In the preceding sections, we have seen that animals contain the same essential cell types, have a similar collection of genes, and share many of the molecular mechanisms of pattern formation. But how can we square this with the radical differences that we see in the body structures of animals as diverse as a worm, a fly, a frog, and a mouse? We asserted earlier, in a general way, that these differences usually seem to reflect differences in the regulatory DNA that calls into play the components of the conserved basic kit of parts. We must now examine the evidence a little more closely.
When we compare animal species with similar basic body plans—different vertebrates, for example, such as fish, birds, and mammals—we find that corresponding genes usually have similar sets of regulatory elements: the regulatory DNA sequences have been well conserved and are recognizably homologous in the different animals. The same is true if we compare different species of nematode worms or insects. But, when we compare vertebrate regulatory regions with those of worms or flies, it is hard to see any such resemblance. The protein-coding sequences are unmistakably similar, but the corresponding regulatory DNA sequences appear mostly very different, suggesting that the differences in body plans mainly reflect differences in regulatory DNA. Although variations in the proteins themselves also contribute, differences in regulatory DNA would be enough to generate radically different tissues and body structures even if the proteins were the same.
It is not yet possible to trace the genetic steps that have led to all the spectacular diversity of animals. Their lineages have diverged over hundreds of millions of years, and in most cases too many changes have occurred for us to be able to say that this or that feature results from this or that mutation. The picture is clearer, however, for more recent evolutionary events. Studies of both closely related animal populations and plant populations whose members have different morphologies have revealed that dramatic developmental effects can result from subtle changes in regulatory DNA.
A well-studied example is the morphological diversity found in stickleback fish. After the last ice age ended about 10,000 years ago, marine sticklebacks colonized many newly formed freshwater streams and lakes. Marine sticklebacks extend sharp spines from their pelvic skeleton. These spines are thought to help protect the fish from soft-mouthed fish predators. In contrast, several populations of freshwater sticklebacks have lost these spines, usually in lakes that lack such predators. The different morphologies reflect differences in control of the expression of a transcription regulator called Pitx1. Whereas marine sticklebacks express the Pitx1 gene in the pelvic bone precursor cells that will form the spikes,
Figure 21–36 Neuroblasts and asymmetric cell division in the central nervous system of a fly embryo. the neuroblast originates as a specialized ectodermal cell. It is singled out by lateral inhibition and emerges from the basal (internal) face of the ectoderm. It then goes through repeated division cycles, dividing asymmetrically to generate a series of ganglion mother cells. each ganglion mother cell divides just once to give a pair of differentiated daughters (typically a neuron plus a glial cell).
freshwater sticklebacks have lost this expression as a result of a change at the Pitx1 locus. These changes do not lie in the coding sequence. Instead, each is a small deletion of a block of adjacent regulatory DNA that controls Pitx1 expression specifically in the pelvic cells (Figure 21–37).
The Pitx1 protein has important functions elsewhere in the body, so that the DNA sequences that encode this protein must be retained. The regulatory DNA responsible for Pitx1 expression at these other sites is also unchanged in the two populations of sticklebacks. The evolution of pelvis development in sticklebacks shows how the modular nature of regulatory DNA elements that we encountered in Chapter 7 (see Figure 7–29) allows independent modification of the different parts of the body, even when formation of those body parts depends on the same proteins.
In the recent evolution of plants, changes of body structure can be traced in a similar way to changes in regulatory DNA. For example, these account for a large part of the dramatic difference between the wild teosinte plant and its modern descendant, maize, through some 10,000 years of mutation and selection by Native Americans.