Evo-devo Flashcards
What does the Von Baer developmental hourglass tell us?
Study image and explain to self
Name the three Evo-devo principles
- Deep homology of conserved toolkit loci
- pleitropy
- modular CREs
Describe the first Evo-devo principle
- Deep homology of conserved toolkit loci; distantly related species share deep homology of toolkit loci, not structural homology
- example: hox genes, pax6, Dll
Describe the 2nd evo-devo principle
- pleiotropy: The condition in which variation at a locus affects more than one phenotype.
- example: pitx 1 sticklebacks
Describe the 3rd evo-devo principle
- modular CREs
- example: pitx 1 stickleback
- note: also allows for variation within and between species (ex: Bmp4 abd finch beak dimension)
Describe Pitx1 in saltwater vs. freshwater sticklebacks
Pitx1 is a pleiotropic master regulator. It is expressed in several regions
of the stickleback embryo, corresponding to different adult structures (pelvis,
eye, jaw pituitary).
)evo-devo 3):
Pelvic spine increases fitness in marine environment since they can’t be swallowed by predator fish.
NO pelvic spine increases fitness in freshwater environment since they can’t be grabbed by dragonfly larvae.
Describe bmp4 and the beak morphology of Darwin’s Finches
Development constrains nauutral selection
o Narrow range of new alleles are tolerated (or the whole “system” is torn apart)
o Example: beaks, either grow or shrink together(evo-devo principle 2)
evo-devo principle 3:
o Various beaks have different fitness in their as needed evnironments
o Looked at toolkit loci of beaks
Adult with heavy beak shows robust expression in toolkit locus
Adult with smaller beaks have little expression in toolkit locus
Describe edf and Dollo’s Law of fish teeth
Due to natural selection and developmental constraints associated with
Eda (a developmental toolkit) expression, the loss of oral and upper pharyngeal
teeth in zebrafish supports Dollo’s Law. We will return to Eda expression in evo-devo
Eda is a toolkit locus. It’s activity/expression in naïve mesenchyme cells ctivates gene regulatory networks (GRNs) the lead to different teeth. Analysis of Eda mutants show that in zebrafish, selective pressures only tolerate a limited “range” of cells that can respond to Eda, resulting in the (irreversible) loss of teeth.
Describe distal-less
- A genetic network that includes the Distalless (Dll) gene triggers outgrowths during the development of starfish and insects.
- There is NO structural homology of the different outgrowths thatserve as appendages but all species share Deep homology of Dll.
Describe importance of distal-less
it follows deep homology (evo-devo principle 1)
leads to ability to have outgrowths in organisms
Name the traits associated with pax6
- eye development
- studies done in drosophila and mouse embryos
Name the traits associated with Hox
a subset of the Hox genes is expressed in the limbs of tetrapods, where they determine the proximal–distal differentiation of the limb
Why is UBX (a hox) gene significant in evo-devo studies?
it is an example of a rare case in which protein-coding DNA evolution causes phenotypic diversity
: Ubx acts with other genes to determine the length of the middle legs in water striders, insects that skate on the surface of water [59]. The Ubx transcription factor can play diverse roles because it can bind the enhancers of diverse genes. Ubx and some other genes that encode transcription factors are like a hammer or wrench that can be used for a wide range of different tasks.
What lineage is considered the closest lineage to the common ancestor in animals?
Choanoflagellates!
Why are Choanoflagellates significant in evo-devo?
- they come before the Cambrian and have toolkit loci
- after this group, diversity in toolkit loci is seen
- infers all animals (extinct and extant) have toolkit loci in genome
What do fossil Records do for us?
- provides insights into the temporal sequence of morphological changes from deep in time to present day.
- provides evidence of morphological characteristics in extinct taxa that are absent in extant lineages. Such data provide insights into the direction of change, e.g. were traits lost or gained?
- provides dates for the origin of traits
What does evo-devo do for us?
