Lecture 25 Flashcards
1
Q
Definitions of Novelty
A
- 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
2
Q
Ways of Seeking Novel Features in Evolution
A
- comparative anatomy
- features of fossil organisms
- features in development
- evolution of genes
- where an organism sits in the phylogenetic tree
3
Q
Some Major Novelties in Evolution
A
- origins of animal body plans
- mouth parts of insects
- tetrapod limbs
- body plan of whales
- hominid upright anatomy and walking
- human brain
4
Q
Organisms Arise From Process of Development
A
- 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
5
Q
Development Itself
A
- 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
6
Q
Symmetry Breaking
A
- 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
7
Q
Standard Selection
A
- 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
8
Q
Developmental Constraints
A
- 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?
9
Q
All Animal Phyla
A
- 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
10
Q
Genes in the Control of Development
A
- 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
11
Q
What Embryo’s Have to Accomplish
A
- 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
12
Q
Developmental Regulatory Genes and the Evolution of Development
A
- 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
13
Q
Developmental Regulators
A
- transcription factors
- growth factors
- membrane-bound signal receptors
- signal cascades
- enzymes that regulate protein functions by protein modification-kinases and phosphatases
14
Q
Instructions to Build a Fly
A
- 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)
15
Q
Hox Genes
A
- 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
16
Q
Function of Hox Proteins
A
- 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
17
Q
Signaling
A
- 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
18
Q
Simple Positive Transcription Output
A
- transcription factor + start site on DNA
- turns gene on
19
Q
Simple Negative Transcription Output
A
- transcription factor + block
- turns gene off
20
Q
Complex Positive Transcription Output
A
-two negatives result in a positve
21
Q
Butterflies vs. Flies
A
- 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
22
Q
Drosophila Haltere
A
- 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
23
Q
Paleozoic Primitive Wings Insects
A
- 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
24
Q
Cateripllars
A
- 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
25
Q
Dll Expression in Young Caterpillar
A
-holes in AbdA and AbdB expression when Dll will be allowed to be expressed and lead to legs
26
Q
Two Wings vs. Four
A
- 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
27
Q
Butterflies
A
- 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
28
Q
Pathway Evolution Summary
A
-see notes to memorize pathways
29
Q
Gene Regulatory Network
A
- 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
