A quick review from earlier in the course.
To the extent that we saw development it was as a constraint
- The number of cervical vertebrae in mammals
- Which digits are lost in lineages that evolve fewer digits Almost all mammals have 7 cervical vertebrae, it is not for a functional reason (swans have 22-25, ducks 16)
-Hox genes are involved in axial patterning & regulating cell proliferation -Human children with embryonic cancers have a 125x risk of cervical ribs -Human children born with cervical ribs have a 120x risk of early childhood cancer -Taxa with more cervical vertebrae have lower metabolic and cancer rates (birds, reptiles, amphibs, manatees, sloth) -Pleiotropy of Hox genes prevents the evolution of novel # of cervical vertebrae in taxa with cancer risk
Galis, F. 1999. Why do almost all mammals have seven cervical vertebrae? Developmental constraints, Hox genes, and cancer. J. Exp. Zool. / Mol. Dev. Evol. 285: 19-26. http://wwwbio.leidenuniv.nl/~galis/ Patterns of digit loss in tetrapods correlate 3 4 2 1 5 inversely with order of digit development
3 4 2 1 5
lost last lost first (perrisodactyls, horses in particular) Patterns of digit loss in tetrapods correlate 3 4 2 1 5 inversely with order of digit development
Skink limbs
3 4 2 1 5 Chick limbs
lost last lost first (skinks and chickens too) Patterns of digit loss in tetrapods correlate 3 4 2 1 5 inversely with order of digit development 1 2 3 4 5
1 2 3 4 5
lost last lost first (caudata)
Exception that proves the rule A quick review from earlier in the course.
To the extent that we saw development it was as a constraint
- The number of cervical vertebrae in mammals
- Which digits are lost in lineages that evolve fewer digits
But the study of the connection between development and evolution goes far deeper than this. Phenotype: what an organism looks or acts like.
Phenotypic evolution is usually what we care about the most.
Genotype: the DNA of an organism
- The phenotypes of organisms are determined (in part) by their genotypes. - Heredity is of genotypes not phenotypes
Variation GENOTYPIC Heredity Selection PHENOTYPIC
We must understand both genotypes and phenotypes; and their relationship. The process of development connects genotypes and phenotypes.
Phenotype
Development
Genotype
The relationship between genotype and phenotype is sometimes called the genotype:phenotype map.
Studying this has been a long-standing part of biology in general; evolutionary biology as well.
We will look at 3 approaches: 1. Comparative embryology - Haeckel, 1800s 2. Allometric studies - Gould & D'Arcy Thompson, 1900s 3. Molecular genetics - 1990s + The process of development connects genotypes and phenotypes.
Phenotype
Development
Genotype
The relationship between genotype and phenotype is sometimes called the genotype:phenotype map.
Studying this has been a long-standing part of biology in general; evolutionary biology as well.
We will look at 3 approaches: 1. Comparative embryology - Haeckel, 1800s 2. Allometric studies - Gould & D'Arcy Thompson, 1900s 3. Molecular genetics - 1990s + First approach - Haeckel, 1800s
The first evolutionary approach to development was by Ernst Haeckel in the 1800s. His idea was that ontogeny recapitulates phylogeny; the developing organism develops through its previous evolutionary stages until it reaches its final form.
By this model, evolution works by adding more developmental steps onto the end of the ancestral developmental program.
Haeckel saw evidence for this in the similarity of developing mammal embryos to fish and amphibians.
Next slide - Haeckel's famous drawing First approach - Haeckel, 1800s First approach - Haeckel, 1800s The first evolutionary approach to development was by Ernst Haeckel in the 1800s. His idea was that ontogeny recapitulates phylogeny; the developing organism develops through its previous evolutionary stages until it reaches its final form. By this model, evolution works by adding more developmental steps onto the end of the ancestral developmental program.
Problems with Haeckel's model:
- Adult features of ancestors not really seen (as predicted by Haeckel) - Some species skip developmental steps (e.g., direct developing sea urchins) - Development of traits doesn't proceed at same relative rates in all organisms (heterochrony) - Some species appear to cut development short instead of adding terminal steps (paedomorphic)
These weaknesses were known fairly quickly; but better tools (math, DNA) to explain development had to wait until 20th century. The process of development connects genotypes and phenotypes.
