Hox Genes & Mall Chicks

After six decades I still enjoy watching a young woman stroll, stride, or sometimes march through the mall. Each performs her unique symphony: her hips and waist, shoulders and arms sway in point and counterpoint, the head targets here and there and sometimes her eyes get larger and her lips smile when she sees another woman, a guy, or a bargain. She stops at "sale" before her conscious mind translates the sign.

Raff (1996), Gould (2002), and Carroll (Carroll et al, 2001), however, induce a different vision in me. A woman's symphony still plays but I now find that her orchestra has sections like those of a crayfish and that each moves according to both internal and external batons. Her segments are clustered like sections of strings and woodwinds: nearby instruments are closely synchronized by the direct connections that Sherrington described so well and her more distant ones by the sparse go-and-stops from a central pacemaker. Her arms and legs often consult with each other rather than with her brain.

Yes, a mall chick has "segments."

A sequence of 8 Hox genes arose by duplication, variation, isolation, and specialization. This set of transcription factors took one path to the external skeleton of a fly with a nervous chord that runs down its stomach and a second path to an internal skeleton and a spine that runs down the back. Meg Ryan not only has a segmented backbone and brainstem, she is inside out and backwards from a fly! Nonetheless, her lovely abdomen, thorax, and head emerged from a 500 million year collaboration not between Gilbert and Sullivan but Ultrabithorax and Antennapedia and their duplicated, mutated, add-ons that we call Proboscipedia, Labial, and Abdominal A and B. (God knows what she does with her homologs for Deformed and Sex Combs Reduced!)

Rudy Raff describes parallel, loosely connected, developmental cascades, each of which does its own thing but remains in a changeable synchrony with other chains. Look at a girl and you see the outcomes of multiple evolutionary histories, a separate one for her head and mind, chest and stomach. Each of her compartments travels its unique course both through evolutionary time and during her personal development. And developmental events can skew the unfolding of any one of her segments without disrupting that in others. Her mind can be more or less mature than her gut or heart. A mall chick can have incredible hips, legs, chest, and arms but a face that would break glass on a city bus. When I admire a waist, I also admire the junction of two different ancestral genes. Likewise, when I notice a long neck, tapered chin, large eyes, flushed cheeks, and pink lips, I also imagine seams between the developmental plates that once arranged just a mouth and two antennae. I also wonder if her bottom half is a mirror image of her top half, the two sections divided by her navel? And the shape of her legs may have only a modest relationship to the quality of her mind: Mother Theresa could act like a saint but look like a toad.

Furthermore, each of her sections, organs, and perhaps each of her cells, has its own evolutionary fitness whether defined by Darwin as length of life and numbers of children or by modern network theorists who measure the number of links that a node attracts. Few of us, however, are ideally proportioned and similarity is a large component of mate selection. Apart from kindness, apparent fertility, and resources, is there a trend for barrel-chested, peg-legged individuals to be attracted to each other? Or is there a special magnetism between individuals with larger-than-average heads? (Are spindles such as me most comfortable with other reeds? I tend to think so when I search the personals for the face and hair that were Farrah's but now happily travel atop a broom!) And what is the effect when head proportions match between two people but none of the other segments do so? "I take this cranium to have and to hold but not the rest of her...?"

As to environment, I thank mutagens and gods for a small Oriental girl. I watched her 52 years ago when I lived in Yokohama: she spoke only Japanese, wore white socks each with two separate compartments, one for her big toe and the other for her four smaller ones, wooden getas, and a blue and white baggy kimono. She often had little formal education and her mother could pedal a bicycle with a 6-foot stack of wood balanced over its rear wheel. The same girl now sits with her books one table distant in the cafe. She wears a ponytail, denim shorts that are not only very short but also very snug, and a light pink bare-midriff blouse, all of which accompany her intent, symmetrical face, a mouth that probably speaks better English than I do, and a brain that adsorbs organic chemistry through eyes that once sought only blades of new grass...

References
Carroll, Sean, Grenier, J., & Weatherbee, S. (2001) From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design. Malden, MA: Blackwell.
Gould, S. (2002) The Structure of Evolutionary Theory. Cambridge, MA: Harvard Belknap.
Raff, R. (1996) The Shape of Life. Chicago, IL: University of Chicago Press.

James Brody, copyright, 2004, all rights reserved.

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