X. ruwenzoriensis, with a sixfold reduplication of the original genome and six times as much DNA per cell (108 chromosomes, compared with 36 in X. laevis, for example). These species are estimated to have diverged from one another within the past 120 million years (Figure 1–42).
The Frog and the Zebrafish Provide Accessible models for vertebrate development
Frogs have long been used to study the early steps of embryonic development in vertebrates, because their eggs are big, easy to manipulate, and fertilized outside of the animal, so that the subsequent development of the early embryo is easily followed (Figure 1–43). Xenopus laevis, in particular, continues to be an important model organism, even though it is poorly suited for genetic analysis (Movie 1.6 and see Movie 21.1).
The zebrafish Danio rerio has similar advantages, but without this drawback. Its genome is compact—only half as big as that of a mouse or a human—and it has a generation time of only about three months. Many mutants are known, and genetic engineering is relatively easy. The zebrafish has the added virtue that it is transparent for the first two weeks of its life, so that one can watch the behavior of individual cells in the living organism (see Movie 21.2). All this has made it an increasingly important model vertebrate (Figure 1–44).
The mouse Is the Predominant mammalian model organism
Mammals have typically two times as many genes as Drosophila, a genome that is 16 times larger, and millions or billions of times as many cells in their adult bodies. In terms of genome size and function, cell biology, and molecular mechanisms, mammals are nevertheless a highly uniform group of organisms. Even anatomically, the differences among mammals are chiefly a matter of size and proportions; it is hard to think of a human body part that does not have a counterpart in elephants and mice, and vice versa. Evolution plays freely with quantitative features, but it does not readily change the logic of the structure.
Figure 1–42 Two species of the frog genus Xenopus. X. tropicalis, above, has an ordinary diploid genome; X. laevis, below, has twice as much dnA per cell. From the banding patterns of their chromosomes and the arrangement of genes along them, as well as from comparisons of gene sequences, it is clear that the large-genome species have evolved through duplications of the whole genome. These duplications are thought to have occurred in the aftermath of matings between frogs of slightly divergent Xenopus species. (courtesy of e. Amaya, m. offield, and R. Grainger, Trends Genet. 14:253– 255, 1998. With permission from elsevier.)
Figure 1–43 Stages in the normal development of a frog. These drawings show the development of a Rana pipiens tadpole from a fertilized egg. The entire process takes place outside of the mother, making the mechanisms tail bud involved readily accessible for experimental studies. (From W. shumway, Anat. Rec. 78:139–147, 1940.)
For a more exact measure of how closely mammalian species resemble one another genetically, we can compare the nucleotide sequences of corresponding (orthologous) genes, or the amino acid sequences of the proteins that these genes encode. The results for individual genes and proteins vary widely. But typically, if we line up the amino acid sequence of a human protein with that of the orthologous protein from, say, an elephant, about 85% of the amino acids are identical. A similar comparison between human and bird shows an amino acid identity of about 70%—twice as many differences, because the bird and the mammalian lineages have had twice as long to diverge as those of the elephant and the human (Figure 1–45).