Lec 9 Flashcards
Developmental abnormalities are widespread
Observations of development found abnormalities are widespread
Homeotic transformations
Certain mutations in insects and vertebrates result in one body part replacing another or duplicating - these are callled homeotic transformations
Homeotic transformations seem to be most common in parts of the body that were either repeated (appendages, ribs, and so on), segmented, or both
What can homeotic transformation tell us about evolutionary change?
Evolutionary Development (Evo-devo_
Structural variation in body shape and form depends in part on when and where certain genes are expressed
This field is a fusion of developmental and evolutionary biology
If we can understand this process, we can understand the origin and evolution of morphological variation and innovation - major transitions in the evolution of life
History of Evo-devo
Ontogeny: Development of an individual over its lifetime
Pre-Darwinian naturalists noticed that the development of an individual started with “simple” traits and moved on to complex traits later in development
Meckel-Serres Law (early 1800s): Embryos display characteristics of embryos from species that came before them in the scala naturae - the classification of life forms from the “highest” to the “lowest”
von Baer’s law (1828): General traits appear early in development, and more specific traits that separate species appear later
Von Baer: Characteristics that unite species appear _______
Early
Stage 1 embryos are most similar to each other
Stage 4 embryos are least similar to one another
Haeckel’s Theory of Recapitulation
Ernst Haeckel expanded on the Meckel-Serres law with his Biogenetic Law: Ontogeny is a precise and compressed recapitulation of phylogeny
First theory to tie development to evolutionary theory
Put development in terms of phylogeny
Thought that evolution produced novelty by “tacking” new structures on to the terminal part of development of an ancestor
-This is NOT true: Evolution acts on ALL stages of an organism’s life, including the embryological stages. Development, like all traits, is thus a fusion of phylogenetic history and ongoing adaptive change
Evo-devo and the Modern Synthesis
Experimental work on evolutionary genetics in the 1930s and 40s demonstrated that genes not only code for physical traits, but also control the rate and timing of development
-When different structures appear during embryogenesis
Heterochrony: The time in the developmental process at which a trait is first expressed, relative to when that trait is first expressed in the ancestor
-Puts developmental stages (when things appear) into an evolutionary lens
Types of heterochrony
2 categories: Changes that affect time of onset of reproductive traits, and changes that affect timing and appearance of somatic traits
Heterochrony: Recapitulation via acceleration
A trait that appears late in development in an ancestral species, but earlier in development in the descendant species (AKA peramorphosis or overdevelopment). Genetic change can lead to: Somatic trait appearing earlier (acceleration)
Appearance of somatic = accelerated
Appearance of Reproductive = unchanged
Heterochrony: Recapitulation via hypermorphosis
Genetic change can lead to : Reproductive trait appearing later (hypermorphosis)
Somatic - unchanged
Reproductive - retarded
Heterochrony: Paedomorphosis via progenesis
A trait that appears EARLY in ancestors but LATER in descendants
Reproductive trait appearing earlier (progenesis)
Somatic - unchanged
Reproductive - accelerated
Heterochrony: Paedomorphosis via neoteny
Somatic trait appearing later (neoteny)
Somatic - retarded
Reproductive - unchanged
Recapitulation
A trait appears EARLIER in descendent species
Paedomorphosis
Trait appears later in descendent
Concept of heterochrony was a significant step forward in our understanding of the evolution of development
Incorporates evolutionary history by comparing ancestral and descendent species
Focus on genetic change
Recognizes that traits associated with reproduction are fundamentally different from somatic traits
The concept of heterochrony allows us to:
a) Think about development within an evolutionary framework
b) Compare the time at which traits appear during development in ancestors and descendants
c) Construct a phylogeny base don when traits appear during development
d) A and B
d) A and B
Best studied example of heterochrony occurs in neotenic axolotls
Salamanders spend juvenile stage in the water
They then lose juvenile traits and move onto the land
The axolotl never loses its juvenile traits (gills, flattened tail) and stays in the water
Reproductive traits appear at same time as ancestors, but somatic traits never appear at all
What causes neoteny?
Most salamanders have a spike in thyroid hormone associated with metamorphosis, but axolotls don’t
Adding thyroid hormone to water makes juvenile axolotles metamorphose into terrestrial-like forms
Mechanism may be reduced expression of TH regulatory genes in axolotls
Why would neoteny be favored by natural selection?
Maybe staying in the water is safer
Some other salamander species can stay neotenous under certain environmental conditions (“facultative neoteny”)
Facultative = under certain conditions
Axolotls are OBLIGATE neonates
Maybe staying in the water is safer
Some other salamander species can stay neotenous under certain environmental conditions (“facultative neoteny”)
This occurs more frequently when there are few predators in ponds, water levels are stable, and there is little competition
Supports the idea that neoteny is favored when these conditions are met
What observation might support the hypothesis that neoteny occurs because it’s safer to stay in the water?
Facultatively neotenous species stay in the water when the water level is stable and there are few predators in the ponds
Axolotls
Native to Mexico
When Spanish settled in 1521, drained lakes = first step to pushing axolotls to extinctions
Neotenic: Reach adulthood without undergoing metamorphosis
Inject iodine in lab to stimulate metamorphosis
Live around 15 years in the wild
Able to regenerate PERFECTLY; can regenerate limbs, spinal chord, jaw, and skin with NO scarring
1000% more resistant to cancer than any other animal
Cells become pluripotent after limb cut off; can then be used to differentiate as needed to regrow limb
Can also place removed limb from one axolotl onto another and it will grow that into another limb
How do multicellular creatures differentiate into so many different forms?
