Topic: Pattern formation/Compartmentalisation
Garcia-Bellido A, Ripoll A, Morata G.
Developmental Compartmentalisation of the Wing Disk of Drosophila.
Nature New Biology 245, 251-253, 1973

“How do the cells of the embryo generate the cells of the adult”, is an old and important question in developmental biology. There are many possible answers, all of which must lie between two extremes. At one extreme, there would be no defined pattern of cell growth; cells would just divide to create cell clumps and these cells would be used in mass to make the adult organs with no regard to their ancestry. At the other extreme, embryonic cells could divide in a fixed pattern to generate a lineage tree and organs could arise rigidly from specific sets of related cells.

One way to distinguish these two scenarios is “clonal analysis”; a cell is labeled at one developmental stage to visualize its cell descendants (clone) at a later stage. Parameters such as size, shape and location of the clone provides information on the growth characteristics as well as the developmental restriction of the cell’s property at the time of clone production. In 1970s, before molecular biology was introduced to developmental biology, Garcia-Bellido and colleagues had been performing such clonal analysis on the adult thorax of Drosophila. Insect thorax comprises of multiple distinct regions (such as wing blade and body parts called notum or pleural), which arise from a single cell population called “wing imaginal disk”. Imaginal disk proliferate as “undifferentiated” tissue during larval stages and differentiate during metamorphosis. By artificially inducing mitotic recombination (also called “somatic cross-over”) by X-ray irradiation, single cells in the developing imaginal disk can be randomly marked by making cell homozygous for a recessive marker gene (such as jv and mwh), and the behavior of the resulting clone can be analyzed in the adult thorax.

Prior to this work the authors had found two important characteristics of these clones.

1) The frequency of clones induced at earlier stage of development was lower than that of clones induced at later stage. In contrast, the size of clones decreased as clones were induced later during development.

2) The clones generated at earlier stage expanded to several parts of the organ, but those from later stage were limited to specific part(s).

The first result likely reflects the increase in the cell number of the organ primordia at the time of irradiation. The second observation may indicate that the restriction of developmental fate occurs progressively in this organ. Although such developmental restriction can imply an important patterning mechanism, interpretation of such results is limited due to the small size of the clone; clone may not have covered multiple regions simply because the clone did not contain sufficient number of cells to have a chance to grow beyond regional borders.

In the paper we will be discussing this week, the authors solved this problem by employing a special genetic trick called “Minute technique”. Minute (M) is a mutation that reduces cell proliferation rate in heterozygous situation (M/M+). By inducing mitotic recombination to produce a cell that is homozygous for the wild type allele of M (M+/M+), they could provide growth advantage to the marked clone and then observe the features of the clone. They found that M+/M+ clone could occupy a large portion of the thorax, but yet there was a border that clones never crossed. They proposed that wing imaginal disk is subdivided into “compartments” with distinct cell fates, and cells proliferate only within the compartment and cannot cross its boundary. They further proposed that the imaginal disk is subdivide progressively during development. These concepts had an enormous impact on deciphering the patterning strategy of the adult body.

This paper contain many technical terms, such as mitotic recombination technique, cell markers jv and mwh, and specific names of body parts. You do not have to understand what they are; they will be explained in this lecture. We will discuss how we can extend this “compartment theory” in cellular and molecular level to understand the mechanism controlling the body plan during development.