Concept 25.5: Major changes in body form can result from changes in the sequences and regulation of developmental genes How can we understand life’s diversity? Fossils – evidence of past biodiversity Continental drift, mass extinction, adaptive radiation – environmental changes influence biodiversity Genetic Change – changes in DNA sequences and regulation modify bodies/cells

Evolutionary Effects of Development Genes Developmental genes guide the formation of the body from embryo to adult Even a small change in rate, timing, and spatial pattern can produce major morphological differences between species Let’s look at some examples!

Changes in Rate and Timing Heterochrony - evolutionary change in the rate or timing of developmental events Ex. human and chimpanzee skull differences due to small changes in relative growth rates

(b) Comparison of chimpanzee and human skull growth Fig. 25-19b Chimpanzee fetus Chimpanzee adult Figure 25.19 Relative growth rates of body parts Human fetus Human adult (b) Comparison of chimpanzee and human skull growth

Changes in Rate and Timing Different parts of our bodies grow at different rates

Changes in Rate and Timing Paedomorphosis – retention of juvenile traits features in the adult This adult salamander retains the gills of the larval form – adults usually have lungs Development of reproductive organs accelerates compared to other organs

Changes in Spatial Pattern – placement and organization Homeotic genes – control placement and spatial organization of body parts Ex. Where do legs develop, where does the head form, how are the parts of a flower arranged They are master switch genes which activate/regulate other genes needed for formation of body structures Hox genes provide positional information in animal embryos

“Scarce as hens teeth”

Changes in Spatial Pattern – placement and organization The transition from invertebrate to vertebrate may have been influences by alterations of Hox genes In particular duplication of hox genes may have played an important role

Hypothetical vertebrate ancestor (invertebrate) Fig. 25-21 Hypothetical vertebrate ancestor (invertebrate) with a single Hox cluster First Hox duplication Hypothetical early vertebrates (jawless) with two Hox clusters Second Hox duplication Figure 25.21 Hox mutations and the origin of vertebrates Vertebrates (with jaws) with four Hox clusters

The Evolution of Development Changes in developmental genes can result in new morphological forms This may answer the puzzle of the Cambrian Explosion WE JUST DISCUSSED GENE DUPLICATION – BE SURE TO LOOK AT Figure 25.22 NEXT CHANGES IN GENE REGULATION First we need to see how you think about how genes are expressed in cells!

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ANOTHER ANALOGY EVERYDAY – NO MATTER WHAT THE WEATHER – I PUT ON SOCKS AND UNDERWEAR BUT……………….. But weather conditions influence how I select the rest of my clothing

Changes in Gene Regulation Changes in the form of organisms are often by changes in the regulation of developmental genes instead of changes in their sequence For example three-spine 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

Stickleback Fish and a Gene Called Pitx 1 Why do marine stickleback have spines on their lower surface while freshwater stickleback have none (or few)? Hypothesis A: Developmental gene Pitx 1 had changed (nucleotide sequence changed) Test – compare DNA for Pitx 1 in both kinds of fish Hypothesis B: Regulation of the gene Pitx 1 had changed Test – monitor expression of Pitx 1 in developing embryo

Differences in the coding sequence of the Pitx1 gene? Result: No Fig. 25-23 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 ? Pitx1 is expressed in the ventral spine and mouth regions of developing marine sticklebacks but only in the mouth region of developing lake stickbacks. Result: Yes Marine stickleback embryo Lake stickleback embryo Figure 25.23 What causes the loss of spines in lake stickleback fish? Close-up of mouth Close-up of ventral surface

Marine stickleback embryo Lake stickleback embryo Fig. 25-23a Marine stickleback embryo Lake stickleback embryo Close-up of mouth Figure 25.23 What causes the loss of spines in lake stickleback fish? Close-up of ventral surface

Regulatory Genes Pitx 1 gene Pitx 1 protein

CONCLUSION LOSS OF SPINES DUE TO CHANGE IN REGULATION OF GENE NOT THE NUCLEOTIDE SEQUENCE OF THE GENE ITSELF