The history of fruit flies is considered a tradition. Research of these flies initially entered labs 100 years ago. Thomas Hunt Morgan, who lived from 1866 to 1945, was the founder of drosophila genetics. Thomas preformed his research in Morgan lab at the Columbia University in 1910. Here was when they found a famous mutation, know as the white-eyed fly. Quite an accomplishment was this discovery, but the end of the 1980’s there were 3,000+-recorded mutations. Now drosophila is very popular; so popular, it would be almost impossible to list the number of things that are being done with it. However, fruit-fly research relates to human genetics as well. It conveys that genes were related to proteins, therefore referring to the study the rules of genetic inheritance. Currently, it is used mostly in biology, focusing on how a complex organism matures from a fairly simple fertilized egg (embryonic development).
Aside from the fact that drosophila can be found throughout numerous biology labs, they don’t originate in these labs. Drosophila is found all around the world, more species in the tropical regions; whether it is deserts, tropical rainforests, cities, swamps, or alpine zones. More often than not they are found living in habitats that have fermenting or rotting vegetation, caused by various yeasts and bacteria. Breeding environments consist of varies between decaying fruits, plant material, mushrooms, slime fluxes, flowers, etc. These breeding environments initiate the life cycle of the next drosophila generation. Just like humans female fruit flies produce eggs and males produce sperm. When the egg and sperm join, the egg becomes fertilized and starts to develop. In fruit flies, sperm is deposited from the male fruit fly into the female fruit fly, enabling the female to store sperm inside of her. The eggs are fertilized when they pass through the oviduct on their way to being placed in a food source. Fruit flies begin their lives as an embryo in an egg. This stage lasts for about one day.
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During this time, the embryo develops into a larva. The first instar larva hatches out of the egg, crawls into a food source, and eats. The larva in each stage eats as much as possible. After a day, the first instar larva transforms and becomes the second instar larva. Again, the larva in this stage eats. After two days in this stage, the larva transforms again to become the third instar larva. After three days of eating in this stage, the larva crawls out of the food source and transforms again. Following this transformation, the larva stops moving and forms a pupa. Drosophila stays in the pupa for about five days. During this time, the metamorphosis, from larva to adult is occurring. Adult characteristics, like wings, legs, and eyes develop. When the adults emerge from the pupa, they are fully matured. They gain reproductive attributes after about 15 hours. Soon they breed then the females lay eggs, and the cycle begins again. The whole life cycle takes about 12-14 days.
While focusing on the topic of reproduction and breeding, sex determination and how it works in drosophila is a sensible topic. We being humans, makes it easier to elucidate what sex determination in drosophila is, because both of us are complex organisms. In an easier perspective, sex determination in humans and drosophila are the same. To make sex determination more comprehensible though, I will explain how it occurs in humans. In humans the diploid number of chromosomes is 46 (23 pairs).
There are 22 pairs of homologous chromosomes called autosomes. Homologous autosomes look alike. The 23rd pair of chromosomes differs in males and females.
These two chromosomes, which determine the sex of an individual, are called sex chromosomes and are indicated by the letter X and Y. If you are female, your 23rd pair of chromosomes are homologous, XX. However, if you are male, your 23rd pair of chromosomes, XY, looks different. Males usually have one X and one Y chromosome and produce two kinds of gametes, X and Y. Females usually have two X chromosomes and produce only X gametes. It is the male gamete that determines the sex of the offspring. After fertilization, a 1:1 ratio of males to females is expected. Because the laws of probability govern fertilization, the ratio usually is not exactly 1:1 in a small population.
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Sex Determination Punnet Square Male (XY)
X Y
Female X XX XY
(XX) X XX XY
The Punnet Square above shows that any mating between humans or drosophila will have a 50-50 chance of having a female or male.
As stated earlier drosophila inherit sex chromosomes in the same way as humans. Traits controlled by genes located on sex chromosomes are called sex-linked traits. The alleles for sex-linked traits are written in superscripts of the X or Y chromosome. Because the X and Y-chromosomes are not homologous, the Y chromosome has no corresponding allele to one on the X chromosome and no superscript is used. Also, any recessive allele on the X chromosome of a male will not be masked by a corresponding dominant allele on the Y chromosome. Thomas Hunt Morgan discovered traits linked to sex chromosomes. Morgan noticed one day that one male fly had white eyes rather than the usual red eyes. He crossed the white-eyed male with a homozygous red-eyed female. All of the F1 offspring had red eyes, indicating that the white-eyed trait is recessive. Then Morgan allowed the F1 flies to mate among them. According to simple Mendelian inheritance, if the trait were recessive, the offspring in the F2 generation would show 3:1 ratio of red eyed to white eyed flies.
