Genetics - A. V. Sivolob 2008

Genetics of Multicellular Eukaryotes
Genetics of Individual Development
Genetic Control of Drosophila Development

Insects of the genus Drosophila are characterized by a life cycle with complete metamorphosis: The Development of an adult Organism (imago) from a fertilized egg proceeds through several larval stages separated by molting periods. During the egg stage and the three larval stages, the axes of the future embryo are established, and the main segments of the three body PARTS OF THE adult—the HEAD, Thorax, and abdomen—are laid down and developed. Thus, the embryonic development of Drosophila can be conventionally divided into three stages: 1 - Determination of the anteroposterior and dorsoventral axes of the embryo, 2 - determination of the number of body segments and their orientation (polarity), and 3 - specification of the segments, i.e., the acquisition of individual characteristics unique to that specific segment. All developmental stages are under the strict control of a specific set of genes.

A characteristic feature of Drosophila development is The formation of a syncytium during the initial Cleavage Stages of the zygote: through successive mitotic divisions, cytoplasmic cleavage is incomplete, and the newly formed nuclei share a common Cytoplasm. This facilitates the free movement of cytoplasmic determinants and plays a crucial role in establishing the embryonic axes. Later, the nuclei migrate to the periphery, Cell membranes are formed, and a layer of approximately 6,000 Cells is created on the outer surface of the embryo.

The ESTABLISHMENT OF THE embryonic axes is controlled by a set of genes known as egg-polarity genes. The products of these genes (mRNAs and their corresponding Proteins) form concentration gradients from one pole of the egg to the other, thereby ensuring the directed determination of the embryonic axes—the dorsoventral and anteroposterior axes. The uneven distribution of these cytoplasmic determinants (also referred to as morphogens) is maintained through various mechanisms. First, certain RNA molecules possess localization signals—elements of nucleotide sequence that, by forming a specific Spatial Structure, direct The transport of RNA to the required site in the oocyte cytoplasm and anchor it there. Second, the Translation activity of such RNAs varies in different parts of the oocyte, which also leads to differences in morphogen concentrations. As already noted, the presence of cytoplasmic determinants and their distribution within the egg depend solely on The activity of maternal genes. Consequently, The First stage of Drosophila development relies exclusively on maternal genes.

Among the genes controlling the dorsoventral polarity of the egg, the most critical is dorsal. The protein product of this Gene is a METABOLISM/31.html">Transcription factor that regulates the activity of several other genes. Differences in the nuclear concentrations of the Dorsal protein—peaking in the ventral region of the egg and dropping to a minimum in the dorsal region (with the cytoplasmic concentration changing in the opposite direction)—lead to the specific activation of genes that drive the Development of the corresponding dorsal and ventral structures.

Among the genes controlling the anteroposterior polarity of the egg, two principal ones stand out: bicoid and nanos. bicoid mRNA is synthesized during oocyte maturation and, following Fertilization, is transported to the anterior region of the zygote. The directed transport of bicoid mRNA is ensured by a specific localization sequence element in its 3'-untranslated region, which is recognized by protein factors. The expression of bicoid mRNA in the anterior part of the fertilized egg establishes a concentration gradient of the Bicoid protein that decreases along the anteroposterior axis. Like Dorsal, the Bicoid protein acts as a transcription factor and drives the expression of genes required for the development of the head and thoracic regions. Similarly to bicoid mRNA, nanos mRNA is synthesized during oocyte maturation. During zygotic development, a concentration gradient of the Nanos protein (an RNA-binding protein) is established, opposing that of the Bicoid protein: with a maximum at the posterior end of the embryo and a minimum at the anterior end (Fig. 6.17).

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Fig. 6.17. Localization of Bicoid and Nanos proteins and the main axes of the Drosophila embryo

With the participation of other genes as well, the two described gradients play a decisive role in determining the Formation of the main abdominal structures.

Determination of the number of body segments and their polarity. Once the main embryonic axes are established, segmentation genes are activated. Their activity is observed after egg fertilization; therefore, they are not maternal-effect genes. Approximately 30 segmentation genes are known, comprising three groups:

✵ Gap genes determine the formation of large embryonic structures that roughly correspond to the three body parts of adult insects. Mutations in these genes can lead to the complete absence of all thoracic segments, several abdominal segments, etc. (Fig. 6.18, a).

✵ Pair-rule genes determine the formation of structures from which individual body segments develop. Mutants for these genes may lack either all even-numbered or all odd-numbered segments (Fig. 6.18, b).

✵ Segment-polarity genes are responsible for the Organization and mutual orientation of specific structures within a single segment. Mutants for these genes may lack certain segmental structures or exhibit mirror-image duplication of identical structures within a given segment (Fig. 6.18, c).

Fig. 6.18. Examples of mutations in gap genes (a, Krüppel mutation - elimination of thoracic and the first five abdominal segments), pair-rule genes (b, even-skipped mutation - elimination of odd thoracic and even abdominal segments), and segment-polarity genes (c, gooseberry mutation - mirror-image duplication of the anterior parts of segments in their posterior region) in Drosophila. Г - thoracic, Ч - abdominal segments.

The expression of gap genes is controlled by the Bicoid and Nanos proteins. The most important gap gene is hunchback, which is essential for determining the development of the head and thoracic regions. The activation of its transcription is controlled by the Bicoid protein, whereas the blocking of its translation is controlled by the Nanos protein. This means that structures characteristic of the head and thorax form at the site of Bicoid localization, whereas their formation is inhibited at the site of Nanos localization, leading to the development of the abdominal region. In turn, gap genes act as regulators of pair-rule gene activity, which subsequently control segment-polarity genes. Thus, the development of Drosophila exhibits a clear hierarchical Introduction/30.html">Regulation of Gene activity, with each successive tier defining progressively smaller elements of the adult body (Fig. 6.19).

Fig. 6.19. Cascade of gene interactions in determining the polarity and developmental Specificity of Drosophila segments. Specification of individual characteristics of discrete segments.

After the segmentation genes have established the number of segments and their polarity, genes that determine the individual characteristics and subsequent developmental pathways of each segment—homeotic genes—are activated (Fig. 6.19). The control of homeotic Gene Expression is executed by segmentation gene proteins. The products of homeotic genes are various regulatory proteins capable of interacting with DNA and regulating the work of genes that will determine the formation of appropriate morphological structures (antennae, legs, eyes, wings, etc.) from the cells of each individual segment. Mutations in homeotic genes cause disruptions in the development of individual segments: for example, legs instead of antennae are observed to form from the head segment of Drosophila. Consequently, the activity of homeotic genes, which depends on the cellular environment and the segment in which The Cell resides, dictates the final differentiation.

In Drosophila, homeotic (or selector) genes form two clusters (the Antennapedia complex and the bithorax complex) on the third chromosome, collectively constituting the homeotic complex (HOM-C). The Antennapedia complex contains five genes responsible for the differentiation of cells in the head and anterior thoracic segments. The bithorax complex comprises three genes that determine the ultimate development of the posterior thoracic and abdominal segments. Each homeotic gene contains a conserved sequence of 180 Base Pairs (the homeobox), which corresponds in the gene's protein product to a DNA-binding domain (the homeodomain) consisting of 60 amino acid residues.

Hox genes (derived from homeobox-containing genes) have been discovered in all investigated species of plants and animals, indicating the universality of the systems regulating Cell Differentiation in eukaryotic organisms. In mammals and other vertebrates, Hox genes also form a cluster of 9–11 genes whose products—transcription factors—determine the development of specific Regions of the embryo along the anteroposterior axis.