Phenotype
Development
Genotype
The relationship between genotype and phenotype is sometimes called the genotype:phenotype map.
Studying this has been a long-standing part of biology in general; evolutionary biology as well.
We will look at 3 approaches: 1. Comparative embryology - Haeckel, 1800s 2. Allometric studies - Gould & D'Arcy Thompson, 1900s 3. Molecular genetics - 1990s + Second approach - allometry, 1900s
As organisms grow (develop) different parts of them grow at different rates - this leads to different shapes.
Maybe small changes in the relative growth rates (due to genotypic changes) can cause big differences in the final form (phenotype)...
We can plot the sizes of height traits over time (age) and the resulting curve is termed an ontogenetic trajectory.
length Second approach - allometry, 1900s
We can plot the sizes of traits over time (age) and the resulting curve is termed an ontogenetic trajectory.
Different developmental rates or durations would result in different final shapes
head head
body body Second approach - allometry, 1900s Hypermorphosis
4 types of Acceleration developmental change:
- Hypermorphosis - Progenesis Log(head) - Acceleration - Neoteny
2 phenotypic results: Progenesis
- Peramorphosis
- Paedomorphosis Neoteny
Log(body) Second approach - allometry, 1900s Many salamanders have gills when juveniles and then as they mature they lose the gills. Some species of salamanders have gills as adults - neoteny causing paedomorphosis
juvenile Ambystoma mabeei
adult Ambystoma mabeei
adult Necturus spp. Second approach - allometry, 1900s Irish elk antlers are HUGE
But they are proportional to the size of the body along a height:antler allometric curve - hypermorphosis creating peramorphosis
Antler size
Shoulder Height Second approach - allometry, 1900s Second approach - allometry, 1900s A more sophisticated prediction of relative growth rates can be done by using a grid and altering growth in X and Y directions in each grid. (D'Arcy Thompson)
This approach can help us understand the mola mola, one of the weirdest fish in the world Second approach - allometry, 1900s A more sophisticated prediction of relative growth rates can be done by using a grid and altering growth in X and Y directions in each grid. (D'Arcy Thompson)
Pufferfish and the mola mola are in the same order, fairly closely related.
Similarities in the skeleton are apparent. Second approach - allometry, 1900s A more sophisticated prediction of relative growth rates can be done by using a grid and altering growth in X and Y directions in each grid. (D'Arcy Thompson)
Pufferfish and the mola mola are in the same order, fairly closely related.
Developmental mutations that change rates of growth in different regions can "transform" one into the other. Second approach - allometry, 1900s If we reverse previous head:body ratio; head starts relatively large and grows slower than body - neoteny would therefore lead to relative larger head for the body Second approach - allometry, 1900s If we reverse previous head:body ratio; head starts relatively large and grows slower than body - neoteny would therefore lead to relative larger head for the body
Selection for increased intelligence (via larger relative brain size in adults) could act via neoteny leading to paedomorphosis - the adult looking like the younger version of ancestral species.
Do human heads look like juvenile chimpanzees/human ancestors?
Thompson's grid? Second approach - allometry, 1900s Second approach - allometry, 1900s
Human faces may have arisen via neoteny from ancestral primate
Cause or correlation?
Other correlations? The process of development connects genotypes and phenotypes.
Phenotype
Development
Genotype
The relationship between genotype and phenotype is sometimes called the genotype:phenotype map.
Studying this has been a long-standing part of biology in general; evolutionary biology as well.
We will look at 3 approaches: 1. Comparative embryology - Haeckel, 1800s 2. Allometric studies - Gould & D'Arcy Thompson, 1900s 3. Molecular genetics - 1990s + Third approach - Molecular, 1990s
Molecular developmental evolution
Using modern molecular tools to look directly at genotypic differences and their effect on phenotype - specifically how when and where genes are expressed is altered.
Regulatory sequences control gene expression during development.
Mutations in these sequences can change the function of a protein.
biochemical function vs evolutionary function
It is now thought that the evolution of regulation may be a more potent force for evolution than evolution of protein biochemical function.