Every multicellular organism develops from a single cell
Except for sperm and eggs, every cell in the body of a multicellular creature contains the same set of genes
yet skin cells function very differently than do the cells in muscles, cells in the liver, and so on
Regulation, expression and switches
Very early in development each cell in an embryo is totipotent - it could develop into any kind of cell
We talked about how some cells give up the ability to be reproductive in favor of performing other tasks
Which type of cell they become depends on how their genes are regulated and expressed, and the environment surrounding the cell
Homeotic genes
Tells genes what to become
Genes that play key role in development and construction of the phenotype
Encode proteins that switch other genes on and off in a specific sequence
This affects cell size, shape, division,a dn positioning within an organism’s body plan
Act as a map for where structures should develop
In plants the most important homeotic genes are called the MADS-box genes
In animals the key homeotic genes are the HOM (inverts) or Hox (vertebrates) genes
Hox genes and the Drosophila body plan
Hox genes are responsible for development of overall regions and segments
Development of overall body regions and segments within regions controlled by 8-13 Hox genes
These genes determine the fate of cells in the head, thorax, and abdomen
Mutations in Hox genes are responsible for abnormalities like legs growing out of heads
Hox genes and the evolution of animal body plans:
- Hypothesis: Diversity of body plans due to mutations in Hox genes
- First looked at what Hox genes turned on, then turned them off one by one to see what would happen
- Abd A Hox changed backwards walking legs and swimming legs into normal legs; Abdominal A essential for differentiating leg type
Homeotic genes affect spatial development in plants too
MADS-box genes determine which cells become which structures
Small changes to these genes have large effects on phenotype and reproductive success
You are studying Hox genes in crustaceans. you notice that when you knock out the Abd-A gene, your organism develops walking legs where it normally develops backwards walking legs, and anchoring legs where it normally develops swimming legs. you can infer that
Abd-A is needed to develop backwards walking legs and swimming legs
Hox gene evolution
There are remarkable homologies in Hox genes of different groups of organisms
This was discovered/realized at the same time as the homeobox
The same 180 base pair sequence, called the homeobox, is found in ALL homeotic genes in a wide array of animals
Once this was identified, other homeotic genes could be found in a variety of species
Position of Hox genes is conserved along chromosomes
Position on chromosome corresponds to position on body, with genes on anterior end of chromosome are associated with anterior structures, and those on posterior end associated with posterior structures
This chromosomal ordering is conserved (occurs in invertebrates and vertabrates)
-i.e. posterior chromosome same in mouse and fly
We don’t know why this occurs
Molecular genetic tools let us study Hox genes
We can deactivate Hox genes and look at the consequences for limb and body plan development
We can swap homologous Hox genes between species
The Hox-2.2 in mice is similar to the Antp gene in Drosophila
Mutations in Antp cause flies to grow legs out of their heads
If Hox-2.2 from mice is inserted into flies and expressed in the head area, flies grow legs out of thier heads
if you have a fly that is missing the eyeless gene, it doesn’t produce eyes
If you insert the homologous gene from a mouse, it grows normal compound insect eyes
-Sequence similar enough so that fly still has fly eyes even when given mouse eye genes
These deep homologies persist across the animal kingdom
Why are homeotic genes so deeply conserved?
Developmental changes can lead to radical new body plans
However, the programs underlying EARLY development are very resistant to change
Homeotic genes appear to be fundamental to the early stages of development, and mutations in these genes are very likely to be lethal
We therefore see strong conservation of these genes over evolutionary time
Homeotic genes have shown us that
The mechanisms that organize animal body plans are broadly conserved across species
Regulatory enhancers as switches
Homeotic genes encode transcription factors that guide development
Transcription factors bind to stretches of DNA called regulatory enhancers
Regulatory enhancers are not part of the exon, but they regulate the timing and level of gene expression
-Upstream of gene: Enhancer sequences tell gene when to turn on and off
When transcription factors bind to the regulatory enhancer, it is like turning on a switch
This triggers the RNA polymerase to start transcribing the gene
Regulatory enhancers are a type of cis-regulatory element - elements that control expression of nearby genes
These are how the cells of multicellular organisms can do different things despite containing the same set of genes
These are also important to diversity of life - organisms with very similar genes can differ a lot due to differences in gene expression and regulation
Regulatory enhancers are important because
They determine when a gene is turned on and at what level it is expressed
Evo-devo and neural crest cells
Neural crest cells are embryonic stem cells controlled by a set of regulatory genes (Hox, snail, Dlx)
Positioned near neutral tube early in development, then migrate to new locations
After migration, they form or contribute to critical tissues and organs
Neural crest cells have dramatic effects on craniofacial development
Darwin noted that beak proportions of birds are constant during development
These differences have fitness consequences related to feeding
Neural crest cells
If you move embryonic neural crest cells from a duck to a quail, the quail develops a duck-like beak (and vice versa)
Neural crest cells differentiate into specific cell types (duck neural crest cells differentiate into specific duck bill)
These cells were once thought to be unique to vertebrates, but similar cells are found in ascidians and amphioxus
May have evolved through a series of gene duplications that ultimately led to vertebrate neural crest
Evo-devo and genome duplications
Gene duplications are important to the evolution of developmental pathways
Gene families are genes related due to duplication
Paralogs are duplicates that continue to function
These can create new developmental pathways that allow diversification of forms
