The Sex Lives of Scales
Normark, Benjamin B., Natural History
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.
Bird-of-paradise flies-which are not really flies at all, but members of the sca/e-insect family Margarodidae-are a rarely seen kind of scale insect: mature males. Here are two of them on one enormous female. On reaching adulthood, the typical male scale insect lives just a few days. The genetic system of many margarodids, unlike that of most scale insects, is "conventional": both mother and father contribute half of their genome to each of their offspring.
Female scale insects of the family Ortheziidae have sunk their mouth parts into a leaf, where they will stay, feeding on the plant sap, for much of their sedentary, parasitic lives. The sap gives the insects access to a practically unlimited supply of sugars.
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.
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 HughesSchrader 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. …