- provides a mechanistic understanding for the origin of traits (e.g. wings, lungs, feathers, body segments)
- provides insights into repeated patterns of evolution observed in different times and in different lineages
- provides a mechanistic understanding of how traits are modified through time consistent with gradual vs. punctuated models
What does Evo-devo help us understand?
- Difference between structural homology and deep homology
- Repeated evolution (convergent evolution)
a. pleiotropy
b. developmental bias/constraint [Flexibility of genetic variation in a pleiotropic master regulator is limited because a major genetic change will affect several traits simultaneously] - Rapid vs. gradual evolution
- Evolution of novel traits
What did Karl Ernst von Baer study?
studied embryos of fish, reptiles, amphibians, birds, and mammals and noted in early embryonic development, there is a remarkable similarity in structure
differentiation occurs later on
Describe toolkit loci
- Protein coding regions are deeply evolutionarily conserved in metazoans
- CRE/cis sequences show flexibility- species specific characteristics
- Two general functions: transcription factors or signalling factors
- Genomic loci expressed/transcribed ONLY during embryonic life in metazoans
- (discrete instruction all important)
Describe a toolkit genomic locus
- has CRE for where, when and how much
- gene proper region with protein and mRNA
- CRE = cis regulatory element/sequence
- CREs are bound by transcription factor
proteins to control gene transcription/expression
Give an example of homoplasy due to similar ecological/environmental pressures
the structures of the skull in sabertooths vs hyenas and dogs
What is the relationship between protostome appendages and vertebrate appendages
analogous
Describe CRE in freshwater sticklebacks
In freshwater stickleback fishes,
The CRE module that gives specific
instructions to Pitx1 for expression in
Embryonic pelvic cells is DISABLED.
So Pitx1 isn’t expressed in these embryonic
cells and adults don’t have a pelvic spine.
However, because of the module CRE,
the other CREs function normally.
Remember since Pitx1 is pleiotropic, you can’t
lose the Pitx1 genomic locus just to cause loss of the pelvic
spine. Pitx1 LOF is embryonic lethal (same for all pleiotropic master regulators).
no pitx 1 expression in embryos, no pelvic spine in adults
Describe deletion of pitx 1 overtime in freshwater stickleback
Deletion of the Pitx1 modular CRE (pelvic switch) has occurred repeatedly in different times and places on Earth, always associated with an ocean to freshwater environmental shift. This is due to a “simple” genetic mutation at one (Pitx1) genomic locus. The fossil record has a punctuated equilibrium rate. The simple and repeatable genetic mutation explains the PE rate AND repeated saltwater to freshwater shifts.
How do the Evo-Devo principles explain evolution of novel traits?
Modular CREs can place a pleiotropic master regulator in a new spatial and/or temporal
context in the embryo without interfering with the ancestral spatial and temporal contexts.
Once a pleiotropic master regulator toolkit locus is in a cell, it will activate the GRN, composed of thousands of genomic loci.
In closely related taxa that have acquired a new CRE module, further diversity can occur by sequence diversity in a given CRE.
explain developmental evolution in tetrapod limbs
The Pitx1 gene: encodes a transcription factor that is important in specifying whether a limb develops into a forelimb or hind limb.
* When Pitx1 is underexpressed and another gene (Tb5) is overexpressed, feathers that normally develop on the wings (forelimbs) of a pigeon also develop on the legs (hind limbs)
* Site specified for hox genes
Describe the impacts of Fdf and Shh signaling in tetrapod limbs
A region (the apical ectodermal ridge) at the end of the limb bud emanates Fgf signaling proteins: induce growth in length of the limb.
Fdf: upregulates the Shh gene in the zone of polarizing activity, located at the rear side of the bud. Shh produces a diffusing protein that elicits the development and affects the identity (form) of the digits.
reduction in the number of digits, and of entire legs in snakes and some lizards, caused by reduced Shh signaling
* loss of even these rudiments in most snakes, is caused largely by greatly reduced expression of Shh, due to the degenerated sequence of a key enhancer
How can autopod elements be affected?
- Heightened Fgf signaling
- apoptosis—programmed cell death—is a major sculptor of hands and feet
