Sex Determination in Drosophila: Definition, Meaning, examples, Chromosomes, Genes, Hormones
Learn how sex determination in Drosophila melanogaster is controlled by the X chromosome to autosome ratio, unlike mammals where the Y chromosome dominates. Understand the role of key genes like Sex-lethal (Sxl), transformer (tra), and doublesex (dsx) in controlling sexual differentiation. Includes tables, genetic pathways, and NEET MCQs for Class 12 biology.
- Introduction – Sex Determination Mechanisms in Animals
- Drosophila melanogaster as a Model Organism
- Mechanism of Sex Determination in Drosophila
- Special Conditions – Metafemale, Metamale, Intersex
- Life Cycle and Genetic Simplicity of Drosophila
- MCQs on Sex Determination In Drosophila
- FAQs on Sex Determination In Drosophila
- Recommended Video on “Sex Determination In Drosophila”
Mammals and fruit flies have XX females and XY males, but the sex-determining mechanisms are very different. In mammals, the Y chromosome has a major role in determining the male sex. Therefore, the XO mammals are females and XY are males. The sex determination in Drosophila is achieved by a balance between the female determinants on the X chromosome and male determinants on the autosomes. The flies have either one or two X chromosomes and two sets of autosomes. For being a male, the fly should have one X chromosome (1X:2A). For being a female, two X chromosomes should be present. Sex determination in Drosophila helps to understand the Principles of Inheritance and Variation in the environment.
Introduction – Sex Determination Mechanisms in Animals
Sex determination determines the fate of an organism to be either male or female. This is very important in reproduction, survival, and hence the continuation of a particular species. Sex determination across animal kingdom is controlled by various mechanisms such as chromosomal, environmental, and genetic. These mechanisms help in the study of developmental biology, genetics, and evolution.
Drosophila melanogaster, commonly known as the fruit fly, is a tiny insect that has been the cornerstone in the study of genetics during the past century. It is a simple model organism, with a very short life cycle, making it perfect for studying such complicated biological processes as the determination of sex. Insight from Drosophila research unlocked the intricacies of genetics and helped form a critical subject in basic and applied sciences.
Drosophila melanogaster as a Model Organism
Drosophila contains only four pairs of chromosomes, three pairs of autosomes and one pair of sex chromosomes, X and Y. The females are normally XX, while the males are XY.
Aside from being genetically simple, the genome of Drosophila is also fully sequenced. This has helped a lot in identifying key genes that govern the major biological processes, including those involved in sex determination. Such genes identified are experimentally induced to study their functions and have come up with vital findings on genetics and developmental biology.
Genome of Drosophila
The genome of D. melanogaster is compact but very informative, containing approximately 165 million base pairs and probably about 15,000 genes. Although smaller in size compared to the human genome, the fruit fly has many genes and is homologous to humans. This makes this organism very useful for studying gene regulation and expression.This genomic information has made an enormous contribution towards understanding and researching genetic diseases, developmental processes, and evolutionary biology.
Human- Like Qualities
One of the most exciting research areas in Drosophila is the genetic and physiological similarities between the fly and humans. For almost every gene linked to a human disease, a matching or similar gene can be found in Drosophila. Scientists can, therefore, model and study many diseases using the fly. Examples include genes related to neurological disorders, cancer, and metabolic diseases that are expressed in Drosophila, giving information about their function that could yield possible therapeutic targets for treatment.
Significantly, the basic cellular and developmental mechanisms in Drosophila are very similar to those of humans. Cellular mechanisms, such as signal transduction pathways, cell division, and differentiation, are conserved, making Drosophila very useful for studying basic biological phenomena.
Mechanism of Sex Determination in Drosophila
The mechanism of sex determination in Drosophila depends on the ratio of X chromosomes to sets of autosomes, known as the X:A ratio. A fly with a ratio of 1.0 i.e., having two X chromosomes and two sets of autosomes, develops as a female. A fly with a ratio of 0.5 i.e., having one X and two sets of autosomes, develops as a male. It is this ratio that is crucial for the turning on of the master regulatory gene, Sex-lethal (Sxl), which controls downstream processes leading to sexual differentiation.
X:A Ratio Table with Examples
The table below summarises the X ratios and resulting sexes in Drosophila:
X Chromosomes | Autosomes Sets | Ratio of X to Autosomes | Sex |
2 | 2 | 1.0 | Female |
1 | 2 | 0.5 | Male |
3 | 2 | 1.5 | Metafemale (sterile) |
2 | 3 | 0.67 | Intersex |
1 | 3 | 0.33 | Metamale (sterile) |
Role Of Sex-lethal (Sxl) Gene
The Sex-lethal gene is the determining factor for sex. The two X chromosomes in females activate Sxl. It then cleaves its pre-mRNA in such a way to give rise to female-specific splicing patterns. Subsequently, functional Sxl protein is generated to regulate female development. In males, Sxl is not turned on because the X-linked signal is too weak. Hence, male-specific splicing and development ensue.
