Chapter 9: From Genes to Traits Flashcards

Chapter 9: From Genes to Traits

Regulatory Networks Across Nature (Section 9.1)

Complex Adaptations Defined:
- Consist of multiple components that work together to perform a specific function.
- They are encoded and controlled by many different genes.
- Novel Traits:
  1. Are not inherited but arise from mutations within a specific lineage.
  2. Usually require multiple mutations to reach a state that provides a fitness advantage.
  3. Lack obvious counterparts (homologs) in related lineages.
Categories of Genes in Adaptations:
1. Protein-coding genes: These encode the proteins physically involved in the adaptation.
2. Regulatory elements: These are the control regions (e.g., enhancers, promoters) that determine when and where genes are expressed.
Typical Regulatory Network Organization:
- Environmental factors (signals) trigger the expression of transcription factor genes.
- Transcription factors produce regulatory elements that bind to the control regions of other genes.
- This initiates a cascade that eventually activates specific protein-coding genes.
- Hierarchy: Networks are organized such that a few elements at the top can trigger the expression of hundreds of genes downstream.
Developmental Gene Hierarchies:
1. Hierarchies allow a single-celled zygote to develop into a complex mature individual with diverse organs.
2. Hox Genes: A critical family of transcription factors that determine the identity of body parts and their positions along the body axis.
  - Mutations in Hox genes cause massive deformities. Example: The Antennapedia mutation in fruit flies results in legs growing where antennae should be.
3. Spore Formation: In bacteria, a similar hierarchical system of genes controls the transition into a resting spore state to survive harsh environments.
Questions and Discussion:
- Review Question: What is a transcription factor?
- Response: Transcription factors are activators that bind to the enhancer region of a gene's control region to activate transcription.

Generating Innovations (Section 9.2)

Horizontal Gene Transfer (HGT):
1. A major source of new genes for adaptations, particularly in prokaryotes (though it occurs in some eukaryotes).
2. Genes are acquired from the environment or other organisms without sexual breeding.
3. This occurs within the same generation (—horizontal on the family tree vs. vertical movement to the next generation).
4. If the acquired gene is similar to an existing one, it functions like a duplication; if it is different, it can be recruited into an existing network to facilitate new adaptations.
5. Eukaryote Example: Aphids (chewing insects) feeding on crops can transfer crop genes to nearby weeds via saliva, potentially spreading herbicide or pest resistance.
Gene Duplication:
1. One of the most common sources of new genetic material.
2. Mechanism: Often occurs through unequal crossing-over. This happens when homologous chromosomes align incorrectly during meiosis, which is more likely if sequences (like transposable elements) repeat in multiple locations.
3. Paralogs: Duplicated genes within the same organism.
4. Gene Family: A group of related paralogs.
5. Redundancy: Organisms can tolerate mutations in one duplicate if the other remains functional. This allows the duplicate to evolve new functions, such as being expressed in a new organ or responding to a different signal.
Gene Recruitment:
1. A gene is added to a different network of genetic interactions, fostering a new adaptation.
2. Triggered by mutations in control regions (changing location/condition of expression) or mutations in a paralog.
3. Evolution can recruit single genes or entire modules/networks of genes.
Promiscuous Proteins:
1. Proteins that perform two different functions.
2. They may bind strongly to one target and weakly to a second target.
3. If environmental conditions change and the second function becomes advantageous, natural selection will favor mutations that improve the protein's performance for that secondary role.

The Origin of New Adaptations in Microbes (Section 9.3)

The Industrial (R)evolution:
1. Industrial pollution introduces bacteria to novel chemicals.
2. Case Study: Pentachlorophenol (PCP):
  - PCP was introduced in $1936$ to kill wood-rotting fungi.
  - The bacterium Sphingobium evolved to digest PCP as a food source.
  - The process uses a suite of $5$ proteins to strip $5$ Chlorine ( $Cl$ ) atoms from the molecule to get to the Carbon ( $C$ ), Hydrogen ( $H$ ), and Oxygen ( $O$ ) atoms.
  - One enzyme, $PcpC$ , also breaks down amino acids. Natural selection favored mutations that improved its ability to break down PCP specifically.
  - This highlights the roles of protein promiscuity, duplication, and recruitment in microbial adaptation.
Questions and Discussion:
- Thought Question: What is the evidence for an active gene?
- Response: The presence of messenger RNA ( $mRNA$ ) for that gene, which indicates the gene is being expressed/translated.