What is a gene anyway? Third approach - Molecular, 1990s Transcription factors (trans elements) are proteins that bind to binding sites in the DNA (cis elements), turning the gene on or off.
The sequence of each binding site is critical to the binding of the trans elements. Evolution of these binding sites can lead to changes in gene expression during development.
The trans elements present in each region of the embryo are critical Third approach - Molecular, 1990s The regulatory genes most studied are the HOX genes
Hox genes are transcription factors, trans elements for a whole series of "downstream" genes.
The absence/presence of particular HOX genes in tissues during development controls other genes and creates structures.
bithorax mutant antennapedia mutant Third approach - Molecular, 1990s The absence/presence of particular HOX genes in tissues during development controls other genes and creates structures.
Manipulation in which a gene product is added to the organism (ectopic expression) can reveal these effects as well. Eyes can be created on legs, wings and other places not normally producing eyes
ectopic expression ectopic expression of Drosophila ey of mouse sey Third approach - Molecular, 1990s The absence/presence of particular HOX genes in tissues during development controls other genes and creates structures.
These patterning genes are correlated with the presence of certain structures
Brine shrimp
Insects Third approach - Molecular, 1990s The absence/presence of particular HOX genes in tissues during development controls other genes and creates structures.
These patterning genes are correlated with the presence of certain structures Third approach - Molecular, 1990s Gene duplications: - Increased the number of genes per HOX complex (ancient duplications, during origin of bilateria) - Increased the number of HOX clusters (more recent duplications, during vertebrate radiation)
Do the presence of more genes allow more complexity in the structures? Third approach - Molecular, 1990s
Models of gene duplication may explain HOX gene evolution
- Do we see evidence of multiple functions (pleiotropy)? cervical vertebrae & cancer? - Do we see evidence of multiple genes for one function (epistasis)? Hox a-11 and Hox d-11?
Traditional model of evolution by gene duplication
F
New genes arise from novel mutations in copies created by gene duplication.
But beneficial mutations are very F F rare compared to deleterious ones. :(
F ψ Recall: globin pseudodgene vestigial traits F G DDC, duplication degeneration complementation model
FG
The diversification of the genes is driven by deleterious mutations. :) FG FG Easiest to envision with mutations in the regulatory sequences controlling gene F FG expression during development.
F G
Zebrafish engrailed gene eng1 and eng1b
Top: expression in pectoral appendage
Bottom: expression in brain
Force et al, Genetics 1999 Third approach - Molecular, 1990s HOX 9-13 are also involved in the development of the vertebrate limb
These genes have pleiotropy Third approach - Molecular, 1990s HOX 9-13 are also involved in the development of the vertebrate limb
Pictures of mouse limbs: HOX A-11 WILDTYPE/knockouts HOX D-11 WILDTYPE/knockouts CAPITAL/lower case
Note: the redundancy of the duplicated genes - epistasis
Davis et al; Nature 375: 791-5 (1995) Third approach - Molecular, 1990s
Hox genes A-11 and D-11 don't code for proteins to make bones, they code for trans elements that turn those bone development genes on and off.
Details of how many developmental genes work together is the study of developmental regulatory networks - can be very complicated.
Gene1 Gene2 Gene5
Gene3 Gene1: Turns 2 and 5 ON Turns 3 and 4 OFF
Gene4 a, Part of the network of transcription factors and their interactions with the control regions of other transcription factors. Genes are indicated by horizontal lines; arrowheads indicate activation; 'perpendicular' symbols indicate gene repression. b, An enlargement of the promoter region of a gene, called endo 16, that helps modulate the development of the endoderm. It contains 34 binding sites (rectangles) for 13 different transcription factors and cofactors (illustrated as rectangles or lollipops, respectively). Six modules (A–G) of transcription factors and binding sites carry out discrete functions to developmentally regulate endo 16. c, Diagram depicting the logical structures of the A and B control circuits during sea urchin development. Nature 421, 444-448 (23 January 2003)
insects worms echinoderms chordates
Mutational targets of genes (variation possible):
Protein structure: mutations in exons change amino acid sequence; change biochemical function
Regulation: mutations in regulatory sequences change expression timing and location; change physiological or developmental function.
non coding coding
regulatory protein "function" "function" Both are evolutionary "functions"