Arrowhead

DEVELOPMENTAL BIOLOGY

Embryonic

Awh transcription is first detected in cells located at the position of the neuroblasts in stage 9 embryos, when neuroblasts begin to segregate. Loss-of-function mutations in the neurogenic gene Delta result in hypertrophy of the nervous system. Embryos carrying a loss-of-function mutation in Delta have about twice as many Awh-expressing cells as wild-type embryos, suggesting that the Awh expressing cells are neuroblasts. Awh is also transcribed at stage 11 in cells in the procephalon that may correspond to cells of the supraesophageal ganglia (brain). During embryogenesis, Arrowhead is expressed in each abdominal segment and in the labial segment, consistent with its role in establishing the proper numbers of abdominal histoblasts and salivary gland imaginal ring cells (Curtiss, 1995). AWH mRNA expression during stage 10 of embryogenesis, the extended germ band stage, is found in bilateral stripes in the three thoracic segments and in all 10 abdominal segments. As the gnathal buds become apparent during stage 11, the Awh transcript is also detected in a stripe and a few additional cells at the center of the labial bud. At stage 14, Awh is not expressed in cells in the region of the thoracic segments in which the developing prothoracic, wing, haltere, and leg discs are located. escargot, a gene involved in imaginal disc cell cycle regulation. is expressed in the developing discs from which Awh expression is excluded. Late in embryonic development, expression is refined to the abdominal histoblasts and salivary gland imaginal ring cells themselves (Curtiss, 1997).

Larval

By the late third instar, most excorporate imaginal cells have completed an intense period of proliferation and are undergoing the final stages of organization before the dramatic events of metamorphosis begin. The abdominal histoblasts, which have remain mitotically quiescent throughout the larval stages, are about to enter a period of especially intense proliferation, migration, and differentiation to generate the adult abdominal epithelium. At this stage, the Awh transcript is expressed in cells on both sides of the border separating the larval salivary gland from the salivary gland imaginal ring. Awh is also transcribed in specific areas of wing, leg and eye-antennal discs. Awh is transcribed on the medial edge of the wing disc, in a stripe on the anterolateral edge of the leg disc, which extends in a spiral toward the center of the disc, and in cells extending down the ventral edge of the eye-antennal disc, from approximately the middle of the antennal portion, to approximately the middle of the eye portion. No defects in the structures derived from these imaginal discs have been detected in Awh mutants (Curtiss, 1997).

Effects of mutation or deletion

Metamorphosis in Drosophila melanogaster requires synchronization of numerous developmental events that occur in isolated imaginal precursor tissues. The imaginal primordia are established during embryonic stages and are quiescent for much of larval life. The Arrowhead gene is necessary for the establishment of proper numbers of cells within a subset of imaginal precursor tissues. Loss-of-function mutations in Arrowhead reduce the number of abdominal histoblasts and salivary gland imaginal ring cells before the proliferative stages of their development. The number of abdominal histoblasts in mutant animals is approximately half that of wild-type, as might result from failure of a single early division of these cells. A neomorphic Arrowhead allele, Awh1, results in the specific loss of the retinal precursors by the early third instar, before they have begun to differentiate. Since Arrowhead mutations affect only subsets of imaginal tissue, there must be distinctions in the developmental regulation of different imaginal precursors. Arrowhead may be part of a regulatory pathway responsible for establishing the proper number of abdominal histoblasts and salivary gland imaginal ring cells. The neomorphic Arrowhead allele, which may cause misexpression of the Arrowhead gene in the eye-antenna imaginal disc, interferes with the establishment or proliferation of retinal precursor cells (Curtiss, 1995).

Curtiss, J. and Heilig, J. S. (1995). Establishment of Drosophila imaginal precursor cells is controlled by the Arrowhead gene. Development 121(11): 3819-3828.

Curtiss, J. and Heilig, J. S. (1997). Arrowhead encodes a LIM homeodomain protein that distinguishes subsets of Drosophila imaginal cells. Dev. Biol. 190(1): 129-141.

Kylsten, P and Saint, R. (1997). Imaginal tissues of Drosophila melanogaster exhibit different modes of cell proliferation control. Dev. Biol. 192: 509-522.

Sagasti, A., Hobert, O., Troemel, E. R., Ruvkun, G. and Bargmann, C. I. (1999). Alternative olfactory neuron fates are specified by the LIM homeobox gene lim-4. Genes Dev. 13(14): 1794-806. 10421632

date revised: 23 January 98  

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