smallminded

DEVELOPMENTAL BIOLOGY

Embryonic

Probing with the smallminded cDNA indicates that a 3.2 kb transcript is expressed at all key developmental stages: its expression being highest during embryogenesis and at lower but similar levels in larvae, pupae and adults (Long, 1998a).

During the earliest stages of embryogenesis (stages 1-10), SMID mRNA is expressed ubiquitously at high levels. The early ubiquitous expression suggest that the mRNA is maternally supplied. Expression is absent from the amnioserosa during germ band extension. After stage 10, smid expression becomes restricted to two specific tissues. Most obvious is the neurogenic ectoderm, where by stage 12 expression of smid is localized to the NBs that have enlarged and are actively dividing. All NBs express smid at this stage. Expression can be detected in NBs up to stage 15, but throughout this period there is a gradual decline in the number of NBs expressing smid. At stage 13 high expression can be seen in NBs of the thoracic and abdominal NBs. By stage 16 expression can no longer be detected in these NBs, but the cephalic NBs continue to express smid. The other cells expressing high levels of smid are in the gonads, which show expression through to hatching and into larval life. Expression of smid in the non-neurogenic ectoderm decreases markedly post stage 12, coincident with the final divisions of the epidermal cells. Small islands of smid expression can be seen in the epidermis post stage 12 in sensory precursor cells (Long, 1998b).

Larval

The GAL4-induced lacZ expression of line C161 was examined. Line C161 contains a P-element inserted into the smallminded gene. At early larval stages, lacZ is expressed in neuroblasts and in sensory neurons. Neuroblast expression is seen in the cephalic and thoracic neuromeres of the central nervous system. Expression is also seen in imaginal discs. Reporter gene expression is not detected in embryonic stages (Long, 1998a).

In larvae, smid expression is again detected in NBs as they begin their post-embryonic program of cell division. As early as 24 h after hatching, smid expression can be detected in the outer proliferation center (OPC) and in a small number of NBs in the cephalic neuromeres, although none is seen in the thoracic neuromeres. By 96 h after hatching, smid is expressed at high levels throughout the cephalic and thoracic neuromeres and in small segmentally repeated sets in the abdominal neuromeres. smid expression is restricted to the NBs, in a pattern that mirrors exactly the pattern of NB activity in larvae. Smid is also expressed in imaginal discs, in adepithelial cells (the precursors of adult mesoderm), and in segmentally repeated histoblast nests (the precursors of the adult ectodermal derivatives in the abdomen). During late larval stages, high smid expression is detected in small clusters of cells associated with muscles and sensory neurons. These cells are progenitors of the adult mesoderm and are the so-called persistent twist cells (Long, 1998b).

Effects of Mutation or Deletion

The CNS of wild-type and homozygous C161 larvae are comparable in size and indistinguishable. Later in development ( post 24 h), significant differences in the size of the CNS become apparent. Over the next 96 h both wild-type and C161 nervous systems increase in size but the rate and extent of the growth of the wild-type CNS is far greater than its C161 equivalent. As early as 36 h after hatching the difference is such that the wild-type CNS is significantly larger than the equivalently aged CNS from C161 larvae (P<0.001). The wild-type CNS can be seen to be significantly larger than the C161 CNS at all subsequent stages of larval development. It is for this reason that the mutated gene has been named smallminded. This observation supports the hypothesis that mutations of smid disrupt the normal patterns of neurogenesis (Long, 1998a).

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date revised: 18 March 99

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