The History of Venom (Section 9.4)

Venom Composition:
1. Studies of venom gland cells show that approximately $50\%$ of active genes are "housekeeping" genes (basic life support).
2. The remaining genes encode for venom molecules.
3. Venom cocktails are unique to each species but are most similar between closely related species.
Evolution of Crotamine:
1. Defensins: These genes originally fought infections and were expressed in the pancreas. They are found in mammals (humans, pigs, mice) as well as reptiles.
2. Duplication: In snake ancestors, the defensin gene underwent duplication. One copy specialized in attacking pathogens in the pancreas.
3. Regulatory Mutation: A mutation changed the location of expression from the pancreas to the mouth.
4. Functional Shift: Further mutations allowed the protein to damage muscle tissue in prey. Subsequent duplications created a whole family of venom genes.
Venom Delivery Systems:
1. Timing: Venom genes evolved over $200$ million years ago, long before specialized delivery systems like fangs.
2. Evidence: Venom genes are found in lizards (iguanas, Komodo dragons) and "non-venomous" snakes (garter snakes).
3. Primitive Systems: Early venoms were likely delivered via mucus glands in the mouth. They weren't fatal but caused bleeding or slowed prey.
4. Specialization: As snakes lost their legs (~ $60\,\text{mya}$ ), selection favored stronger venoms and efficient delivery like grooved or hollow fangs.
Questions and Discussion:
- Question: What is the order of evolution for snakes, venom, and venom delivery systems?
- Response: 1st: Venom; 2nd: Snakes; 3rd: Venom delivery systems.

The Genetic Tool Kit for Development (Section 9.5)

The Shared Toolkit:
1. Studies of mice, chickens, and flies reveal a nearly identical underlying gene network for body planning, known as the "genetic toolkit."
2. Hox Gene Family: These transcription factors assign body sections to specific parts. Mice have similar Hox genes to flies but possess four sets due to two lineage-specific rounds of duplication (some copies were subsequently lost).
3. Divergence: These lineages have been separated for over $570\,\text{my}$ .
MicroRNA (miRNA):
1. Small molecules that bind to specific $mRNA$ sequences to enhance or prevent translation.
2. They function to keep certain Hox genes "shut down" in specific regions of an embryo. Some $\text{miRNA}$ molecules are nearly identical between flies and mice.
Axis Development:
1. Bodies are built front-to-back and top-to-bottom.
2. Inversion of Expression:
  - Mice: Ventral (bottom) side expresses $Bmp4$ (a Hox gene) to develop the digestive system. The dorsal (top) side develops the nervous system ( $Chordin$ ).
  - Flies: Dorsal (top) side expresses $Dpp$ (ortholog to $Bmp4$ ) to develop the gut. The ventral side develops the nervous system ( $Sog$ ).
Paralogs vs. Orthologs:
1. Paralog: Homologous genes that resulted from gene duplication within the same species.
2. Ortholog: Homologous genes separated by a speciation event (the same gene in different species).

