scale insects have taken the parasitic lifestyle to the farthest extreme: the females of some lineages have evolved into legless, eyeless blobs that are permanently attached to their hosts.
Even among the most casual keepers of houseplants, most people's reactions to scale insects run from mere distaste to full-blown disgust. But if you take the trouble to look beneath the surface, scale insects turn out to be quite fascinating creatures. In particular, the laws of genetics--the rules that describe how the DNA of one generation is passed on to the next--seem to have gone totally haywire as the scale insects have evolved. The group encompasses more weird variations on the laws of genetics than does any other group of animals.
But surely, weirdness is in the eye of the beholder. Just because mammals don't have such varied genetic systems, is that grounds for calling scale insects strange? Well, consider this: From a genetic point of view, a typical multicellular animal is an assemblage of cells that are nearly all clones, or genetically identical to one another. Yet in most species of scale insects, not all the cells of an individual get the same genes from the insect's mother. Furthermore, scale-insect fathers vary widely in the genes they contribute (or, often, do not contribute) to their sons: In some species the males are the product of asexual reproduction and have no father at all. In other species the males have fathers, but all the chromosomes they get from the fathers are deactivated, in still other species the chromosomes from the fathers are present in some cells but not in others.
Much of this is not news. Thanks to the pioneering work of the American geneticist Sally Hughes-Schrader and others, many of these facts have been known since the 1920s. Only recently; however, has evolutionary theory begun to catch up with those facts, and to describe them with concepts powerful enough to explain the data in a satisfactory way. Only now do biologists have an inkling about what might be causing the apparently staid world of blobby little plant parasites to be convulsed by so many sexual revolutions.
It is not readily apparent why scale insects, of all I life-forms, should exhibit such a diversity of genetic systems. After all, other, related groups of insects show nothing approaching the same degree of variation in their genetic machinery. Particularly puzzling is the scale insects' patrilineal inheritance. Why is it so frequently sabotaged? Those are unsolved mysteries, but quite recently, suspicion has fallen on some unusual suspects: bacteria.
The bacteria in question are not disease organisms; rather, they live symbiotically with their scale-insect hosts, a relationship that is vital to both organisms. The bacteria derive their very livelihoods from the insects, and the insects in turn depend on their resident bacteria for help surviving on a diet that is conspicuously lacking in protein. Yet despite the aid the bacteria render the insects, the bacteria may still interfere with the insects' genetics. To understand why, it is helpful to understand something about the lives of scale insects and the bacteria that live reside them.
Immature scale insects do not look much like the little bumps on plants that many of them are destined to become. During the first, or "crawler," stage in the life cycles of scale insects, the animal looks a bit like a small potato bug. The typical crawler, after hatching from its egg, walks a short distance on its natal plant, then inserts its mouthparts into the plant and becomes immobile. There the insect generally remains through subsequent immature stages, regardless of its sex.
The life and form of an adult scale insect, how ever, depends heavily on its sex. The males are not exceptionally strange-looking insects. Most people would probably mistake them for small flies, such as gnats or midges. Their strangeness becomes apparent only when you look more closely: they have only two wings (like true flies but unlike almost all other insects) and simple, rather than compound, eyes, and they always lack functional mouthparts; grown males never feed. Much more unusual looking are the adult females; they are the bumps you may have seen on your houseplants. Adult females never have wings, and they often lack legs and eyes as well; for antennae, many possess only the tiniest nubs.
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