We all know that the Colonel's lady and Molly O'Grady are
sisters under the skin; and the more discerning of us voters recognise that, in January 1999 in Barbados, blackish people are much better than whitish. But my interest last week was in far more distant relatives, in primitive
vertebrates and in arthropods, of which our local fortyleg is a good example. They have many things in common, such as being bilaterally symmetrical, having a dorsum or back and opposite the back a belly. They have a front end,
concerned with feeding and a rear end concerned with excretion. Eyes and other sensors are at the front end in both families. And both are segmented, the fortyleg more obviously than the vertebrate.
But where they differ
tremendously is that the vertebrate keeps his nervous system near his back and above his intestine, while the arthropod's nervous system is below his intestine and close to his belly-wall .
The first chordate, or primitive
vertebrate, appeared on earth about 500 million years ago, but the arthropods had been around much longer. Could the vertebrates have evolved from the arthropods? No intermediate forms have been discovered, and anyway, an
intermediate form is unlikely to have an intestine both above and below his nervous system.
What about turning the arthropod upside down to make the chordate? That would provide a nervous system and a gut in the correct
human-type relationship. A little problem is that in arthropods the mouth is rather sensibly positioned towards the ground, where vegetation and dead or dying prey are likely to be found. An upside-down arthopod has a mouth
pointing stupidly at the stars. Furthermore, when the arthropod's mouth is full his swallowing arrangements have to go through a hole in his nervous system to reach his stomach. Clumsy!
Much argument about this matter went on in
the last century. It is only in the last decade or so that any new advances have been made, and from an improbable direction, the new, detailed study of genes and what they can do. The names of genes are so strange that they
deserve italics, like a foreign language. And here, once again, I am indebted to an essay by Stephen Jay Gould, entitled Brotherhood by Inversion.
As usual, much of the work was done on Drosophila, the fruit
fly, easy to breed, cheap to feed (on bananas), never a close friend, and whose anatomy is already very well known to scientists.
The group of genes which concerns us here is the group controlling what structure or organ develops
where. When they go wrong a leg may appear on the top of the head. They are known as homeotic genes, to be clearly distinguished from homoerotic feelings.
The eyes of flies, with their many tiny facets, are very different from
our own job, with its modifiable lens, and from the eyes of octopi, like our own and basically better, but Pax-6 is the gene which makes them all. Our gene works for flies and their gene for vertebrates.
In the toad,
Xenopus, a gene called chordin, makes the spinal cord grow at the back, and this gene is almost the same as the Drosophila gene called sog, which makes the nervous system grow near the ventral surface of
the fly's larva. So vertebrate backs are really arthropod bellies.
And listen to this! At UCLA De Robertis found that a gene called dpp specifies the dorsal side of flies, and there is a similar gene in toads,
Bmp-4, which specifies their ventral side. Furthermore, vertebrate, toadish Bmp-4 can antagonise chordin, preventing the development of the toadish nervous system, just as fruit-fly dpp can block the growth
of the fly's ventral nerve cord which was stimulated by sog.
Showing even more clearly the close family relationship between vertebrate toad and arthropod fly, vertebrate chordin can stimulate nerve development in
flies and arthropod sog can do the same in toads. Like the Colonel's lady and Molly O'Grady they are cousins under the skin.
And now, Class, for next Sunday kindly let me have 500 words on your relationship to the
sea-egg.
And when you next step on a fortyleg please apologise to your cousin.
