The Profound Impact of Drosophila Melanogaster, the Fruit Fly, and Danio Rerio, the Zebrafish, on Understanding the Human Genome and its Role in Disease.
Drosophila melanogaster
Animal models have been essential to medical research for millennia. Ethical concerns with their use has led to a decrease in use of large animals (e.g. dogs, cats). Perhaps, the smallest of research “animal models” is Drosophila melanogaster, the lowly fruit fly, 3 millimeters in length, which has contributed, dramatically and profoundly, to our understanding of genetics and gene mutations for approximately 125 years. In the wild, they are found in homes, restaurants, grocery stores, warehouses and food processing plants, anywhere in which food is accessible. What makes them useful for genetic studies? Fruit fly populations are inexhaustible, simple to breed and inexpensive to maintain. The female lays approximately 30-50 eggs per day and, perhaps, 300-600 over her lifetime. Their reproductive cycle is approximately 10 days, so that multiple generations can be studied over a few months. They are tiny and thousands can be kept in a laboratory. Whereas a genetic experiment in a mouse may take several years, one in the fruit fly may take only a few months. The fruit fly genetic make up has been unraveled and there is a 60% homology between the fly and the human genome. Approximately 75% of the genes identified as causing human disease are represented in the fruit fly genome. Remarkably, although a few millimeters in length, it has 14,000 protein-coding genes (of which 8000 have human analogues). Humans have 19,000 protein-coding genes.
Thomas Hunt Morgan, a scientist of unusual insight, started using fruit flies in 1904 as they were cheap and took up little space, conducive to his very limited resources. After 24 years at Columbia University, he was recruited to the California Institute of Technology where he remained until 1945. In 1920, the former Throop College in Pasadena, California was renamed and repurposed with the goal of making it one of the outstanding institutions of higher learning committed to science. The recruitment of outstanding faculty began. Morgan was one of those persons of promise. He was the first to describe the importance of genes arrayed on chromosomes and for confirming that the gene is responsible for determining identifiable, hereditary characteristics. He explained gene linkage and the distance between genes was measured in a unit, which became designated the centimorgan in recognition of his discovery. The centimorgan represents a one percent chance that a gene will be separated (unlinked) from its neighboring gene during crossing-over, the process of swapping genetic material during the formation of an egg or sperm cell. He, notably, described genes linked to the sex chromosomes.
The female of a mammalian species is determined by expressing two X chromosomes, the XX genotype. The male is determined by expressing the XY sex chromosomes. All chromosomes except the X and Y are numbered and in humans are identified as from 1 to 22 pairs or 44 autosomes. Thus, with the two sex chromosomes, humans have 46 paired chromosomes, although in males the X and Y are not really a pair but are usually arrayed together in a gene display called a karyotype. (Figure 1) The chromosomes are arrayed by (i) length with chromosome 1 being the longest and 22 the shortest and (ii) the position of their centromere a constriction that separates the long arm (designated “q”) from the short arm (designated “p”) of the chromosome. Actually, and little known, chromosome 21 is the shortest (slightly shorter than 22) but has been kept where it is in the karyotype because chromosome 22 was designated to be the site of a mutation leading to a blood cell neoplasm before the chromosomes were carefully studied in humans. The relationship between that disease and chromosome 22 was deeply embedded in the scientific literature. The relationship was left undisturbed by agreement among geneticists. The X chromosome is large and the Y chromosome is very small. It evolved from an autosome over millennia by losing genetic material. The Y chromosome carries the SRY gene that determines “maleness” in the embryo. The small Y chromosome has only 69 genes, including those that govern sperm formation.