However, he noticed that the trait of white eyes appeared only in male flies. Morgan hypothesized that the red eye allele was dominant and the white-eye allele was recessive. He also reasoned that the gene for eye color was located on the X chromosome and was not present on the Y chromosome. In heterozygous females, the dominant allele for red eyes masks the recessive allele for white eyes. In males, however, a single recessive allele is expressed as a white-eyed phenotype. When Morgan crossed a heterozygous red-eyed female with a white-eyed male, half of all the males and half of all the females inherited white eyes. The only explanation of these results is Morgan’s hypothesis: the allele for eye color is carried on the X chromosome and the Y chromosome has no corresponding allele for eye color. The genes that govern sex-linked traits follow the inheritance pattern of the sex chromosome on which they are found. Eye color in fruit flies is an example of an X linked trait. Y linked traits are passed only from male to male offspring because genes for these traits are on the Y chromosome.
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Now there should be a clear understanding of Drosophila genetics and its backgrounds history. With this in mind, we preformed a cross between 2 wild virgin females {(XRXR)(XRXR)} and two white males {(XrY)(XrY)}. This cross was our P1 generation, which would create our F1 generation. Using the Punnet Squares below we were able to predict the phenotypic and genotypic ratios, for the F1 and F2 generation.
F1 Punnet Square White Eyed Male (XrY)
Xr Y
Red Eyed Female XR XRXr XRY
(XRXR) XR XRXr XRY
F1 are all red eyes as you can see in above Punnet Square. Then for the F2 generation we took 2 females (XRXr), which were carriers or the white-eye gene and 2 males (XRY), which were red eyed, from the F1 generation.
F2 Punnet Square Red Eyed Male (XRY)
XR Y
Red Eyed Female (White Eyed Carrier) XR XRXR XRY
(XRXr) Xr XRXr XrY
In this F2 Punnet Square all the females are red eyed and there is a 1:1 ratio of red and white-eyed males. Our lab consisted of the same procedure and fly cross that Morgan preformed.
The final stage of the Drosophila lab is to analyze and present your results. My colieges’ results and me were as follows.
F1 GENERATION
Groups Wild Male Wild Female White Eyed Male White Eyed Female
1 26 28 0 0
2 55 43 0 1
Me, Nate, and Philip 19 14 0 0
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4 40 40 41 5
5 0 81 10 1
6 88 92 5 6
Our F1 vial was created on Jan. 31. The P1 flies were 2 wild virgin females and 2 white-eyed males. The dates when we gathered the F1 data for the number of drosophila in our vial were Feb. 16, Feb. 17, Feb. 21, and Feb. 25.
F2 GENERATION
Group Wild Male Wild Female White Eyed Male White Eyed Female
1 40 46 0 0
2 54 24 1 4
Me, Nate, and Philip 32 48 26 0
4 23 39 35 30
5 63 60 17 11
6 40 82 32 45
Our F2 vial was created on Feb. 17. The P1 were flies chosen from the F1 generation, they were 2 females XRXr which were each carriers or the white-eye gene, and 2 red eyed males which were XRY. The dates when gathered the F2 data for the number of drosophila in our vial was March 6, March 8, March 10, and March 13.
In conclusion since F1, should be all wild type, we will use the F2 results to do a X2 test to see if the sex ratio was 1:1.
Red Homozygote (Male) x Heterozygote
32 Red Eyed (Male)
26 White Eyed (Male)
————————
58 (Male)
Expected = 29
X2 => 95.5, D.F. = 1
White Homozygote (Male) Red Heterozygote (Female)
Red Eyed Male White Eyed Male Red Eyed Female White Eyed Female
181 85 175 100
Total: 266 Total: 275
Expected: 133 Expected: 137.5
X2, .95, D.F. = 1 X2, .95, D.F. = 1
The data above states that we did receive a 1:1 ratio between the male and female Drosophila.
