1 18.4 Embryonic development involves cell division, cell differentiation, and morphogenesis An organism arises from a fertilized egg cell as the result of three interrelated processes: cell division, cell differentiation, and morphogenesis. From zygote to hatching tadpole takes just one week. Fig. 21.1
2 Cell division alone would produce only a great ball of identical cells. During development, cells become specialized in structure and function, undergoing differentiation. Different kinds of cells are organized into tissues and organs. The physical processes of morphogenesis, the creation of form, give an organism shape. Early events of morphogenesis lay out the basic body plan very early in embryonic development. These include establishing the head of the animal embryo or the roots of a plant embryo.
3 The overall schemes of morphogenesis in animals and plants are very different. In animals, but not in plants, movements of cells and tissues are necessary to transform the embryo. In plants, morphogenesis and growth in overall size are not limited to embryonic and juvenile periods.
4 Fig. 21.2
5 Apical meristems, perpetually embryonic regions in the tips of shoots and roots, are responsible for the plant s continual growth and formation of new organs, such as leaves and roots. In animals, ongoing development in adults is restricted to the differentiation of cells, such as blood cells, that must be continually replenished. The importance of precise regulation of morphogenesis is evident in human disorders that result from morphogenesis gone awry. For example, cleft palate, in which the upper wall of the mouth cavity fails to close completely, is a defect of morphogenesis.
6 Sequential Regulation of Gene Expression During Cellular Differentiation Determination commits a cell to its final fate Determination precedes differentiation Cell differentiation is marked by the production of tissuespecific proteins 2011 Pearson Education, Inc.
7 Myoblasts produce muscle-specific proteins and form skeletal muscle cells MyoD is one of several master regulatory genes that produce proteins that commit the cell to becoming skeletal muscle The MyoD protein is a transcription factor that binds to enhancers of various target genes 2011 Pearson Education, Inc.
8 Figure Nucleus Embryonic precursor cell DNA Master regulatory gene myod OFF Other muscle-specific genes OFF mrna OFF Myoblast (determined) MyoD protein (transcription factor) mrna mrna mrna mrna Part of a muscle fiber (fully differentiated cell) MyoD Another transcription factor Myosin, other muscle proteins, and cell cycle blocking proteins
9 21.6. Homeotic genes direct the identity of body parts In a normal fly, structures such as antennae, legs, and wings develop on the appropriate segments. The anatomical identity of the segments is controlled by master regulatory genes, the homeotic genes. Discovered by Edward Lewis, these genes specify the types of appendages and other structures that each segment will form.
10 Mutations to homeotic genes produce flies with such strange traits as legs growing from the head in place of antennae. Structures characteristic of a particular part of the animal arise in the wrong place. Fig
11 Like other developmental genes, the homeotic genes encode transcription factors that control the expression of genes responsible for specific anatomical structures. For example, a homeotic protein made in a thoracic segment may activate genes that bring about leg development, while a homeotic protein in a certain head segment activates genes for antennal development. A mutant version of this protein may label a segment as thoracic instead of head, causing legs to develop in place of antennae.
12 Homeobox genes have been highly conserved in evolution All homeotic genes of Drosophila include a 180- nucleotide sequence called the homeobox, which specifies a 60-amino-acid homeodomain. An identical or very similar sequence of nucleotides (often called Hox genes) are found in many other animals, including humans. Related sequences are present in yeast and prokaryotes. The homeobox DNA sequence must have evolved very early in the history of life and is sufficiently valuable that it has been conserved in animals for hundreds of millions of years.
13 Proteins with homeodomains probably regulate development by coordinating the transcription of batteries of developmental genes. In Drosophila, different combinations of homeobox genes are active in different parts of the embryo and at different times, leading to pattern formation. Fig
14 In fact, the vertebrate genes homologous to the homeotic genes of fruit flies have even kept their chromosomal arrangement. Fig
15 Most, but not all, homeobox-containing genes are homeotic genes that are associated with development. For example, in Drosophila, homeoboxes are present not only in the homeotic genes but also in the egg-polarity gene bicoid, in several segmentation genes, and in the master regulatory gene for eye development. The polypeptide segment produced by the homeodomain is part of a transcription factor. Part of this segment, an alpha helix, fits neatly into the major groove of the DNA helix. Other more variable domains of the overall protein determine which genes it will regulate.
