Multi Cellular Organisms and Homeobox Genes

Even single-celled microorganisms communicate with each other by sending a variety of signals. For example, yeast cells (see above) and gram-positive bacteria (see Ch 18) both secrete pheromones into the external medium when they are ready to mate. Multi-cellular organisms depend on coordinating the activities of many different cells and this requires constant communication. In addition to internal signals between cells, multi-cellular organisms send signals from one organism to another.

Higher animals and plants form permanent multi-cellular structures. As a consequence, their cells differentiate forming different tissues in which different sets of genes are expressed. Development and differentiation involve complex patterns of gene regulation. Despite the great diversity of structures, the genetic mechanisms for laying out the overall body plan have some common themes. Specifying overall body layout, including such things as the number of limbs and segmentation patterns, is the role of the homeobox genes. These encode transcription factors containing a homeodomain of about 60 amino acid residues forming a helix-turn-helix region that binds DNA and is highly conserved. The regions of the transcription factors outside the homeodomain vary greatly from protein to protein. Homeobox proteins are found in all multi-cellular animals and also in higher plants.

The best-known family of homeobox genes is the Hox genes. The Hox proteins are transcription factors that control the expression of many other regulatory proteins, including other transcription factors. They were first identified in Drosophila as a result of bizarre developmental mutations. For example, mutations in the antennapedia subgroup of Hox genes may result in production of a leg where an antenna is normally supposed to grow (hence the name antenna-pedia) and mutations in the bithorax cluster can produce four-winged flies.

The Hox genes of animals are found in clusters and the order of Hox genes along the chromosome corresponds to their expression in the developing Drosophila fruit fly embryo (Fig. 19.23). Hox genes at the 3'-end of the cluster are expressed in the head region whereas Hox genes at the 5'-end are expressed in the tail region. Consequently, these are known as the anterior and posterior Hox genes, respectively. Between these lie the group 3 and central genes. Furthermore, Hox genes at the 3'-end are expressed earlier in development than those at the 5'-end. Thus the Hox genes homeobox genes Genes encoding transcription factors containing a homeodomain that help specify the body plan of multicellular organisms homeodomain Conserved region of about 60 amino acid residues found in homeobox proteins that that binds DNA by a helix-turn-helix motif Hox genes Family of homeobox genes that control overall body layout by regulating the expression of many other regulatory genes, including those for other transcription factors

Multi-Cellular Organisms and Homeobox Genes 531

FIGURE 19.23 Cluster of Hox Genes in Drosophila

Different Hox genes control development of each segment of the Drosophila fruit fly embryo. For example, the embryo segments T1, T2, and T3 express the anfennapedia gene, encoding a transcription factor of the Hox family. This protein controls the development of the adult legs from the T1, T2 and T3 embryonic segments. The Antennapedia transcription factor is expressed as a gradient in these three segments with the highest concentration in T1. If this protein is expressed in a different segment of the embryo, a leg will grow in the wrong place. When this transcription factor was expressed in the head region, the fly's antenna turned into a leg.

More advanced organisms have several sets of homeobox genes due to ancient DNA duplications.

control the expression of groups of other genes both spatially and temporally during development.

The Hox cluster has evolved by duplication of one or several genes and by loss of individual members in some lineages (Fig. 19.24). Jellyfish and other cnidarians have only anterior and posterior Hox genes. The ancestral Hox cluster of bilateral animals is thought to have contained two anterior, one group 3, four central and one posterior Hox gene. The protostomes, which includes arthropods, annelids, molluscs, flatworms protostomes Grouping of animal phyla that includes arthropods, annelids, molluscs, flatworms etc

Flatworm

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