ABSTRACT
As mentioned in Chapter 16, Hox genes are not the only genes that control development in animals. The maternal factors, gap genes, pair-rule genes, segment polarity, and many other homeotic genes exist in multicellular organisms. Each of the aspects of the development is tightly controlled by the products of these genes. They include proteins and RNAs. Some of them show a degree of similarity to some of the genes that control development in plants. The products of these genes, as well as many others, control the organization of various tissues, as well as the placement of organs on the plants. This indicates either a very ancient origin for these genes that predated the separation of the Archaeplastida (plants) from the Opisthokonta (animals and fungi), or that there was an ancient horizontal transfer of one or more versions of these genes that occurred after the divergence of the two groups.