16 Macroevolution can also result from changes in gene that control the placement and spatial organization of body parts. Example: genes called homeotic genes determine such basic features as where a pair of wings and a pair of legs will develop on a bird or how a plant s flower parts are arranged.
17 One class of homeotic genes, Hox genes, provide positional information in an animal embryo. Their information prompts cells to develop into structure appropriate for a particular location.
18 One major transition in the evolution of vertebrates is the development of the walking legs of tetrapods from the fins of fishes. The fish fin which lacks external skeletal support evolved into the tetrapod limb that extends skeletal supports (digits) to the tip of the limb. This may be the result of changes in the positional information provided by Hox genes during limb development. Fig
19 Key events in the origin of vertebrates from invertebrates are associated with changes in Hox genes. While most invertebrates have a single Hox cluster, molecular evidence indicates that this cluster of duplicated about 520 million years ago in the lineage that produced vertebrates. The duplicate genes could then take on entirely new roles, such as directing the development of a backbone.
20 A second duplication of the two Hox clusters about 425 million years ago may have allowed for even more structural complexity. Fig
21 Concept 25.5: Major changes in body form can result from changes in the sequences and regulation of developmental genes Evo-devo is a field of interdisciplinary research that examines how slight genetic divergences can become magnified into major morphological differences between species. A particular focus are genes that program development by controlling the rate, timing, and spatial pattern of changes in form as an organism develops from a zygote to an adult.
22 Changes in Rate and Timing Heterochrony is an evolutionary change in the rate or timing of developmental events It can have a significant impact on body shape The contrasting shapes of human and chimpanzee skulls are the result of small changes in relative growth rates 2011 Pearson Education, Inc.
23 2011 Pearson Education, Inc. Animation: Allometric Growth Right-click slide / select Play
24 Allometric growth tracks how proportions of structures change due to different growth rates during development. Fig a
25 Figure Chimpanzee infant Chimpanzee adult Chimpanzee fetus Chimpanzee adult Human fetus Human adult
26 Evolution of morphology by modification of allometric growth is an example of heterochrony, an evolutionary change in the rate or timing of developmental events. Heterochrony appears to be responsible for differences in the feet of tree-dwelling versus ground-dwelling salamanders. Fig
27 Heterochrony can alter the timing of reproductive development relative to the development of nonreproductive organs In paedomorphosis, the rate of reproductive development accelerates compared with somatic development The sexually mature species may retain body features that were juvenile structures in an ancestral species 2011 Pearson Education, Inc.
28 Figure Gills
29 Changes in Spatial Pattern Substantial evolutionary change can also result from alterations in genes that control the placement and organization of body parts Homeotic genes determine such basic features as where wings and legs will develop on a bird or how a flower s parts are arranged 2011 Pearson Education, Inc.
30 Hox genes are a class of homeotic genes that provide positional information during development If Hox genes are expressed in the wrong location, body parts can be produced in the wrong location For example, in crustaceans, a swimming appendage can be produced instead of a feeding appendage 2011 Pearson Education, Inc.
31 Figure 25.23
32 Changes in Gene Regulation Changes in morphology likely result from changes in the regulation of developmental genes rather than changes in the sequence of developmental genes For example, threespine sticklebacks in lakes have fewer spines than their marine relatives The gene sequence remains the same, but the regulation of gene expression is different in the two groups of fish 2011 Pearson Education, Inc.
34 Figure 25.25b RESULTS Test of Hypothesis A: Differences in the coding sequence of the Pitx1 gene? Result: No The 283 amino acids of the Pitx1 protein are identical. Test of Hypothesis B: Differences in the regulation of expression of Pitx1? Result: Yes Pitx1 is expressed in the ventral spine and mouth regions of developing marine sticklebacks but only in the mouth region of developing lake sticklebacks. Marine stickleback embryo Lake stickleback embryo Close-up of mouth Close-up of ventral surface
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