Development and Evolutionary Change

20 Development and Evolutionary Change 20.1 How Does a Molecular Tool Kit Govern Development? 20.2 How Can Mutations with Large Effects Change Only One Part of the Body? 20.3 How Can Differences among Species Evolve? 20.4 How Does the Environment Modulate Development? 20.5 How Do Developmental Genes Constrain Evolution?

20.1 How Does a Molecular Tool Kit Govern Development? Developmental genes were first studied in the fruit fly, Drosophila. Using comparative genomics, scientists discovered a similar set of genes in vertebrates. The amazing diversity of organisms is produced by a modest number of regulatory genes.

20.1 How Does a Molecular Tool Kit Govern Development? The molecular toolkit—the transcription factors, extracellular signals, and developmental genes, that govern pattern formation. A few tools can be used to build many structures.

20.1 How Does a Molecular Tool Kit Govern Development? Plants and animals share many regulatory genes. Similar gene families, MADS in plants and Hox in animals, govern cell differentiation in similar ways. These lineages diverged in the far distant past. The regulatory genes have been conserved—they have changed little.

20.1 How Does a Molecular Tool Kit Govern Development? The same kind of homeobox-containing genes direct the anterior-posterior axis orientation of both insect and mammalian embryos. Mutations of these genes have similar effects in both insects and mice.

Figure 20.1 Regulatory Genes Show Similar Expression Patterns

Figure 20.2 Altering Homeotic Genes Changes Morphology

The form of each module can be changed independently of other modules. 20.2 How Can Mutations with Large Effects Change Only One Part of the Body? Embryos are made up of modules— functional entities consisting of genes, the signaling pathways the genes stimulate, and physical structures that result. The form of each module can be changed independently of other modules.

Genetic switches control how the molecular toolkit is used. 20.2 How Can Mutations with Large Effects Change Only One Part of the Body? Genetic switches control how the molecular toolkit is used. These DNA sequences determine where and when genes will be turned on or off. Multiple switches control each gene; and most elements in the toolkit influence more than one developmental process.

20.2 How Can Mutations with Large Effects Change Only One Part of the Body? In an embryo, genetic switches integrate positional information, and play a key role in making different modules develop differently. Genetic switches control the activity of Hox genes by activating each Hox gene in different zones of the body.

Figure 20.3 Segments Differentiate under Control of Genetic Switches

20.2 How Can Mutations with Large Effects Change Only One Part of the Body? Modularity also allows the timing of developmental processes to be independent—heterochrony. Example: In most salamanders, the webbing between the toes disappears; but a mutation that inhibited the genes that trigger apoptosis allowed an arboreal species to evolve.

Figure 20.4 Heterochrony Created an Arboreal Salamander

20.2 How Can Mutations with Large Effects Change Only One Part of the Body? The webbed feet of ducks results from an altered spatial expression pattern of a developmental gene. Duck and chicken embryos both have webbing; and both express BMP4, a protein that instructs cells in the webbing to undergo apoptosis.

But in ducks, the gremlin gene is also expressed in the webbing cells. 20.2 How Can Mutations with Large Effects Change Only One Part of the Body? An BMP inhibitor protein, Gremlin, is produced in both ducks and chickens around the digits. But in ducks, the gremlin gene is also expressed in the webbing cells. Experimental application of Gremlin to chicken feet results in a webbed foot.

Figure 20.5 Changes in gremlin Expression Correlate with Changes in Hindlimb Structure

Figure 20.6 Changing the Form of an Appendage (Part 1)

Figure 20.6 Changing the Form of an Appendage (Part 2)

20.3 How Can Differences among Species Evolve? Action of genes controlled by genetic switches underlie both the development of an individual organism and the evolution of differences among species. In the arthropods, morphological changes in species have evolved through mutations in genes that regulate differentiation of segments.

20.3 How Can Differences among Species Evolve? All arthropods have the Hox gene Ubx. Insect Ubx has a mutation; Ubx protein from this gene inhibits expression of the distal-less gene (dll) which is essential for leg formation. Insect Ubx is expressed in the abdomen segments—result: insects have only six legs, none on the abdomen.

Figure 20.7 Mutation in a Hox Gene Changed the Number of Legs in Insects

20.3 How Can Differences among Species Evolve? Similar processes govern development of the vertebral column. Vertebral column has anterior-to-posterior regions (cervical, thoracic, lumbar, caudal). The regions are controlled by Hox genes. The characteristic numbers of vertebrae in different species result from genetic changes that expand or contract the expression domain of different Hox genes.

20.4 How Does the Environment Modulate Development? Developmental plasticity is the ability of an organism to modify its development in response to environmental conditions. Some environmental signals accurately predict future conditions. Developmental processes should respond to these signals.

20.4 How Does the Environment Modulate Development? Organisms do respond to signals that are accurate predictors, such as seasonal changes in day length or wet and dry seasons. The West African butterfly Bicyclus anynana has wet and dry season forms. Temperature during pupation determines which form develops.

20.4 How Does the Environment Modulate Development? In late larval stage, transcription of distal- less gene is restricted to small areas of the wing that may become eyespots. In pupal development, the area over which distal-less protein is expressed expands with increasing temperature, resulting in conspicuous eyespots.

