Placental and marsupial opposite numbers

In animals, unlike bacteria, gene transfer seems almost entirely confined to sexual congress within species. Indeed, a species can pretty well be defined as a set of animals that engage in gene transfer among themselves. Once two populations of a species have been separated for long enough that they can no longer exchange genes sexually (usually after an initial period of enforced geographical separation, as we saw in Chapter 9), we now define them as separate species, and they will never again exchange genes, other than by the intervention of human genetic engineers. My colleague Jonathan Hodgkin, Oxford's Professor of Genetics, knows of only three tentative exceptions to the rule that gene transfer is confined within species: in nematode worms, in fruit flies, and (in a bigger way) in bdelloid rotifers.

This last group is especially interesting because, uniquely among major groupings of eucaryotes, they have no sex. Could it be that they have been able to dispense with sex because they have reverted to the ancient bacterial way of exchanging genes? Crossspecies gene transfer seems to be commoner in plants. The parasitic plant dodder (Cuscuta) donates genes to the host plants around which it is entwined.*

I am undecided about the politics of GM foods, torn between the potential benefits to agriculture on the one hand and precautionary instincts on the other. But one argument I haven't heard before is worth a brief mention. Today we curse the way our predecessors introduced species of animals into alien lands just for the fun of it. The American grey squirrel was introduced to Britain by a former Duke of Bedford: a frivolous whim that we now see as disastrously irresponsible. It is interesting to wonder whether taxonomists of the future may regret the way our generation messed around with genomes: transporting, for example, 'anti-freeze' genes from Arctic fish into tomatoes to protect them from frost. A gene that gives jellyfish a fluorescent glow has been borrowed from them by scientists and inserted into the genome of potatoes, in the hope of making them light up when they need watering. I have even read of an 'artist' who plans an 'installation' consisting of luminous dogs, glowing with the aid of jellyfish genes. Such debauchery of science in the name of pretentious 'art' offends all my sensibilities. But could the damage go further? Could these frivolous caprices undermine the validity of future studies of evolutionary relationships? Actually I doubt it, but perhaps the point is at least worth raising, in a precautionary spirit. The whole point of the precautionary principle, after all, is to avoid future repercussions of choices and actions that may not be obviously dangerous now.

I began the chapter with the vertebrate skeleton, which is a lovely example of an invariant pattern linking variable detail. Almost any other major group of animals would show the same kind of thing. I'll take just one other favourite example: the decapod crustaceans, the group that includes lobsters, prawns, crabs and hermit crabs (which are not crabs, by the way). The body plan of all crustaceans is the same. Whereas our vertebrate skeleton consists of hard bones inside an otherwise soft body, crustaceans have an exoskeleton consisting of hard tubes, inside which the animal keeps and protects its soft bits. The hard tubes are jointed and hinged, in something like the same way as our bones are. Think, for example, of the delicate hinges in the legs of a crab or lobster, and the more robust hinge of the claw. The muscles that power the pinch of a large lobster are inside the tubes that make up the claw. The equivalent muscles when a human hand pinches something attach to the bones that run through the middle of the finger and

Like vertebrates, but unlike sea urchins or jellyfish, crustaceans are left/right symmetrical, with a train of segments running the length of the body from head to tail. The segments are the same as each other in their underlying plan, but often differ in detail. Each segment consists of a short tube joined, either rigidly or by a hinge, to the two neighbouring segments. As with vertebrates, the organs and organ systems of a crustacean show a repeat pattern as you move from front to rear. For example, the main nerve trunk, which runs the length of the body on the ventral side (not the dorsal side, as the vertebrate spinal cord does), has a pair of ganglia (sort of mini-brains*) in each segment, from which sprout nerves supplying the segment. Most of the segments have a limb on each side, each limb again consisting of a

Bdelloid rotifer

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