Question

Discuss the morphological differences between the parasegments and segments of Drosophila. Discuss the evidence, providing specific examples, that suggests the parasegments of the embryo are the subdivisions for the organization of gene expression.

Short answer

In Drosophila, parasegments and segments are developmental units distinguished by the position of their boundaries: parasegments' posterior boundaries align with the anterior boundaries of segments. Significant evidence demonstrates that parasegments are central for organizing gene expression early in embryonic development. Genes like even-skipped, hairy, and runt express in bands matching parasegments' territories. Specific examples such as Wingless-Hedgehog regulatory relation further corroborates the crucial role of parasegments.

Step-by-step solution

Understanding of Morphological Differences

Parasegments and segments are developmental units in Drosophila (fruit fly) embryos, the first appearing transiently during early embryo development. Parasegments are defined by transient gene expression territories, while segments become apparent later during morphogenesis and are defined by morphological landmarks. The basic morphological difference between a parasegment and a segment is the positioning and overlapping of boundary lines in their units. The posterior boundary of a parasegment aligns with the anterior boundary of a segment.

Evidence of Parasegments – Gene Expressions

Biological research has provided substantial evidence that the parasegments, rather than segments, of the Drosophila embryo are the primary subdivisions for the organization of early gene expression. If we consider the example of the pair-rule genes, they are initially expressed in a series of seven bands, each corresponding to a parasegment. Each gene band regulation results in the formation of a stripe of proteins within each parasegmental unit. This expression of pair-rule genes, including even-skipped, hairy, and runt, that cross the boundaries of future segments but align with the boundaries of parasegments, provides evidence for the importance of parasegments in gene regulatory organization.

Specific examples of Gene Expression in Parasegments

A classic example to understand the role of parasegments is the mutual exclusion of the 'Wingless' and 'Hedgehog' signaling pathways in adjacent cells. The Hedgehog (Hh) protein is expressed in the posterior part of each parasegment, whereas the Wingless (Wg) protein is expressed in the anterior portion. Hh stimulates the production of Wg in the neighboring cell to the front, while conversely, Wg induces Hh production in the parasegment's posterior neighbors. This kind of regulation is crucial for the correct development, and mistakes in this delicately balanced system often lead to developmental anomalies.