The Animal Tree
There are about 35 living animal phyla. To understand the origin and evolution of any feature found in one or more of these groups, it is necessary to have a picture of the phylogenetic relationships among animals. Ideally, the fossil record would present a complete, ordered, unambiguous picture of the branching pattern of the animal tree. Unfortunately, it does not. As the divergence of most bilaterian phyla appears to have predated the emergence of recognizable members of modern phyla in the...
The Toolkit And Animal Design
The visualization of fields, compartments, and organizers as domains of selector gene expression and as sources of signaling proteins with demonstrable long-range effects on the patterning of embryonic fields has provided concrete molecular evidence of the fundamental roles that these units of organization play in embryonic development. The identification of genes that affect the formation and function of these units of animal body organization represents a first step toward understanding the...
Sharing Of The Genetic Toolkit Among Animals
One of the most exciting and unexpected discoveries that occurred soon after the cloning of the Hox genes was the detection of genes with related sequences in all sorts of animals. Using the homeobox to search for similar sequences in other genomes by hybridization, researchers isolated Hox-related genes from a broad sample of other animals. The similarity between the sequences of the homeodomains of genes isolated from frogs, mice, and humans and the original Drosophila Hox sequences was...
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Most signaling pathways operate through similar logic but have different proteins and signal transduction mechanisms. Signaling begins when membrane-bound receptors bind a ligand, leading to the release or activation of associated intracellular proteins. Receptor activation often leads to the modification of inactive transcription factors that are translocated to the cell nucleus, bind to c s-regulatory DNA sequences or to DNA-binding proteins, and regulate the level of target gene...
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from other animals. Thus the systematic inventory of the Drosophila genome is representative of what is generally known about the types of molecules with large-scale effects on animal patterning. A few dozen Drosophila genes are required for proper anteroposterior patterning of the embryo and larva. These genes are grouped into five classes based on their realm of influence on embryonic pattern. Each class represents a progressively finer subdivision of the developing embryo. The first class...
The Genetic Toolkit for Development
The foremost challenge for embryology has been to identify the genes and proteins that control the development of animals from an egg into an adult. Early embryologists discovered that localized regions of embryos and tissues possess properties that have long-range effects on the formation and patterning of the primary body axes and appendages. Based on these discoveries, they postulated the existence of substances responsible for these activities. However, the search for such molecules proved...
Drosophila Images
Methods for visualizing gene expression in developing animals The two most common means of visualizing where a gene is transcribed and its protein product is synthesized are left in situ hybridization of complementary RNA probe to mRNA and right immunolocalization of protein expression. The procedures for each method are indicated. Gene expression patterns are visualized as the product of enzymatic reactions left or with fluorescently labeled compounds right . The relationship between the...
The Genetic Toolkit
Animal genomes contain thousands of genes. Many of these genes encode proteins that function in essential processes in all cells in the body for example, metabolism, biosynthesis of macromolecules and are often referred to as housekeeping genes. Other genes encode proteins that carry out specialized functions in particular cells or tissues within the body for example, oxygen transport, immune defense or, to extend the housekeeping metaphor, in specific rooms in the house. But here we are...
General Features Of Animal Design And Diversity
One of the most outstanding features of animal design, particularly of larger bilaterians, is their construction from repeating structures or modules . The segments of arthropods and annelids and the vertebrae and associated processes of vertebrates are the basic units of body plan organization in these phyla Fig. 1.5a-c . Similarly, many body parts such as the insect wing Fig. 1.5d and the tetrapod hand Fig. 1.5e are composed of repeated structures. An important trend in the morphological...
Animal Origins And The Fossil Record
The fossil record is our primary window into the history of life. It provides many kinds of information that cannot be inferred from living animals. Fossils give us pictures of extinct forms that may be ancestors of modern animals, provide minimal estimates of the time of origin or divergence of particular groups, reveal episodes of extinctions and radiations, and, in favorable circumstances, offer detailed accounts of the evolution of important structures. The search for the origins of modern...