Lecture 25

Question 1

Definitions of Novelty

Answer 1

• morphologically or physiologically new and qualitatively distinct feature
• a key feature that allows the lineage carrying it to enter a new adaptive zone
• new homolog. Evolved from homologous fins, but contain new homologs shared with other limbs
• ex: ancestral globin gene gave rise to new family member myoglobin and hemoglobin family of new gene homologs

Question 2

Ways of Seeking Novel Features in Evolution

Answer 2

• comparative anatomy
• features of fossil organisms
• features in development
• evolution of genes
• where an organism sits in the phylogenetic tree

Question 3

Some Major Novelties in Evolution

Answer 3

• origins of animal body plans
• mouth parts of insects
• tetrapod limbs
• body plan of whales
• hominid upright anatomy and walking
• human brain

Question 4

Organisms Arise From Process of Development

Answer 4

• evolutionary change has to occur through generation by generation of individuals
• development must change for evolution to occur
• gene control of development must evolve for development to evolve-heritable change
• mechanisms of development are under selection and may influence or constrain evolutionary possibilities

Question 5

Development Itself

Answer 5

• will have features that constrain what evolution can do
• selection doesn’t just operate at a single stage in an organism’s life
• operates all the way through

Question 6

Symmetry Breaking

Answer 6

• mostly homogenous sphere egg–>development must produce symmetry breaking to produce body axis and differentiation of cells etc.
• genes and signaling systems exist for all three axes D/V set up first, A/P set up second and L/R set up last
• commonly shared systems-shared by echinoderms, vertebrates, mollusks and likely others

Question 7

Standard Selection

Answer 7

• phenotypes are the sorting of genes so genes contribute to appearance of phenotypes and there is selection for phenotypes
• if development is included genes–>development (has rules)–>phenotype and there is selection for both development and phenotype

Question 8

Developmental Constraints

Answer 8

• can selection do anything or is it limited by existing genetics and developmental mechanisms?
• can organisms actually always be moved by variation and selection to optima or are some apparently possible phenotypes unreachable?

Question 9

All Animal Phyla

Answer 9

• studied share families of regulatory genes used in development
• share similar requirements for development from a basic set up, eggs plus sperm; i.e. development of a multicellular body from a single cell
• share similar gene expression and morphogenetic strategies (i.e. alternate ways of wiring shared gene systems (networks) produces specifically distinct final development outcomes
• distinct phylogenetic pattern among developmental modes -that’s what we expect from a common descent from a single animal ancestor

Question 10

Genes in the Control of Development

Answer 10

• seen in effects of mutations of genes that specifically affect development
• produce phenotypes-are thus targets of natural selection
• can be cloned and their roles and evolution studied

Question 11

What Embryo’s Have to Accomplish

Answer 11

• set up body axes, anterior-posterior, dorsal-ventral, left-right
• turn on an ordered pattern of gene expression
• establish body regions and control cell identities and positions in those regions
• differentiate cells that have specific locations, have defined fates, or talk to each other

Question 12

Developmental Regulatory Genes and the Evolution of Development

Answer 12

• evo-devo is really only possible if there are relatively few key control genes
• evo-devo only possible if pre-existing genes can be co-opted and used in a new way to construct a new developmental pathway
• kinds of genes involved: all elements of basic gene regulatory machinery

Question 13

Developmental Regulators

Answer 13

• transcription factors
• growth factors
• membrane-bound signal receptors
• signal cascades
• enzymes that regulate protein functions by protein modification-kinases and phosphatases

Question 14

Instructions to Build a Fly

Answer 14

• egg born already knowing which end is front and which is back.
• due to Maternal gene in anterior end (14 total)
• start to set up patterns of differentiation in center of embryo
• gap genes (6)–>pair rule genes (8; broad divisions set up)–>segment polarity genes (8; segment facing forward or backward?)–>hox genes (identify body segments)

