Lec 9

Question 1

Developmental abnormalities are widespread

Answer 1

Observations of development found abnormalities are widespread

Question 2

Homeotic transformations

Answer 2

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?

Question 3

Evolutionary Development (Evo-devo_

Answer 3

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

Question 4

History of Evo-devo

Answer 4

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

Question 5

Von Baer: Characteristics that unite species appear _______

Answer 5

Early

Stage 1 embryos are most similar to each other

Stage 4 embryos are least similar to one another

Question 6

Haeckel’s Theory of Recapitulation

Answer 6

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

Question 7

Evo-devo and the Modern Synthesis

Answer 7

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

Question 8

Types of heterochrony

Answer 8

2 categories: Changes that affect time of onset of reproductive traits, and changes that affect timing and appearance of somatic traits

Question 9

Heterochrony: Recapitulation via acceleration

Answer 9

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

Question 10

Heterochrony: Recapitulation via hypermorphosis

Answer 10

Genetic change can lead to : Reproductive trait appearing later (hypermorphosis)

Somatic - unchanged

Reproductive - retarded

Question 11

Heterochrony: Paedomorphosis via progenesis

Answer 11

A trait that appears EARLY in ancestors but LATER in descendants

Reproductive trait appearing earlier (progenesis)

Somatic - unchanged

Reproductive - accelerated

Question 12

Heterochrony: Paedomorphosis via neoteny

Answer 12

Somatic trait appearing later (neoteny)

Somatic - retarded

Reproductive - unchanged

Question 13

Recapitulation

Answer 13

A trait appears EARLIER in descendent species

Question 14

Paedomorphosis

Answer 14

Trait appears later in descendent

Question 15

Concept of heterochrony was a significant step forward in our understanding of the evolution of development

Answer 15

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

Question 16

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

Answer 16

d) A and B

Question 17

Best studied example of heterochrony occurs in neotenic axolotls

Answer 17

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

Question 18

What causes neoteny?

Answer 18

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

Question 19

Why would neoteny be favored by natural selection?

Answer 19

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

Question 20

What observation might support the hypothesis that neoteny occurs because it’s safer to stay in the water?

Answer 20

Facultatively neotenous species stay in the water when the water level is stable and there are few predators in the ponds

Question 21

Axolotls

Answer 21

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

Question 22

How do multicellular creatures differentiate into so many different forms?

Answer 22

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

Question 23

Regulation, expression and switches

Answer 23

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

Question 24

Homeotic genes

Answer 24

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

Question 25

Hox genes and the Drosophila body plan

Answer 25

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

Question 26

Homeotic genes affect spatial development in plants too

Answer 26

MADS-box genes determine which cells become which structures

Small changes to these genes have large effects on phenotype and reproductive success

Question 27

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

Answer 27

Abd-A is needed to develop backwards walking legs and swimming legs

Question 28

Hox gene evolution

Answer 28

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

Question 29

Position of Hox genes is conserved along chromosomes

Answer 29

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

Question 30

Molecular genetic tools let us study Hox genes

Answer 30

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

Question 31

Why are homeotic genes so deeply conserved?

Answer 31

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

Question 32

Homeotic genes have shown us that

Answer 32

The mechanisms that organize animal body plans are broadly conserved across species

Question 33

Regulatory enhancers as switches

Answer 33

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

Question 34

Regulatory enhancers are important because

Answer 34

They determine when a gene is turned on and at what level it is expressed

Question 35

Evo-devo and neural crest cells

Answer 35

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

Question 36

Neural crest cells have dramatic effects on craniofacial development

Answer 36

Darwin noted that beak proportions of birds are constant during development

These differences have fitness consequences related to feeding

Question 37

Neural crest cells

Answer 37

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

Question 38

Evo-devo and genome duplications

Answer 38

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