The X chromosome is the location for the genes that encode for two key clotting factors and when mutated result in hemophilia A or B in males. Since males have one Y and one X chromosome, a mutation of a gene on the maternal X chromosome is not compensated for by the same
Fig. 1. Normal male Karyotype. During the mitotic phase of the cell mitotic cycle chromosomes are separated and spread out and can be individually identified. For analysis, they are cut out, digitally, and the 22 autosomes are arrayed in pairs from largest to smallest with the two sex chromosomes following: either XX in females or XY in males. This array is called a karyotype and is a key technique of analyzing the chromosomes of cells in persons with malignancies or with abnormalities that are related to chromosome disorders, such as Down syndrome.
gene on the paternal X chromosome, as is the case in women whose chromosome composition is XX. There are two major types of hemophilia. Hemophilia A occurs when a mutation on the X chromosome that encodes for the coagulation protein Factor VIII is present and hemophilia B occurs when a mutation on the X chromosome that encodes for coagulation Factor IX is present. Hemophilia is classically confined to males who inherit a mutant Factor VIII or Factor IX gene on the X chromosome from their mother and the Y chromosome from their father. Since daughters get an X chromosome from mother and father, the normal paternal X results in the production of sufficient coagulation Factor VIII or IX to provide normal coagulation of the blood, even with severe injury. This inheritance of a disease as a result of a male offspring receiving a mutant gene on mother’s X chromosome is referred to as sex-linked inheritance and there are many examples of such. There are 13 clotting factors, each designated by a Roman numeral, Factor I through XIII. Although deficiencies in one or another of those factors may cause bleeding, the term “hemophilia” is restricted to the severe deficiency of Factor VIII or Factor IX. Approximately, two-thirds of the cases of hemophilia result from a male child inheriting an X chromosome with the gene mutation from mother who is the unaffected carrier of the mutation and the Y chromosome from father. One–third of cases result from a mutation, involving the gene on the single maternal X chromosome that occurs during development of the male embryo for as yet unexplained reasons.
Morgan’s work on gene linkage, sex-linked traits, and gene mapping resulted in his being awarded the Nobel Prize in Physiology and Medicine in 1933, the first American to receive that specific Nobel Prize. He was passed over for several years because the Nobel Foundation was unclear of the importance of genetics and what, if any, relationship it had to Physiology or Medicine! I anticipate, they also wondered what the genes of a fruit fly had to do with mice or men. Six Nobel Prizes in Physiology or Medicine have been awarded to 10 scientists whose groundbreaking biological findings resulted from experiments using fruit flies, starting with Morgan in 1933.
The Bloomington Drosophila Stock Center at Indiana University is the only center of its kind in the United States and the largest in the world. It houses 77,000 different strains of fruit flies and in 2019 shipped over 200,000 vials to laboratories in 49 States and 54 countries. The center has a fly-food chef. Imagine feeding millions of fruit flies and, although their diet is just a mash of cornmeal, the fly-food chef in Bloomington indicates they can be fussy about the preparation. The flies of each of the 77,000 strains must be “flipped”, transferred from an old vial to a clean one, frequently. The flies mate, lay eggs which hatch, pupate and reproduce continuing the cycle. A staff of over 60 fly-keepers at the Bloomington Center manage the vials. They are constantly on the move.
The fruit fly is one exotic, but critical, model for genetic and biomedical research; another is the zebrafish. The zebrafish is a minnow and is popular in home aquariums. They are very important in medical research because (i) they lay many eggs resulting in hundreds of offspring, (ii) they share the same organs as humans and those organs share features of the human organ, e.g. eyes, blood, muscles, kidney, heart and (iii) the zebrafish and its embryos are transparent and develop outside a uterus so it is possible to trace the details of development from fertilization through development of organs visually. The zebrafish reaches adulthood in approximately 3 months making its development and organogenesis relatively easy to study. Approximately 70% of human genes and a higher fraction of disease-causing human genes have a zebrafish analogue. They are fed brine shrimp and can be kept easily in fresh water for years. Although the human genome project resulted in the molecular description of the 19,000 protein-coding genes, it was models like the fruit fly and zebrafish that allowed one to determine the function of such genes and the impact of a mutation of a specific gene on organ function.
It is remarkable that the poet William Blake writing in the 1794’s posed this query in his poem The Fly:
Am not I
A fly like thee?
Or art not thou
A man like me?
Could he inadvertently have sensed that there was some fundamental relationship between species of profoundly different composition as genetics has informed us over a century later?
Written March 2021