Imaginal Discs: The Genetic and Cellular Logic of Pattern Formation by Lewis I. Held, Jr.
Imaginal Discs
by Lewis I. Held, Jr.
Chapter 1: Cell Lineage vs. Intercellular Signaling

Figure 1.1 | Figure 1.2
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Figure 1.2

Figure 1.2
The nonclonal nature of fly development.
The irrelevance of cell clones to pattern formation is seen in the piebald variegation of sexually mosaic 'gynandromorphs' (middle panel) [1370, 2026]. Such flies are typically half-male (gray) and half-female (black) [1715, 2950]. They start life as a heterozygous female (2X) zygote but lose an X chromosome from one nucleus at the first mitosis to create a male (1X) clone (top panel) [1695]. If the X that remains has the yellowLOF (yLOF) allele (enlarged gray circle), then the descendants of that nucleus will make yellow (instead of brown = wild-type) bristles or cuticle in the adult (bottom panel).

The two embryos above (A, anterior; P, posterior) differ in the orientation of the first mitotic spindle [3274, 4021]. This disparity causes the male/female boundary to trace different paths in the cuticle (middle panel) [4649, 4652, 4845].

The adults are bisected in the middle panel, and the cross sections are turned ~90° to a frontal view in the bottom panel (D, dorsal; V, ventral; R, right; L, left). The outer ring of circles (nuclei) schematically represents the thoracic epidermis. The inscribed tree represents an imaginary series of mitoses (branch points) from the initial two nuclei to the adult epidermis. Bristle numbers and cell densities are drastically reduced for clarity.

If the wing disc (dashed outline) were a clone -- i.e., derived from a single nucleus -- then it should be purely yellow or brown because its progenitor nucleus must be one or the other. In actual gynandromorphs, however, the wing disc is often mosaic (R disc in fly #1 and L disc in fly #2), so it cannot be a clone. Moreover, the ability of the male/female boundaries to pass between any two landmarks (e.g., the different pairs of bristles in fly #1 vs. fly #2) argues that the patterns of cell lineage within the disc (inscribed trees) must also vary from fly to fly.

Overall, therefore, such flies reveal a fundamental uncoupling between pedigrees and patterning. This uncoupling is abstractly seen in the ability of the male/female 'hour hands' to lie anywhere on the epidermal 'clockface'. The two flies shown here are only two examples from a large set of possibilities.

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