Drosophila melanogaster, also referred to as “fruit flies,” are one of the most commonly used test subjects in genetic research. In the early 1900s, Thomas Hunt Morgan found a correlation between chromosomal mutations and heredity using fruit flies (Markow, 2015). Throughout the 20th and into the 21st century, research continued using fruit flies bringing forth notable findings such as genetic control during early development of an embryo, and receptor proteins innately activate the immune system in response to bacteria exposure (Markow, 2015). Fruit flies have become increasingly important in human disease research and, as of late, they are used mainly in the study of heart disease, obesity, and mental and neurological related illnesses (Markow, 2015).
Drosophila melanogaster is believed to have originated from an Afrotropical area migrating overtime to the rest of the world, evolving into different subspecies, with different genetic traits based on habitat and climate (David, 2007). This genetic variation based on region is one of the conditions that qualifies them as useful test subjects. Despite their presence for thousands of years, the Drosophila melanogaster were not used until the early 1900s in research by Charles Woodworth, and then later by Thomas Hunt Morgan (Markow, 2015). Drosophila melanogaster became prevalent when Charles Woodworth realized they were inexpensive and efficient in breeding due to their short life cycle (Markow, 2015; Hales et al., 2015). Thus, using fruit flies for experiments provides a quicker method for studying inheritance since many generations can be bred and observed within a short amount of time.
Drosophila melanogaster found in the wild are drastically different than those in the laboratory. Some major differences include temperature, body size, disease exposure, predation, and food availability (Markow, 2015). The lab is a controlled environment kept at a constant temperature where food is provided, and potential dangers are eliminated. These independent variables within the laboratory eliminate any variation in their appearance and results in a smaller body size in comparison to those found in the wild (Markow, 2015). Wild fruit flies eat yeast, rotting fruit and bacteria, while the media for lab fruit flies contains yeast, along with cornmeal and various carbohydrates and preservatives correlating with their natural Afrotropical habitat (Hales et al., 2015).
Regardless of the habitat they live in, Drosophila melanogaster develop in the same manner through metamorphosis: starting as an egg, become larvae, then pupa, and finally an adult (Markow, 2019). After fertilization, it takes 24 hours for the embryo to completely develop then the larval stages begin, each lasting roughly 1-2 days (Hales et al., 2015). Once the larval development is complete, the flies move into the pupal phase for about 5 days initiating the beginning of the breakdown of the larval tissues and the development of the adult structure (Hales et al., 2015). After the adults emerge from the pupal case, they become sexually mature within 12 hours and the life cycle repeats (Hales et al., 2015). In view of their quick life cycle, fruit flies are ideal in studying genetic phenomena such as gene inheritance, mutations, and sex chromosome relationships.
Drosophila can be used to study a wide variety of genetics from the most basic patterns of inheritance, such as dominance and recessiveness, to more complex patterns such as mapping distances between genes on a chromosome. Regardless of the complexity of the genetics being studied, all expressions of genes, including those found in Drosophila, are governed by what scientists term the Central Dogma of Molecular Biology. This states that genes, in the form of DNA, are translated into mRNA then transcribed into proteins which perform specific functions throughout the organism (Marshall 2018). The certain patterns of DNA that code for these proteins are called genes which are passed from parents to offspring. Genes often come in multiple forms called alleles and they are inherited by means of the parental gametes passing on one copy of genetic material from each parent to its offspring. The way in which specific genes are passed on varies. In Mendelian genetics, Gregor Mendel proposed that genes come in two forms: a dominant and a recessive allele which are assorted independently when the offspring receives a copy from each of the parental gametes (Bhatia lecture 3 2019). Depending on the combination an organism receives will determine the function of those genes. For example, if a dominant allele is present at all, such as the wild type red eye color in Drosophila, that dominant trait will be expressed in the offspring. If there are two recessive alleles, then the recessive trait will be expressed. An example of a recessive trait expressed in Drosophila, is a brown eye color. This variation is due to the fact that dominant alleles essentially over-powers the recessive allele. To know if a trait is dominant or recessive, a purebred dominant organism for a trait can be crossed with a purebred recessive organism for a trait and the trait that is expressed in the offspring will be the dominant since each offspring will have a copy of the dominant and a copy of the recessive trait (Bhatia lecture 4 2019).
But not all forms of inheritance are as simple as Mendel’s proposal. Other forms include linked genes, X-linked traits, and Y-linked. Linked genes refer to genes that do not assort independently, but rather, together because they are located so close to each other on the same chromosome. When crossing over happens, they move together and stay together (Bhatia lecture 4 2019). In order to determine if a gene is linked or unlinked, test crosses with purebreds can be done. If the ratio of traits does not follow the Mendeliean ratio, then the genes are linked. If they do follow the Mendeliean ratio, then the genes are unlinked (Marshall 2018). X-linked genes are genes that are located on the X chromosome as opposed to being autosomal, which are genes located on a non-sex chromosome. Y-linked genes are genes located on the Y chromosome (Bhatia lecture 4 2019).
Like humans, Drosophila’s sex chromosomes are XX for females and XY for males. The Y chromosome lacks most genes present on the X chromosome resulting in patterns of inheritance which is different from autosomal genes; this is referred to as X-linkage (Klug et al, 2019). Thomas H. Morgan first documented his findings of X-linked inheritance by observing the mutant white eye color in Drosophila with the wild type red eye color. He conducted reciprocal crosses between the two different eye colored flies and concluded the mutant white eye color was X-linked based off the results shown in the second filial generation of both crosses (Klug et al, 2019).
In our experiment, different colonies of Drosophila melanogaster were used for controlled matings and the different generations were observed to identify if the mutant genes were dominant or recessive, autosomal or X-linked, and for determining gene combination linkage. In the experiment, wild female fruit flies were crossed with wingless, crimson eyed mutant males to determine if the cross would result in X-linked or assorted independent offspring. This would result in a 9:3:3:1 phenotypic ratio where brown eyes and wings are dominant over red eyes and wingless. For the experiment, it was hypothesized that the genes being studied were unlinked, and thus, would result in a Mendelian phenotypic ratio of 9:3:3:1.
References
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Klug, W.S., Cummings, M.R., Spencer, C.A., Palladino, M.A., & Killian, D. (2019). Concepts of genetics, 12th Edition. Upper Saddle River, N.J: Prentice Hall.
Markow, T.A. (2015). The secret lives of Drosophila flies. eLIFE, Vol. 4, e06793. Retrieved on September 19, 2019 from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4454838/.