Chapter 11 Flashcards
What did Von Baer discover in embryos?
o The early embryonic stages of most diverse vertebrates are nearly indistinguishable, and that the special characteristics that distinguish mammals from reptiles, or primates from rodents, develop only at later stages
Describe the impact of homologous genes
o Homologous similarities are often based on shared genes
But that does not tell us how a change in DNA sequence becomes realized as a change in an organism’s form and function
What types of general questions is evo-devo trying to address?
What have been the changes in developmental mechanisms that give rise to different phenotypes?
How do genetic difference among species map onto phenotypic differences?
What is the role of development in enhancing, channeling, or constraining evolutionary change in characters?
How does developmental info help us identify homologus characters, or even define homology?
Can understanding development help us understand the origin of novel characteristics?
Proximate causes in evolution
Developmental causes of phenotype
Mechanisms that operate within an individual organism
Ultimate causes in evolution
Natural selection
Act at the level of populations across generations
Explain an example how proximate and ultimate are not exclusive
webbing in limbs is ultimate, in the sense it provides enhanced fitness in ancestors of ducks; its proximate in the sense of cells being eliminated in chickens and humans through programmed cell death but stays in bats and ducks (Helps us understand how differences between species are generated through developmental biology)
Von Baer’s Law
all tetrapod veretebrate embryos display pharyngeal clefts(gill slits), a notochord, segmentation, and paddle-like limb buds before the features typical of their class or order become apparent
Allometric growth
refers to the differential rate of growth of different parts or dimensions of an organism during its ontogeny
Example: an adult human is not simply a magnified infant
Allometry refers to difference in the SHAPE of organisms or of certain of their parts, such as dimensions of skull or leaf
Allometric relationship often described by equation…..
equation y = bx^a
A = 1 relationship linear
A > 1 y increases faster than linearly, positive
A < 1 y increases more slowly than linearly, negative
Seen within and among species
Heterocrhony
an evolutionary change in the timing or rate of developmental events
Paedomorphosis
some characteristics of the adult of a species have a more juvenile form than in the species’ ancestor
- One way it can happen: neotony
- Neotony example: axolotl, a salamander that grows to full size but does not undergo metamorphosis as most salamanders do
- Sexually mature individuals with juvenile features
Heterotropy
Evolutionary change in the spatial position of a feature within an organism
Feature often expressed at additional novel position
Modules
: distinct units that have distinct genetic specification, developmental patterns, locations, and interactions with other module
Serially homologous: modules repeated on the body
Sometimes lack distinct individual identitites
Individualization and example
evolutionary phenomenon with the acquisition of distinct identities by modules
Example: Permian ancestors of mammals had conical teeth; teeth deifferenated later on into incisors, canines, premolars, and molars with different functions (suggests different genes active in developing primordia of teeth)
Name the four cornerstone discoveries about development
Almost all cells in an organism have the same set of genes, based on replication of the genome during mitosis
Differences among cells, tissues, and organs must result from differences in activity of certain genes
Different cells have properties that affect morphogenesis (the development of form)
* Includes growth of indvidiual cells, change in cell shape, adhesion to certain other cells, etc
Many aspects of growth and differentiation are affect by chemical signals, especially hormones
What can embryo development experiments tell us?
discovered that certain events in an animal’s development depend on preceding events and that the differentiation of one tissue or organ is often influenced by others
What can the genetic discovery of Bacterial operon tell us?
a combo of regulatory sequence and coregulated protein-coding genes
Based on concept, developmental biologists roy britten and eric davison laid foundations of modern view that eukaryotes’ genes have gene regulatory elements, or binding site, to which bind proteins that initiate or stop transcription
gene regulation in eukaryotes
, transcription of a protein-coding gene is controlled by a promoter just upstream of the coding region
Promoter acts as switch that is turned on when RNA polymerase binds to it, which in turn occurs when regulatory proteins called transcription factors (TFs) bind to both polymerase and an upstream region called en enhancer
Genes can be transcribed in specific cell types and at specific times
repressors
block transcription
Cis regulatory elements
enhancers and respressors
* Element regulates a gene on the same stretch of DNA
* Noncoding DNA sequence in or near a gene that influences the gene’s expression, often containing binding sites for transcription factors
trans regulatory elements
- Encoded by DNA distant from the genes that they regulate
- Molecule, usually a transcription factor protein, that binds to a regulatory element of a gene, and is encoded by a gene located elshwere in the genome
Name other factors that effect gene expression
DNA methylation, translation of mRNA
DNA methylation
- Transcription of a gene is repressed by methylation of certain cytosines that are followed by guanine residues
- Methylated state may be maintained by a specific enzyme in newly synthesized DNA during cell division
- Example: crx encodes transcription factors thatare necessary for normal expression of many downstream genes
o This gene and others are repressed in cave-dwelling fish, known to cause human eye disorders
MicroRNAs
- MicroRNAs: together with proteins, binds to the 3’ untranslated region of RNA messages and prevent translation
o Various posttranslational processes can affect the activity of proteins, such as binding a signaling molecule
o Tissue induction: interaction among cells, ,molecules released by one cell type diffuse and are bound by extracellular domain of receptor proetins that span membranes of other cells
Trasgenic organisms
an organism whose genome has been artificially modififed, usually by the intro of genes from diff species
Inserting gene from one organisms into genome of another (often different species))
Method to study developmental genetics
Gene kknockout
Replacing normal gene with mutated sequence, using CRISPR-Cas targeted mutagenesis
Method to study developmental genetics
Hox genes
part of genes’ seuqnce that encodes the DNA-binding domain of the transcription factor is called the homeobox, and the genes are called hox genes
* Aka homeotic selector genes
Hox genes are part of a gene regulatory network
gene regulatory network
set of interacting regulatoy genes and the genes they regulate – that specified the anterior-posterior body pattern
* Example: drosophila
o First genes expressed maternal bicoid, form protein products in egg gradients
o Proteins transcription factors that activate expression of gap gnees such as hunchback and Kruppel, which define broad territories in embryo
what do several groups of genes together determine?
determine the region where each homeotic selector (Hox) gene is transcribed, defining the identity of each segment and initiating developmental pathways that result in the segment’s various features, such as antennae, legs, and wings. Complex regulatory pathways of this kind are characteristic of development of most of the morphological features of multicellular organisms.
What did Hox genes discover about Drosophila?
the Hox genes described in Drosophila are also present in mammals, and some of them occur in all bilaterian animal phyla.
Hox genes are conserved
Name an example of Hox genes interacting with other genes to play other roles
: 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.
genetic tool kit
The set of genes and proteins, often conserved across distantly related organisms, and the developmental pathways that they comprise, by which multicellular organisms are constructed during development
Importance of genetic toolkit
shared widely among animals and can contribute to evolutionary changes in the regulation of diverse genes with diverse developmental roles. In some instances, genetic pathways, not just single genes, have taken on additional roles
