Witnessing Phenotypic and Molecular Evolution in the Fruit Fly

Abstract

This multi-day exercise is designed for a college Genetics and Evolution laboratory to demonstrate concepts of inheritance and phenotypic and molecular evolution using a live model organism, Drosophila simulans. Students set up an experimental fruit fly population consisting of ten white eyed flies and one red eyed fly. Having red eyes is advantageous compared to having white eyes, allowing students to track the spread of this advantageous trait over several generations. Ultimately, the students perform PCR and gel electrophoresis at two neutral markers, one located in close proximity to the eye-color locus, and one located at the other end of the chromosome. Students observe that most flies have red eyes, and these red-eyed flies have lost variation at the near marker, but maintained variation at the far marker, hence observing a "selective sweep" and the "hitchhiking" of a nearby neutral variant. Students literally observe phenotypic and molecular evolution in their classroom!

Keywords: Drosophila; evolution; genetics; hitchhiking; selective sweep.

Figure 1

When an advantageous mutation occurs, it rises to high frequency, dragging along nearby neutral variants. Eventually every individual in the population has the same mutation and the nearby neutral variants that “hitchhiked” with it. All previous diversity is lost. Conversely, neutral variants located far away from the advantageous mutation do not hitchhike, because recombination can break apart the association, and therefore maintain variation.

Figure 2

The initial population consists of white eyed flies that carry two alleles at the Near marker (green or blue), two alleles at the Far marker (purple or yellow), and white eye allele at the eye color gene. A red-eyed fly is introduced that carries the blue allele at Near, yellow at Far, and red at the eye color gene. The red eye allele is advantageous and sweeps to high frequency in the population. The blue allele at Near is located close to the advantageous eye color, and “hitchhikes” with the red eye color. The Far allele is located 22 million base pairs down at the other end of the chromosome. Recombination breaks down linkage between eye color and Far, so both yellow and purple alleles persist.

Figure 3

The eye color gene is located on the X chromosome (one of the sex determining chromosomes of Drosophila). White eye color is recessive. When a red eyed male mates with white eyed females, their daughters will have red eyes, but their sons will have white eyes. Therefore, there can be no red-eyed males in the first generation, but they will appear in the second and later generations. From Heil et al., in press.

Figure 4. A sample student’s gel

At the top of the gel, the eye color of the sample is noted as W (white-eyed), R (red-eyed), or C (control, no DNA present in sample). On the left of the gel are the results for the Near marker, the marker located in close proximity to the eye-color gene. The original red-eyed male had a high band at the Near marker. One can see that all red-eyed flies have the high band at the Near marker, illustrating the concept of a nearby neutral variant hitchhiking with an advantageous mutation (the red-eye color). On the right side of the gel are the results for the Far marker, which is located on the other end of the chromosome from the eye-color gene. The Far marker clearly has less size differential between the high and low bands, so instruct students to us caution in interpreting results. The original red-eyed male had a low band at the Far marker, but one can see that red-eyed flies in the current sample have both high and low bands, illustrating the concept that recombination breaks up the association between the eye color gene and neutral variants located far away on the chromosome.