Role of Transformer (Tra) Gene
The transformer (tra) gene further modulates sexual differentiation. The tra gene under the control of Sxl produces female-specific splice variants responsible for female promoters. In the absence of active tra genes in males, the male-specific splicing pattern prevails. Morphological and physiological differences of the sexes are finally set by the dsx gene, which guides the development of either male or female morphology.
Role of Doublesex (dsx) Gene
Another important gene, doublesex (dsx) gene, which is downstream of Sxl in the hierarchy. further progresses sexual differentiation. Morphological and physiological differences of the sexes are finally set by the dsx gene, which guides the development of either male or female morphology.
Special Conditions – Metafemale, Metamale, Intersex
In Drosophila, normal male and female development occurs when the X : A ratio is 0.5 or 1.0, respectively. However, when this ratio deviates from the normal values, special conditions arise.
Metafemale: Occurs when the X : A ratio is greater than 1.0 (e.g., 3 X chromosomes with 2 sets of autosomes = 1.5). Individuals show female traits but are sterile, and often have poor viability due to gene dosage imbalance.
Metamale: Occurs when the X : A ratio is less than 0.5 (e.g., XO with 3 sets of autosomes = 0.33). These flies show underdeveloped male characteristics, are sterile.
Intersex: Occurs when the X : A ratio is between 0.5 and 1.0 (e.g., 2 X chromosomes with 3 sets of autosomes = 0.67). Individuals display a mix of male and female traits in their body structure.
Life Cycle and Genetic Simplicity of Drosophila
The life cycle of Drosophila is relatively short, and under optimal conditions, it could be about 10 days from egg to adult. Several successive stages are involved in the cycle including egg, larva, pupa, and adult. It just goes to prove why Drosophila can turn out to be one of the best model organisms for conducting genetic experiments, due to its fast development and ease of breeding in high numbers. This will help in observing multiple generations within a short time frame.
MCQs on Sex Determination In Drosophila
Q1. Which chromosome pair determines sex in Drosophila?
1st pair
2nd pair
3rd pair
4th pair
Correct answer: 2) 2nd pair
Explanation:
The second pair of chromosomes in Drosophila is the pair that controls sex. The proportion of X chromosomes to the total number of sets of autosomes (A) in Drosophila is used to identify gender. Drosophila has X and Y sex chromosomes, with females having XX and males having XY. The quantity of X chromosomes in the 2nd chromosome pair determines the ratio of sets of autosomes to X chromosomes. For instance, a female fly is produced when the ratio is 1:1 (i.e., 1 X chromosome to 2 sets of autosomes), whereas a male fly is produced when the ratio is 1:2 (i.e., 1 X chromosome to 4 sets of autosomes). Therefore, in Drosophila, the fly's sex is determined by the 2nd pair of chromosomes.
Hence, the correct answer is option 2) 2nd pair
Q2. What happens if the ratio of X chromosomes to autosomes in Drosophila is 1:1?
The individual develops as a male
The individual develops as a female
The individual develops as a hermaphrodite
The individual fails to develop
Correct answer: 2) The individual develops as a female
Explanation:
The sex of a person is determined by the individual's X chromosome to autosome ratio in Drosophila. One X chromosome and two sets of autosomes, at a ratio of 1:1, produce a female fly. This is because, in Drosophila, the development of a female requires the presence of two X chromosomes, whereas the formation of a male requires the presence of one X chromosome and one Y chromosome.
Hence, the correct answer is Option 2) The individual develops as a female
Q3. In Drosophila, what is the role of the transformer (Tra) gene?
Promotes male development
Promotes female development
Determines the number of X chromosomes
Determines the number of autosomes
Correct answer: 2) Promotes female development
Explanation:
The Transformer (Tra) gene in Drosophila is responsible for promoting female development. It is involved in the regulation of sexual differentiation by controlling the splicing of the doublesex (dsx) pre-mRNA transcript. In females, the Tra protein is present, and it interacts with other proteins to promote the splicing of dsx into a female-specific isoform. This isoform then directs the development of female-specific structures and traits.
In contrast, in males, the absence of functional Tra protein results in the alternative splicing of dsx into a male-specific isoform. This male-specific isoform of dsx directs the development of male-specific structures and traits. Therefore, the presence or absence of functional Tra protein is critical in determining the sexual development and differentiation of Drosophila flies.
Hence, the correct answer is Option 2) Promotes female development
Also read:
FAQs on Sex Determination In Drosophila
What is the X : A ratio in Drosophila sex determination?
In Drosophila melanogaster, sex is determined by the ratio of X chromosomes to sets of autosomes (X : A ratio).
X : A = 1.0 → Female (e.g., XX with 2 sets of autosomes).
X : A = 0.5 → Male (e.g., XY with 2 sets of autosomes).
X : A = greater than 1 → Metafemale (e.g., 3 X chromosomes with 2 sets of autosomes = 1.5)
X : A = less than 0.5 → Metamale (e.g., XO with 3 sets of autosomes = 0.33)
X : A = between 0.5 & 1 → Intersex (e.g., 2 X chromosomes with 3 sets of autosomes = 0.67)
What does the Sxl gene do?