Figure 20.8 Eyespot Development Responds to Temperature

20.4 How Does the Environment Modulate Development? The moth Nemoria arizonaria has two generations per year. Larvae that hatch in the spring feed on oak flowers, and resemble those flowers. The second generation feeds on oak leaves, and the larvae resemble small oak twigs. A chemical in oak leaves induces them to develop in the twig-like form.

Figure 20.9 Spring and Summer Forms of a Caterpillar Differ

20.4 How Does the Environment Modulate Development? Venomous tiger snakes (Notechis) of Australia show developmental plasticity in different areas. Mainland prey is small frogs and mice. On a nearby island, prey is much larger (silver gull chicks). These snakes have a larger body size and larger relative head size, and greater developmental plasticity.

20.4 How Does the Environment Modulate Development? Some environmental signals occur irregularly. Developmental changes can occur in response to presence of predators. Daphnia cuculata grow large “helmets” on their heads when predatory larvae of Chaoborus are present.

Figure 20.10 Predator-Induced Developmental Plasticity in Daphnia

20.4 How Does the Environment Modulate Development? Plant development is influenced by light levels. If grown in low light conditions, plants become spindly—cells elongate, and the plant may be better able to reach available light. Because of undifferentiated meristems, plants can respond to light throughout their life.

Figure 20.11 Light Seekers

20.4 How Does the Environment Modulate Development? Learning is also a developmental response to environmental change. Learning allows individuals to adjust behavior as environment changes. It is especially important in species with complex social systems.

20.4 How Does the Environment Modulate Development? Organisms do not respond to signals that don’t accurately predict future environmental conditions. Plants may respond to their environment by changing size, shape, or number of flowers or seeds, but seed size remains constant. Environmental conditions such as rainfall and temperature in one year are poor predictors of what these conditions will be the next year.

20.4 How Does the Environment Modulate Development? Many plants have genes for seed dormancy. Seeds made in one year may germinate at staggered intervals, in years that will have unpredictable amounts of rainfall, etc. Seed size has evolved in response to average conditions over many generations.

20.4 How Does the Environment Modulate Development? Organisms can respond to signals that have been encountered frequently in their recent evolutionary history; but may not be able to respond to novel signals. Human activities have created many novel signals, such as new chemicals that disrupt normal development.

20.4 How Does the Environment Modulate Development? Examples: Effects of DDT on reproduction of birds by interfering with development of the eggshell. Thalidomide, a drug used as a sedative during pregnancy, had severe effects on limb development of the fetus.

20.5 How Do Developmental Genes Constrain Evolution? Evolution of form has not been a result of radically new genes, but has resulted from modifications of existing genes. Developmental genes constrain evolution in two ways: Nearly all evolutionary innovations are modifications of existing structures. Genes that control development are highly conserved.

20.5 How Do Developmental Genes Constrain Evolution? Example: Wings arose as modifications of existing structures. Wings evolved independently in insects and vertebrates, three times in vertebrates. In each case, they are modified limbs.

Figure 20.12 Wings Evolved Three Times in Vertebrates

20.5 How Do Developmental Genes Constrain Evolution? Organisms also lose structures. Ancestors of snakes lost their limbs as a result of changes in expression of Hox genes. Some snakes occasionally develop rudimentary limbs, suppression of limb genes is not complete. The same process resulted in the evolutionary changes in number of legs in arthropods, and on which segments.

Figure 20.13 Developmental Genes and Lost Structures

20.5 How Do Developmental Genes Constrain Evolution? Many developmental genes exist in similar form across a wide range of species. The Pax6 gene for eye expression in mice is much the same as the eyeless gene in insects. If mouse Pax6 is inserted into the fly genome, an insect compound eye is formed.

Chapter Opener 2 A Mouse Gene Can Produce a Fly’s Eye opening figure of insect eye from mouse Pax6 could be placed here…Opener 2 of chapter

20.5 How Do Developmental Genes Constrain Evolution? Highly conserved developmental genes make it likely that similar traits will evolve repeatedly—parallel phenotypic evolution. Example: three-spined sticklebacks (Gasterosteus aculeatus). Marine populations return to freshwater to breed. Freshwater populations never go into saltwater environments.

20.5 How Do Developmental Genes Constrain Evolution? Freshwater populations have arisen many times from adjacent marine populations. Marine populations have structures that protect them from predation. In freshwater populations, the spines and bony plates are greatly reduced.

20.5 How Do Developmental Genes Constrain Evolution? One gene, Pitx1, is responsible for loss of armor in freshwater sticklebacks, as well as changes in other bones. Pitx1 is inactive in bone-forming regions of freshwater embryos, but active in marine embryos. Pitx1 is expressed in other developing organs in both freshwater and marine embryos.

Figure 20.14 Parallel Phenotypic Evolution in Sticklebacks

20.5 How Do Developmental Genes Constrain Evolution? This change in gene expression has arisen many times and suggests it is adaptive. In the freshwater lakes, fish predators are lacking, but predatory dragonfly larvae are present. Long spines make the fish easier for dragonfly larvae to capture.

20.5 How Do Developmental Genes Constrain Evolution? Calcium is often limited in freshwaters, so less is available to make long spines and bony armor. Lakes have abundant vegetation where sticklebacks can hide, reducing the need for bony armor.