Lecture 25 Flashcards Preview

Evolutionary Genetics and Genomics > Lecture 25 > Flashcards

Flashcards in Lecture 25 Deck (20):

Selective agents on Adh:

- Alcohol
- Temperature
- Alcohol and temperature
- Something else?


Clarke Protocol:

5. Difference in molecular function must be reflected in fitness differences in postulated selective regime
6. Natural populations must be re-examined to seek a comprehensive explanation for observed allele frequency distributions in space and time
7. Test your hypothesis by perturbing (changing) allele frequencies in a natural population


Fitness in the presence of ethanol:

- Population cages AdhF frequency increases to fixation when select with lower concentrations of ethanol
- Mixed success


Fitness in the absence of ethanol:

- Population cages frequencies converge on an intermediate frequency


Chateau Tahbilk Winery:

- Populations of dros. in the cellar and outside are separate for most of the year
- Once a year there is massive unidirectional gene flow from outside via the fermentation area in the cellar
- This replenishes cellar population


What are the conditions in the Tahilk winery?

- Range from 8 - 22 degrees
- The population is temperature dependent, and stop breeding at 15 degrees
- Most flies die over winter and there are tiny sub-populations on each vat


Evidence for natural selection:

- Parallel clines over 5 continents of the S allele decreasing further away from the equator
- Selection for the fast allele in low alcohol environments in cages, selection for a polymorphism even in the absence of alcohol
- Lack of fluctuation in allele frequencies when population size crashes


Chateau Tahbilk Overview:

- Returned to the pre-perturbation levels
- Returned at a rate which was proportional to temperature/ADH activity relationship


Perturbation Experiment 1:

- Release a large population of flies into the cellar 1 with an increased frequency of Adh-F
- Same thing done in cellar 2 with a decreased frequency of Adh-F


Balancing selection maintains F/S polymorphism:

- Lack of noise in allele frequencies in face of potential for drift
- Perturbation experiments
- Parallel clines
- Environmental heterogeneity


Problems with the conclusions drawn:

Unable to generate consistent fitness estimates for the three genotypes
- Type of balancing selection unclear
- Cannot define the selective agent


For other systems:

- Parallel clines can be found
- Selective agents are generally unidentified
- Fitness differences between genotypes are extremely difficult to demonstrate
- Detailed studies of inividual populations has not been done


Clarke approach:

1. Choose gene
2. Attempt to find the selective agent


The reverse to the Clark approach may be more useful:

1. Choose a selective agent which has an impact on viability eg) heat stress, insecticides, ethanol tolerance
2. Identify gene(s) responsible for adaptation (mapping/DNA seq, candidate gene approach)


Benefits of using bacteria for studies:

- Allozymic variation exists
- Short generation time
- Population size is near infinite (drift effects are minor)
- Defined cultural medium, so the environment is controlled and manipulated
- Different alleles can be introduces into an identical background using transduction
- Haploid, no heterozygotes



- Volume of 30ml, this is a big population size
- 600 generations
- Strains with different alleles are competed pairwise
- T5 bacteriophage resistance is used as a genetic marker
- Nutrient medium enters at top, the same amount of bacteria drip out the bottom, allowing us to sample a population through time (lacz1 and lacz2)


Growth rate = fitness:
In(pn/qn) = ln(po/q0)+n x Inw

- Growth rate of two strains in chemostat is a measure of relative fitness: This formula is used to work out the relative fitness of two strains
- po = initial proportion of strain A
- qo = initial proportion of strain B
- n = number of generations
- pn = proportion of Strain A after n generations
qn = proportion of strain B after n generations
- w = w/1 = relative fitness of strain A (relative to strain B)


Five pgi alleles were compared pairwise:

- Activity of PGI allozymes differed when measured under chemostat conditions
- pgi converts glucose-6-p to fructose-6-p and reverse


Observations from pgi experiment:

- within the limits of detection but no selection
- one allele promotes decreased growth with limiting factors


Comparisons in fitness between different types of sugars:

- Strains with different 'natural' lac operons were competed for lactose or one of three different B-galactosidase substrates which were not normally encountered
- Very small fitness differences when the substrate was lactose
- Very big fitness differences when different B-gal substrates were used.