|Wesley R. Elsberry
Joined: May 2002
In another surprising twist, biologists have also discovered many cases in which the same genes help to produce different adult structures.' Consider, for instance, the eyes of the squid, the fruit fly, and mouse. (ee Figure 2:2) The fruit fly has a compound eye, with dozens of separate lenses. The squid and mouse both have single-lens camera eyes, but they develop along very different pathways, and are wired differently from each other. Yet the same gene is involved in the development of all three of these eyes.
What, was it too hard to add a sentence to note that the downstream activation triggered by hox gene expression is different among all of those species? Or, at least that Pax6 is just one gene among many needed for camera eye formation?
Georg Halder discussed the compound eye of Drosophila in this way:
The potential of eyeless to act as a developmental switch was tested by targeted expression of eyeless in tissues that do not normally express it (6). To do so, a transposon carrying the eyeless gene under the control of a transcriptional enhancer that stimulates expression in wing, leg, and antennal primordia was introduced into the fly genome. As a consequence, extra eyes developed on wings, legs, and antennae! These eyes consist of the full complement of different cell types normally found in a compound eye, including photoreceptors, pigment cells, cone cells, and bristles. In addition, the arrangement of the different cell types is the same as in a normal eye and the photoreceptors depolarize upon illumination. Evidently, eyeless can switch on the eye developmental program in which several thousand genes may act, thereby directing the formation of an organ with all its complexity.
Several thousand genes... several thousand more than are brought to the attention of readers of EE.
If this isn't mendacious on its face, then it bespeaks a deep incompetence in handling the data. It certainly isn't an example of teaching students more about evolution than they would learn from standard textbooks.
Let me put up Halder's conclusion, a statement that, unlike the quote from EE, is actually informative:
What do these findings tell us about the evolution of eyes? The parallels in the eye developmental programs lead us to favor the idea that the common ancestor of all higher animals, including vertebrates and insects, already had a primitive eye and that the development of this ancestral eye was regulated by Pax-6 (16). This eye may have been a simple eyespot consisting of a cluster of photo-sensitive cells with no ability to form an image, a type of organ found in many animal phyla. Once a functional light sensing organ had evolved, nature apparently improved on its optical performance in many different ways, leading to the incredible variety of eyes seen today. During this process, Pax-6/eyeless continued to be used to control the development of the evolving eyes.
"You can't teach an old dogma new tricks." - Dorothy Parker