The Sex-lethal (Sxl) gene is the regulator of sex determination in Drosophila. In females (X : A = 1.0), Sxl is activated, producing a functional Sxl protein that activates downstream genes such as transformer (tra), leading to female development. In males (X : A = 0.5), Sxl remains inactive, and male-specific development occurs.
What is the role of the Tra gene in Drosophila?
The transformer (tra) gene works downstream of Sxl. In females, the active Sxl protein promotes the correct splicing of tra mRNA, producing functional Tra protein. This Tra protein helps in female sexual characteristics. In males, inactive Sxl leads to non-functional Tra protein and hence development of male characters.
What is a metafemale in Drosophila?
A metafemale is an abnormal, sterile female in Drosophila. It results from having X : A ratio greater than 1.0. For example, 3 X chromosomes with 2 sets of autosomes, ratio = 1.5. Metafemales have female traits but are usually sterile due to abnormal dosage of sex-determining genes.
Recommended Video on “Sex Determination In Drosophila”
Frequently Asked Questions (FAQs)
Males are smaller, have darker pigmentation on their abdomen, and possess sex combs on their forelegs, while females are larger and lack these features.
Mutations in key genes like Sxl, tra, or dsx can disrupt the normal process of sex determination, leading to intersex or sterile individuals.
While the sex determination system in Drosophila is well-studied, it's not universal among insects. Some insects use different primary signals, such as parental imprinting in wasps or maternal effects in some beetles. However, many insects share similar downstream components of the sex determination pathway, like doublesex.
The fruitless (fru) gene is primarily involved in determining male-specific behaviors, particularly courtship behaviors. It undergoes sex-specific splicing to produce male-specific FRU proteins, which are essential for the development of neural circuits controlling male courtship behavior.
Dosage compensation in Drosophila ensures that males (XY) and females (XX) have equal expression of X-linked genes. This is achieved by doubling the expression of X-linked genes in males. The process is linked to sex determination, as the Sxl gene, which is key in sex determination, also regulates dosage compensation.
Unlike some reptiles, temperature does not directly determine sex in Drosophila. However, extreme temperatures can affect the expression of sex determination genes, potentially leading to intersex phenotypes or skewed sex ratios in a population.
Unlike in mammals, the Y chromosome in Drosophila is not the primary determinant of maleness. Its main role is in male fertility, carrying genes necessary for sperm production. The presence or absence of the Y chromosome does not directly influence the sex determination pathway.
The genic balance theory in Drosophila sex determination suggests that the balance between female-promoting genes on the X chromosome and male-promoting genes on the autosomes determines sex. This is reflected in the importance of the X:A ratio in activating or repressing key sex determination genes like Sxl.
Cell-autonomous sex determination means that each cell in a Drosophila embryo determines its sex independently based on its own X:A ratio. This is in contrast to mammals, where sex hormones play a major role in sexual differentiation. Cell-autonomy allows for the development of gynandromorphs and facilitates the study of sex-specific traits at the cellular level.
Sex determination in Drosophila is the process by which the fruit fly's sex is established during development. It involves a complex interplay of chromosomes, genes, and gene products that ultimately lead to the development of male or female characteristics.
In Drosophila, females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). This is similar to humans, but the key difference is that in Drosophila, the presence of the Y chromosome does not determine maleness. Instead, the ratio of X chromosomes to autosomes (non-sex chromosomes) is crucial for sex determination.
The transformer (tra) gene acts as an intermediate in the sex determination pathway. In females, the Sxl protein promotes the production of functional TRA protein. TRA, along with TRA-2, then regulates the splicing of the doublesex (dsx) gene to produce the female-specific DSX-F protein. In males, where Sxl is inactive, no functional TRA is produced, leading to the male-specific splicing of dsx.
Alternative splicing is crucial in Drosophila sex determination as it allows the production of sex-specific proteins from the same genes. For example, the doublesex (dsx) gene is alternatively spliced to produce DSX-F in females and DSX-M in males, which then direct sex-specific development.
Unlike in mammals, hormones play a minimal role in primary sex determination in Drosophila. The sex of each cell is determined autonomously based on its chromosomal composition. However, hormones do play a role in some secondary sexual characteristics and behaviors.
Intersex in Drosophila refers to flies that exhibit both male and female characteristics. This can occur due to mutations in sex determination genes or abnormal X:A ratios. For example, mutations in the doublesex (dsx) gene can lead to intersex phenotypes.
Gynandromorphs are Drosophila individuals that have both male and female tissues. They typically form due to the loss of an X chromosome during early embryonic cell division, resulting in some cells being XX (female) and others XO (male). This leads to a mosaic of male and female characteristics in the adult fly.
Mutations in the Sex-lethal (Sxl) gene can have severe consequences. In females, loss-of-function mutations can lead to lethality due to failure of dosage compensation. In males, gain-of-function mutations causing Sxl activation can lead to female development or lethality. These effects highlight the central role of Sxl in both sex determination and dosage compensation.
Sex combs are male-specific structures on the forelegs of Drosophila. Their development is controlled by the sex determination pathway, particularly through the action of the male-specific DSX-M protein produced by the doublesex (dsx) gene. DSX-M promotes the development of sex combs, while the female-specific DSX-F protein represses their formation in females.
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