This astonishing degree of evolutionary conservation was discovered not by broad surveys of animal diversity, but through intensive study of a small number of representative species—the model organisms discussed in Chapter 1. For animal developmental biology, the most important have been the fly Drosophila melanogaster, the frog Xenopus laevis, the roundworm Caenorhabditis elegans, the mouse Mus musculus, and the zebrafish Danio rerio. In discussing the mechanisms of development, we shall draw our examples mainly from these few species.
Figure 21–4 The lineage from blastomere to differentiated cell type.
as development proceeds, cells become more and more specialized. Blastomeres have the potential to give rise to most or all cell types. Under the influence of signaling molecules and gene regulatory factors, cells acquire more restricted fates until they differentiate into highly specialized cell types, such as the pancreatic β-islet cells that secrete the hormone insulin.
Genes Involved in cell–cell communication and transcriptional control are especially Important for animal Development
What are the genes that animals share with one another but not with other kingdoms of life? These would be expected to include genes required specifically for animal development but not needed for unicellular existence. Comparison of animal genomes with the genome of budding yeast—a unicellular eukaryote— suggests that three classes of genes are especially important for multicellular organization. The first class includes genes that encode proteins used for cell–cell adhesion and cell signaling; hundreds of human genes encode signal proteins, cell-surface receptors, cell adhesion proteins, or ion channels that are either not present in yeast or present in much smaller numbers. The second class includes genes encoding proteins that regulate transcription and chromatin structure: more than 1000 human genes encode transcription regulators, but only about 250 yeast genes do so. As we shall see, the development of animals is dominated by cell–cell interactions and by differential gene expression. The third class of non-coding RNAs has a more uncertain status: it includes genes that encode microR-NAs (miRNAs); there are at least 500 of these in humans. Along with the regulatory proteins, they play a significant part in controlling gene expression during animal development, but the full extent of their importance is still unclear. The loss of individual miRNA genes in C. elegans, where their functions have been well studied, rarely leads to obvious phenotypes, suggesting that the roles of miRNAs during animal development are often subtle, serving to fine-tune the developmental machinery rather than to form its core structures.
regulatory DNa Seems Largely responsible for the Differences Between animal Species
As discussed in Chapter 7, each gene in a multicellular organism is associated with many thousands of nucleotides of noncoding DNA that contains regulatory elements. These regulatory elements determine when, where, and how strongly the gene is to be expressed, according to the transcription regulators and chromatin structures that are present in the particular cell (Figure 21–5). Consequently, a change in the regulatory DNA, even without any change in the coding DNA, can alter the logic of the gene-regulatory network and change the outcome of development.
As discussed in Chapter 4, when we compare the genomes of different animal species, we find that evolution has altered the coding and regulatory DNA to different extents. The coding DNA, for the most part, has been highly conserved, the noncoding regulatory DNA much less so. It seems that changes in regulatory DNA are largely responsible for the dramatic differences between one class of animals and another (see p. 227). We can view the protein products of the coding sequences as a conserved kit of common molecular parts, and the regulatory DNA as instructions for assembly: with different instructions, the same kit of parts can be used to make a whole variety of different body structures. We will return to this important concept later.