Ch 13 Flashcards
Homeotic mutations are what?
Body parts replacing other ones. This approach of homeotic studies are called evo-devo. (incorporates developmental biology into evolutionary biology)
What did William Bateson do? (3)
Translated Mendel’s works into English
Named science of Genetics
& documented homeotic mutations (and what it tells us about evolutionary change)
What did Bateson recognize about homeotic mutations and their commonality?
Homo mutations were most common in parts of the body that were segmented, repeated, or both.
What did Bateson hypothesize on homeotic mutation?
It would allow for deciphering evolution of animal body plan
What is developmental biology?
Studying development of the animal from egg into multiple cells and then into an embryo with various incipient organs & tissues and finally into adult form with full-grown internal and external structures.
Parallelism & its proponent was who?
Development stages mirrored scalae naturae in going from simple thru complex
JF Meckel
von Baer’s Law
General characteristics of embryos in closely related species develop before specific characteristics, and embryos of higher taxa do not resemble the adult form of ancestral lower-taxa sp.
What is special about embryos appearing similar early on embryonic development?
Resistant to evolutionary change & presumably changes early on have large consequences
When do specialized traits appear that distinguish species?
Later on during development, von Baer agued against Darwin
Ontogeny
a precise and compressed recapitulation of phylogeny; ontogeny of an organism replays its evolutionary history
What did some work in the 30’s and 40’s demonstrate about genes?
Genes code for physical traits, rate of development, and timing at which developmental stages occur
define heterochrony
changes in the rate and timing of development
What was Gavin de Beer interested in? (think of heterochrony)
Interested in whether the time at which a trait was first expressed in a given species occurred EARLIER or LATER than it had occurred in an ancestral species
What are the 4 recognized types of heterochrony?
- acceleration
- progenesis
- neoteny
- hypermorphosis
What are the two categories of heterochrony?
- changes that affect the timing of the onset of reproductive traits
- changes that affect the timing of the appearances of non-reproductive (somatic) traits
Recapitulation observes what?
-observes trait formerly seen in adult stage of an ancestral species now appearing in juvenile stage in species derived from that ancestral species.
What is accelerated in recapitulation?
Somatic trait is accelerated relative to the appearance of reproductive traits
*deals with relative timing: somatic traits can appear earlier in devo (acceleration) while reproductive traits can appear later (be retarded) in devo (hypermorphosis)
What is Paedomorphosis?
Appearances of traits formerly seen in juvenile stage of a species during the adult stage in a descendant species
What are the two possible ways paedomorphosis can occur?
- reproductive traits appearing earlier (progenesis)
- onset of somatic traits is retarded (neoteny)
What serves as an example of neoteny?
Mexican salamander; matures into normal reproductive adult but never loses aquatic traits, adult somatic traits are extremely retarded
What plays a role in neoteny of the Mexican salamander?
Lack of thyroid hormone (TH); spike is never seen at transition to terrestrial life-style as in other salamanders.
If TH is added to developing M. salamander, they develop like other salamanders.
Neotenous devo means what?
Maintaining juvenile characteristics
Why did neotonous developmental pathway develop in the first place?
Paedomorph advantage hypothesis
What is Paedomorph advantage hypothesis
Neotony was favored as a means of remaining in what is a relatively safe aquatic habitat, rather than metamorphosing and facing a new suite or terrestrial predators and a completely different environment
Early on in devo, each cell in the embryo is TOTIPOTENT. What does this mean?
- could differentiate into any cell types that make up the adult organism
- fate depends on the ways genes regulates and expressed within the environment of a cell
What is development in a dynamic process?
Cells receiving local cues from the cellular environment
What are homeotic genes?
Master-switch genes that encode proteins that activate or repress gene expression of particular genes.
*are highly conserved= no change in devo
Regulatory enhancers
DNA that turns on and off the expression of particular genes
How do homeotic genes affect developing embryos and structures?
Delineate where morphological structures will grow within an embryo and play a key role in the development of these structures
What proteins are homeotic genes encoding for?
Encode proteins that control the switching on and off of a cascade of other genes in a set sequence and thereby affect cell size, shape, and division, and the positioning of the cells within the organism’s body.
What happens when mutations occur in homeotic genes of fruit flies?
- affects body plan
- acts as a map of where structures should develop
- genes along the anterior-to-posterior body segments of the insect were responsible for observed phenotypes.
What do hox genes do?
Similar to homeotic genes, they determine ultimate fate of various cells. So any small change can have a big effect.
Colinearity
each gene on the chromosome corresponds to the relative position of the animal body part that it regulates.
How do hox genes operate in flowering plants?
- similar as in animals;
- MADS-box AG group is involved in the positioning of the different types of plant cells
- determines which structure is formed in which location
Homeobox
180bp sequence found in homeotic genes in a wide variety
What does the similar sequences and ordering in both invertebrates and vertebrates suggest in expression patterns?
Suggests that differences in the expression patterns of regulatory genes in their local cellular environment are at least partially responsible for the very different sorts of body plans in vertebrates and invertebrates
What can be inferred about Hox genes being completely conserved across species?
Deep homologies (shared common ancestor) between invertz and vertz
And the most conserved are associated with anterior, central, and posterior body parts
What is involved in developmental pathways in leaf and root systems?
MAD-box genes- which function in flower development and in non-flowering plants.
Why are homeotic genes so conserved?
<> esp during early stages of development
also, mutations that change structure are usually lethal
What do homeotic genes encode and what do they interact with?
Homeotic genes encode TRANSCRIPTION FACTORS that interact with REGULATORY ENHANCERS
Duplicated genes role?
play important role in the evolution of developmental pathways.
- can become a pseudogene
- stay functional and become a PARALOG
- or lost due to NS if it comes at a cost
How can duplication of homeotic genes create new developmental pathways?
Paralogs may evolve differently
& may explain the diversity of form we see today
How does regulatory enhancers and switches work?
- a section of DNA that lies outside of that gene is involved in regulating time and level of gene expression
- acts as a switch to turn O/F and affect amount of product produced by a gene
- with several enhancers, a gene can operate independently of each other
- can increase amount of variation (morphological) which NS can act on
More gene duplication…
more complexity
Duplicate gene and original may be maintained in population if…
duplication genes may influence gene expression levels, increasing production of some critical substances such a ribosomal RNA and histones.
Subfunctionalization
Paralogs may diverge by dividing the work initially undertaken by the gene before duplication.
when paralogs diverge= accumulates mutation
Neofunctionalization
Duplicated genes may diverge, thus allowing for some new, but related, function to evolve.
What gene has duplicated in colobine monkeys?
RNASE1
Colobine monkeys 3 facts?
- herbivorous diets
- use bacteria in gut to metabolize plants
- RNA from bacteria supply needed nitrogen
What did the duplication of RNASE1 gene allow for colobine monkeys?
Duplication event allowed for herbivory in colobine monkeys
What is neural crest cell and what is controlled by?
- cells that initially positioned near the neural tube during early ontogeny and then migrated to new locations during embryo stages
- controlled by a set of homeotic genes
What did the Schnieder and Helms experiment hypothesize?
Hypothesized that beak proportions were determined early in ontogeny by expression of neural crest cells, and that this expression differed between species with different beak proportions.
put a quail’s beak into a duck’s and vice versa
When are neural crest cells have thought to arise?
Thought to have evolved early on in vertebrate evolution
thought to have even represented a new cell type.
