



Small, cute looking fruit flies known as Drosophila, fly around the leftover foods, fruits, vegetables, and juicy substances. Never thought about these flies helping the scientists tremendously in the genetics research. Drosophila, one of the most important model organisms, is used worldwide in various experiments. Drosophila is one such genus of the flies that contains more than 900 described species. Hence, it is the most extensively studied organism in the fields of cytology and genetics. Most of the genetic information available today involves data extracted from Drosophila melanogaster studies. Thousands of mutants are available with this insect. Many of the mutants affect the developmental processes. These genes provide a rich array of information through developmental genetics. The mutants helped us in understanding sex determination and many other traits.





Image 1: Drosophila (fruit fly)





The specialty of using fruit flies as model organisms lies in their ability to reproduce on any media. Obtaining Drosophila cultures is not a tough job. It grows on almost all the foodstuffs. Just take a container having holes for the air to pass inside. Put a piece of banana or any juicy fruit inside it and keep it in your window. After some time, these tiny fruit flies get attracted to the fruit inside the container. Another important specialty of Drosophila lies in the presence of a polytene chromosome, widely used in gene studies. The eukaryotic development completely depends on the precise regulation of a group of genes. The genetic regulation of the development in Drosophila progresses significantly. An important point to be noted regarding the developmental genes in Drosophila is that these flies have counterparts in all the organisms, including humans. The body structure of Drosophila involves many segments. Homeotic genes determine the developmental identity of these segments. Studying the genes accelerates the developmental analysis. Thus, in layman terms, the developmental genetics involves the study of mutations deciding the developmental processes gaining the information of the way normal genes get control over the growth, form, behavior, and reproduction. A well-differentiated organism arises from a single cell. Zygote forms due to the fusion of sperm and egg. The cell exhibits totipotency or a potential to develop into any cell type. The genetic programming determines the fate of the cell. This process is known as cell determination. It, later on, involves differentiation, meaning, the determined cells undergo developmental programs and synthesize specific types of cells. Then morphogenesis comes into the picture.





Developmental stages in Drosophila:

A well-ordered sequence of developmentally programmed events follows a strict genetic control. Particular molecular gradients get established before fertilization. There is a region known as polar cytoplasm at the posterior end. The fertilized egg consists of two parental nuclei that fuse to produce a diploid zygote nucleus. It undergoes nine divisions in the cytoplasm and gives rise to multi-nucleate syncytium. The syncytial blastoderm nuclei further migrate and divide, thereby producing a layer at the egg periphery. The nuclei then divide four times resulting in instructions making other important cellular structures such as membranes. After these events, the initial steps in the embryonic development include axes formed in a tight genetic control.

The segment pattern of the embryo involves an adult segment organization. The development of the body structure includes two main processes. The four axes studied in a Drosophila egg include anterior, posterior, dorsal and ventral axes. Along these axes, a molecular gradient occurs. Expression of genes depends on the position of a nucleus involving the intersecting gradient specific region in the adult body. Study of genes involves a cellular blastoderm. Hence, they are known as Para segments. After ten hours or so, these Para segments further look clear and appear like segments. The maternal genes controlling the development of anterior, posterior and dorsoventral axes determine them. Before fertilization, the genes get expressed outside the egg of the mother fly. A wide number of genes get expressed such as Bicoid, Swallo, Oskar, Torso, and Cauda. The genes express themselves by coding for transcription factors.

The products of these genes, when carried to the egg, establish gradients consisting of RNA and proteins distributed differentially.





The following table describes the genes and their functions:





Name of the gene The function of the gene Bicoid Involved in axial patterning Swallow Plays an important role in bicoid message localization Oskar The gene defines a posterior pole (early embryogenesis) Torso Determining the anterior and posterior terminal structures. Caudal Segmentation of the embryo Snake (snk) Extracellular signaling component Easter Required for the development of all lateral and ventral pattern elements.

Table 1: Developmental genes and their functions.





Image 2: Development of segments in Drosophila





1. Formation of embryo axes:

As discussed, there are anteroposterior and dorsoventral axes. The anteroposterior axis requires regulation of Bicoid, Nanos, Acron, Telson, and Torso. Bicoid gene determines the anterior end. It encodes for a mRNA translating to a protein consisting of a helix-turn-helix protein. Mutation in this gene leads to the formation of an embryo lacking a head and the thorax. The Nanos class of genes determines the posterior end of the abdominal segments of the embryo. Further classification of the anterior and posterior structures involves most anterior and most posterior structures respectively. A separate set of genes regulate them.

The dorsal gene involves a product forming ventral to the dorsal gradient in the syncytial blastoderm. The bicoid gene, a key maternal effect gene involves a product forming anteroposterior axes. The bicoid gene encodes a protein known as a morphogen which controls the development. The bicoid gene affects the expression of the caudal gene. After the translation of the bicoid mRNA, the formation of the caudal protein gradient occurs. The protein is lowest at the anterior end and highest at the posterior end. The behavior of the caudal protein antagonizes the behavior of the bicoid protein. The segmentation phase involves the caudal protein. One more maternal effect gene known as Nanos gene helps to form posterior structures. The Drosophila with null mutations in the Nanos genes exhibits phenotype with no abdomen. A hunchback gene expresses a hunchback protein. This protein correctly carries out the developmental processes. It decreases from the anterior to the posterior. The Nanos gene product increases from anterior to posterior. It is highest at the posterior end.

Antero-posterior structure Anterior end Posterior end Caudal protein: It is lowest at the anterior end and highest at the posterior end. Hunchback protein: It is highest at the anterior end and lowest at the posterior end. Morphogen protein It is highest at the anterior end and lowest at the posterior end. Nanos protein: It is lowest at the anterior end and highest at the posterior end.

Table 2: Proteins playing a crucial role in the development of the anterior and the posterior end.





2. Segmentation genes:

These genes determine the embryonic and adult segments. Mutations in these genes alter the number. Three main genes include gap genes, pair-rule genes, and segment polarity genes. The homeotic genes specify the identity of the genes. Gap genes include Kruppel, Hunchback, Giant, and Tailless. All these genes encode transcription factors. Pair-rule genes include Hairy, Even-skipped, Runt and Fushi Tarazu genes encoding transcription factors. Engrailed gene is an example of the segment polarity gene. The gap genes divide the embryo into broad regions. The pair-rule gene divides the embryo into seven segments along the craniocaudal axis. The segment polarity genes divide the embryo into fourteen segments.





3. Determination of the regional characteristics:

The regional characterization of the individual segments in the embryo involves an important group of genes known as homeotic genes. These genes determine whether the embryonic segment bears antennae, legs or wings. Chromosome 3 consists of a total of eight homeotic genes arranged in Antennapedia Bithorax groups. They encode 60 amino acids. Other genes involved in the regional characterization include Antennapedia complex, Bithorax complex and Ultrabithorax complex. Hence, the study of the developmental stages involves a wide variety of mutants. The genes control the development in a temporal regulatory cascade. Hence, studying the developmental genes in Drosophila helps in comparison with the higher organisms.





References: [1] Genetics, Daniel Hartl, Maryellen Ruvolo [2] Medical Genetics, G.P. Pal [3] Drosophila embryogenesis, Wikipedia [4] Drosophila and the Molecular Genetics of Pattern Formation: Genesis of the Body Plan, NCBI [5] Homology, Genes, and Evolutionary Innovation, Günter P. Wagner

[6] IGenetics, Peter Russell, second edition