Question 15

Hox Genes

Answer 15

• at molecular level each encodes for a protein that binds to DNA
• each has three domains that act together to bind to the wide groove of DNA where they can influence genes and promote transcription of downstream gene
• example of a class of major regulatory genes in development that have played important roles in evolution
• present in animals
• clustered in chromosome organization
• in mammals genome duplication has produced 4 such clusters and gene number up to 13 by amplification of 9-10 group

Question 16

Function of Hox Proteins

Answer 16

• hydrogen bonds between complementary bases
• the three helix domains of the hbox in the wide groove of the DNA
• the helix that lies across the groove is the recognition helix
• the sequence of amino acids in it determines what DNA binding site the protein binds to

Question 17

Signaling

Answer 17

• strategy of development as we saw with limb buds
• one group of cells signals to another
• cell expresses a gene that produces a diffusible ligand
• responding cell has a cell surface receptor for the ligand
• cytoplasmic signal cascade activates transcription factor then turns on transcription in target gene critical for developmental step

Question 18

Simple Positive Transcription Output

Answer 18

• transcription factor + start site on DNA

- turns gene on

Question 19

Simple Negative Transcription Output

Answer 19

• transcription factor + block

- turns gene off

Question 20

Complex Positive Transcription Output

Answer 20

-two negatives result in a positve

Question 21

Butterflies vs. Flies

Answer 21

• adult: flies have single pair of wings and butterflies have two pairs
• developmental styles: three distinct forms of development in insects. Flies and butterflies share the third with an important distinction
• larva: caterpillars have five pairs of abdominal legs as well as three thoracic pairs

Question 22

Drosophila Haltere

Answer 22

• pull off and can’t stabilize
• can’t fly properly
• don’t provide power flight but are somehow balance organisms that have evolved from rear wings
• derived from a wing; not a wing but has a function

Question 23

Paleozoic Primitive Wings Insects

Answer 23

• segment identity not as tightly regulated as in modern insects
• some had legs on abdomen; others had wings on abdominal segments of their larvae
• these identify functions and outcomes are controlled by Hox genes, and their regulation continues to evolve

Question 24

Cateripllars

Answer 24

• re-evolve abdominal legs by trick of double inhibition to get a positive outcome where it was advantageous
• old developmental machinery for growing a leg on an segment was not lost, it was repressed in abdominal segments
• leg outgrowth requires expression of Distal-less gene
• Dll is negatively regulated by Hox AbdA=repression 1
• Drosophila AbdA and AbdB are expressed in abdominal segments, Dll is not

Question 25

Dll Expression in Young Caterpillar

Answer 25

-holes in AbdA and AbdB expression when Dll will be allowed to be expressed and lead to legs

Question 26

Two Wings vs. Four

Answer 26

- primitive number of wings among living insects is 4 but flies have 2 - Hox Ubx is expressed in Drosophila T3 segment where it converts rear wing disc into haltere - wing disc evolved to read Ubx to generate distinct different pattern of development - wing development was under control of genes controlled by Ubx; in flies the downstream regulation system changed. Ubx still upstream regulator but genes downstream read it differently in terms of developmental path they follow

Question 27

Butterflies

Answer 27

- jaws express Dll as a key regulator of outgrowth - thoracic legs express Dll - in each abdominal segment Dll is being expressed in two dots on each size - differs from drosophila larva because there is no Dll expression pattern along abdomen--it's repressed in drosophila

Question 28

Pathway Evolution Summary

Answer 28

-see notes to memorize pathways

Question 29

Gene Regulatory Network

Answer 29

- GRN - regulatory genes act in concert - genes in a network influence the expression of other genes to produce an outcome of gene expression that affects the behavior or identity of cells in a developing organism - can be extremely conserved in evolution and still produce different outcomes because of some modification at the end - networks are flexible at downstream ends especially at what output could be - upstream less likely to be shifted because would produce larger effects if there's a mutation up there - thus we see highly conserved roles for Hox genes in laying out animal body axes, but the structures of these bodies can differ