Restless Creatures by Matt Wilkinson

Author:Matt Wilkinson
Language: eng
Format: epub
ISBN: 9780465098699
Publisher: Basic Books
Published: 2015-01-06T16:00:00+00:00

THE SPICE OF LIFE

Up to now we’ve assumed that once activated, the repeated appendage developmental modules pay no attention to the GPS coordinates given by the Wnt/BMP gradients. But just because the modules can run without such input doesn’t mean they have to. There’s nothing to stop the local morphogen concentrations having a location-specific effect on a module’s developmental trajectory. This is where a very special family of transcription factor genes called Hox genes—found in all animals except sponges—comes into play. Simply put, their job is to translate the fore-to-aft morphogen gradients into a transcription factor expression pattern. There are commonly between six and twelve of them in any one species, and in many animals they nestle together on a single chromosome in a tight cluster.

The Hox gene activation pattern, determined by the specifics of the genes’ switches, serves to divide the fore-to-aft axis into a number of discrete bands, each defined by a unique Hox code—this being the collection of Hox transcription factors expressed in the band. This is the key to the variety of an arthropod’s appendages, for while any repeated structures within one of the Hox regions will tend to look the same, the structures can take on an entirely different character, or may even be shut down completely, wherever the Hox code changes. For example, most centipede segments express the Hox genes Ultrabithorax (Ubx) and Abdominal-A (Abd-A), so the legs in this region all look the same. The segment nearest the head, however, expresses a different Hox code—Sex combs reduced (Scr) + fushi tarazu (ftz) + Antennapedia (Antp). Its pair of legs is accordingly quite different: they face forward and contain poison glands. These venom-laced fangs are the centipede’s secret weapon, and provide a compelling incentive to shake out your boots every morning before hiking in the tropics, where centipedes get big (look up Scolopendra if you have a moment).

The presence of the Hox cluster alongside a system that builds duplicate appendages has truly profound evolutionary implications. Back in Chapter 3, when we first met the insects, I called attention to the evolutionary potential provided by having two pairs of wings. Because flight can work perfectly well with one pair, the other pair can be selectively tinkered with without messing things up—hence the appearance of halteres and beetle wing cases. What’s true of insect wings is true of all arthropod repeated appendages: the duplicate sets give a measure of redundancy to the locomotory system (many arthropods continue to move around despite losing the odd leg), and as long as the developmental program of one set can be tweaked without automatically tweaking all the others as well, the stage is set for some serious evolutionary voyages of discovery. The Hox cluster makes this all possible: an arthropod can make as many different kinds of appendages as it has Hox codes. Furthermore, it is relatively easy for a region of the body to acquire a new Hox code: a shift in the boundaries of a Hox gene’s expression

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