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

The putoids, unlike other scale insects, are apparently not content to simply insert bacteria from the mother's bacteriome cells into the yolk of the fertilized egg. Instead, when the essential but potentially troublesome bacteria move from the mother putoid to her offspring, entire ceils from the mother's bacteriome move with them, taking up residence to form the bacteriome of the embryo.

It is hard to overstate how weird that process seems to a geneticist. (It is so weird, in fact, that some biologists think Buchner must have gotten it wrong.) For the most part, we animals grow our own organs. Each of my organs is genetically "me" Putoids are the only animals I know of that appear to import one of their organs and then pass it down from mother to offspring much the way people pass along the family china or silverware. Normally an animal's germ line--the cells that give rise to gametes-is the only cell line that is, potentially, immortal: the only cell line inherited by offspring. But in putoids the cells of the bacteriome are equally immortal, yet they are independent of the germ line and not even closely related to it.

Biologists still do not really know what originally led to the evolution of such a wholesale migration of maternal cells (though those cells might play a role in provisioning the embryo). But one effect of the migration is that bacteriome cells in males are genetically identical to the bacteriome cells in their sisters. That may make it hard for the bacteria to identify the sex of their host insect--and thus too risky to their own genetic well-being to kill their hosts at all.

Two other families of scale insects form their bacteriomes in bizarre ways. One is the family Pseudococcidae, another ancient, primitive looking group whose females retain their legs. Pseudococcids are commonly known as mealybugs, a name referring to a coating of fine wax on their bodies that makes them look as though they had been rolled in flour. The other family with extraordinary bacteriomes is Diaspididae, the armored scale insects. Whereas mealybugs retain many of their ancestral characteristics--like legs--the armored scales lie at the opposite extreme in the morphological evolution of scale insects. Adult female armored scales lack legs and eyes, and they barely have antennae. Taken together, the mealybug and armored-scale families encompass more than 4,300 species, some 60 percent of all scale-insect species. Yet despite their clearly visible differences, the two families have each evolved, apparently quite independently, similar yet highly exotic systems of bacteriome development.

Most animal cells, including the cells of the germ line, have two copies of each chromosome: one from the animal's mother, the other from the father. Egg and sperm cells, though, which are derived from germ-line cells, each have only one copy of each chromosome. In particular, to make an egg cell, a female animal must throw away one copy of each chromosome. That is accomplished, among other things, during a process called meiosis.