The Deep History Of Limbs (Section 9.6)

Limb Development Patterns:
1. Flies: A disk of cells unfolds during the larva-to-pupa transition. Growth happens from concentric rings (outer ring = thigh, inner ring = distal tip).
2. Mice: Limbs begin as a bulge (limb bud) and grow outward, building proximal structures (femur) before distal ones.
3. Despite these differences, both use the same orthologous regulatory genes.
Key Genes:
1. Engrained (En): Defines the posterior part of the limb bud in mice; its ortholog triggers leg development in flies.
2. Sonic hedgehog (Shh): Associated with the elongation of the limb bud in vertebrates. Its ortholog in insects is Hedgehog (Hh).
Limb Loss and Gain:
1. Limb patterning is a highly autonomous module that can be switched on or off.
2. Reductions in limbs (e.g., in cetaceans like whales/dolphins and snakes) result from shifted levels of developmental gene expression.
3. Dolphins with hind fins occasionally appear, suggesting the pathway can be reactivated, though a full return to land is blocked by other missing modifications.
Fins to Feet:
1. Tetrapod limbs and fish fins both originate from mesodermal tissue limb buds.
2. Ray-finned fish: Skeletal bones are at the base; the rest is ectodermal "fin rays."
3. Tetrapods: Skeletal bones form the entire limb length, including wrists and digits. These bones are considered homologous to the bones in lobe-finned fish.
4. The expression of genes like $Alx4$ and $Hoxd13$ is striking similar between fish and mouse embryos.

Evolving Eyes (Section 9.7)

Diversity and Origin:
1. Complex eyes evolved independently in several lineages (analogous structures).
2. However, they share a common genetic toolkit involving opsins and crystallins.
Opsin Evolution:
1. Opsins belong to the G-protein-coupled receptors (GPCRs), also called serpentine proteins because they snake through cell membranes.
2. Ancient GPCRs were used for detecting pheromones (e.g., in yeast).
3. A mutation allowed them to pick up light signals, creating the first opsin early in animal evolution.
4. Duplicates allowed bilaterians to detect different wavelengths (~ $620\,\text{mya}$ ).
Crystallin Recruitment:
1. Crystallins are the most stable proteins in the body, used for directing light.
2. They were recruited from heat-shock proteins. These proteins originally "cradled" other proteins to prevent clumping during high temperature or $pH$ stress.
3. A regulatory mutation expressed these heat-shock proteins in the developing lens. Because they had light-bending properties by coincidence, natural selection refined them for optical clarity.

Constraints and Imperfections (Sections 9.8 - 9.9)

Constraints on Evolution:
1. Laws of Physics: Insect size is limited by the way air diffuses through tubes rather than lungs. During the Carboniferous period (~ $360-250\,\text{mya}$ ), atmospheric Oxygen was $35\%$ (vs. $21\%$ today), allowing for giant insects (e.g., $3$ -foot dragonflies, $6$ -foot centipedes).
2. Pleiotropy: When one gene affects multiple phenotypic traits.
3. Antagonistic Pleiotropy: A mutation is beneficial for one trait but harmful for another.
  - Pink Bollworms: Mutations in cadherin proteins provide resistance to $Bt$ toxin but make kernels susceptible to gossypol (retarded growth).
  - Cervical Vertebrae: Nearly all mammals have exactly $7$ neck vertebrae. Deviations are linked to higher rates of stillbirth and a $120\times$ increase in pediatric cancer risk. Exceptions (sloths, manatees) have slow metabolisms which might mitigate cancer risks.
Imperfections in Design:
1. Recurrent Laryngeal Nerve: In fish, this nerve takes a short path from the brain to the gills. As tetrapods evolved necks, the nerve remained looped around the aorta, resulting in a massively inefficient path (meters long in giraffes) from the brain, down the neck, under the heart, and back up to the larynx.
2. The Inside-Out Retina: In vertebrates, photoreceptors point away from the light. The optic nerve must pass through the retina, creating a blind spot. Octopuses, having evolved eyes separately, have nerves behind the retina and no blind spot.

Convergent Evolution (Section 9.10)

Convergence and Parallelism:
1. Convergent Evolution: Unrelated groups (like marsupials and placental mammals) evolving similar phenotypes to fill similar niches.
2. Parallelism: Independent mutations resulting in the same phenotype, often utilizing the same underlying genetic pathways.
3. Deep Homology: Underlying similarities in genetic networks that arose long ago in a common ancestor, even if the eventual complex structures (like eyes or limbs) appear different or evolved independently in later lineages.