Week 8: Evolution and Development Flashcards
What are homeotic transformations as per Bateson?
documented abnormalities in invertebrates and vertebrates. He called homeotic transformations - common in regions that were repeated/segmented. Hypothesized that these changes would lead to an understanding of the evolution of animal body plans.
What are homeotic genes?
genes that determine the identity and position of anatomical features during development.
What are segmentation genes?
- influence patterns of body segments during development.
What did the discovery of homeotic genes lead to?
field of Evo-devo - fusion of evolutionary and developmental biology
What is Haeckel’s theory of recapitulation?
ontogeny recapitulates phylogeny - developmental processes of an organism replays the evolutionary history of the organisms - rejected because NS acts at all stages of development - embryonic develop is not a replay of phylogenetic history but a mix of it and adaptation.
What is Von Baer’s law?
embryos in closely related species resemble each other - similar characteristics appear early in development. Specialized traits appear later and distinguish between species.
Describe the genes that control embryonic development
Few genes control embryonic development - similar across species.
What is heterochrony?
Timing of development of physical traits relative to development of traits in ancestral species. May be little difference in DNA sequences but difference in timing of expression of genes.
What are the 4 types of heterochrony?
Acceleration, progenesis, neoteny and hypermorphosis
Describe acceleration
descendant somatic trait occurs earlies but descendent reproductive trait occurs at same time
Describe progenesis
descendent somatic trait occurs at same type but reproductive trait is early.
Describe neoteny
descendant somatic trait is later and reproductive trait is the same. ie. in axolotl. Retention of juvenile features in adult.. In tiger salamanders there is a TH spike that causes it to lose gills and develop tail suited to terrestrial life. The TH spike does not occur in axolotl, but if raised in an environment with the spike, it develops into a terrestrial form.
Describe hypermorphosis
descendant somatic trait occurs at same time but reproductive trait occurs later.
What type of cells are early embryonic cells and what is their development guided by?
Early embryonic cells are totipotent - could develop into any cell type. Development is guided by genetic switches that impact protein production, growth of cells and general body plan of plants and animals. Largely depending on timing of genetic switches.
What are homeotic genes?
proteins from combinations of these genes act as gene activation signatures to create instructional map to determine where structures should develop. They have critical role in development of phenotype. NS acts on phenotype.
What are Hox genes
determine anterior-to-posterior positioning of structures. A subset of homeotic genes code for transcription factors that impact the anterior-to-posterior positioning of structures by regulating gene expression. ie. Abdominal B gene is expressed in abdominal body parts near end of fruit fly.
Describe Hox genes in different species ie. in plants
Hox genes also present in other species ie lateral line in fish p allows it to interpret surroundings.
MADS-box genes - homeotic genes in plants.
What is a homeobox?
same 180bp sequence found in all homeotic genes in various animals - allowed discovery of more Hox genes as it is the starting sequence for Hox genes.
What is colinearity?
position of the Hox gene parallels that of the position of the body part which the hox gene regulates.
What evidence indicates Hox genes are homologous?
Homeobox and similar ordering of chromosomes in vertebrates and invertebrates
What happens when swap Hox genes?
inserted mouse Hox-2.2 gene into head region of fruit fly genome - produced legs instead of antennae on their head. Homeotic genes have deep ancient ancestral homology.
Why is there a deep homology in homeotic genes?
early developmental stages are highly resistant to change, mutations in the genes are likely lethal, homeotic genes are fundamental to early developmental stages that determine overall body plan required for proper functioning of systems.
What are gene switches?
transcription factors operate by binding to regulatory enhancer regions of DNA - act as switches to turn genes on/off and are responsible for differential expression of genes. It is one of the reasons why chimps and humans share 98.7% of the genomes but look and act very differently.
What is gene duplication, pseudogene and paralog?
Gene duplication - establishment of multiple copies of genes within genome - influential for new evolutionary lineages - may be lost due to selection or become a pseudogene (functionless) or may become a paralog (continue to be expressed) and can evolve separately.
Why are duplicated genes maintained? What is subfunctionalization and neofunctionalization? Give example of opsin
Duplicated genes are maintained because they may influence gene expression by increasing the production of histones, ribosomal RNA and other products.
Subfunctionalization- division of labour by paralogs. Neofunctionalization - divergence of paralog and adoption of new, related function.
Examples: humans have 3 copies of opsin allowing us to see reds and greens - dogs have 2. Humans have 400 genes for smell receptors that evolved from a single gene and dogs have 800, cows have 1000.
Describe Neural crest cells
embryonic stem cells that become different types of cells - development controlled by developmental regulatory genes ie. Hox, snail, Dlx. Initially near the neural tube then migrate to new locations giving rise to different tissues.
What did Scneider and Helms propose?
beak proportions are determined early in ontogeny by development of neural crest cells. Transplanted neural crest cells from duck to quail embryo and vice versa and generated a duck with a short beak and a quail with a longer beak.
What did neural crest cells evolve from?
similar ancestral cells - similar cells near notochord in early chordates. They possibly evolved via gene duplication.
Summarize
homeotic genes and regulatory enhancers control a lot of the developmental process
Small changes in timing of spatial positions during ontogeny can lead to large-scale phenotypic changes which are under NS.
NS on phenotypes can lead to speciation.
