Expression assayed at stages 9, 11, 13, and 17. Expression may be continuous between assayed stages in some tissues.
transcript expression is first detected during the cellular blastoderm stage, in presumptive mesodermal cells. Mesodermal expression fades during germband elongation, when sna
expression begins to be detected in the ectoderm. The initial pair-rule pattern of sna
expression gives way to a segmentally repeated pattern of cells interconnected by longitudinal rows of cells also expressing sna
. Neuroblasts will delaminate at approximately the sites of these longitudinal rows of cells during the S1 phase of neuroblast segregation. At about 5.5 hours, sna
expression is seen in both the nascent neuroblasts and in ectodermal cells. As neuroblast segregation nears completion at about 5.5 to 6 hours, sna
expression is restricted to the neuroblast layer. By 6.5 to 7 hours, sna
transcript is expressed in all neuroblasts and in PNS precursors. As in the CNS, sna
transcript expression in the PNS begins in neurectodermal patches. Later, sna
transcript is detected in sensory mother cells. At about 9 to 10 hours, sna
is expressed in the postmitotic cells of the PNS and CNS. Starting at about 11 hours,and continuing into the first larval instar, sna
is expressed in the precursors of the wing and haltere discs.
In wild type embryos at embryonic cycle 13, sna
transcript is found at low levels ventrally in a region 12 to 14 nuclei-wide. At embryonic cycle 14, sna
transcript is expressed at higher levels and in about an 18 cell-wide strip (the presumptive mesoderm), and the borders of expression continue to be fuzzy. By the onset of cellularization, the borders of sna
transcript expression in the 18 ventral cells have sharpened, and end at the mesoderm-neurectoderm boundary. In a twi
mutant background, level of sna
transcript remains low, and is limited to the ventral-most 12-14 cells.
The early accumulation of sna
transcript in wild-type embryos is consistent with the role of sna
in mesoderm formation, but its later expression pattern suggests that sna might have additional functions. Prior to gastrulation, sna
transcript is detected in the mesoderm and anterior midgut anlagen. Before germ band elongation at stages 7-9, sna
transcript is present at the dorsal surface of the amnioproctodeal invagination and in the anterior midgut rudiment. Starting at late stage 8, and persisting through stage 11, sna
transcript is also observed in some large neurectodermal cells, presumably neuroblast precursors and segregated neuroblasts. Later in embryogenesis, sna
is expressed in cells which might be the precursors of the peripheral nervous system (stage 10), P cells (stages 11-14), dorsal mesothoracic disc (stage 13), dorsal metathoracic disc (stage 13), and Bolwig photoreceptor organs (stage 13).
transcript are first detectable during nuclear cycle 12 as a diffuse band half the width of the presumptive mesoderm. Although the expression patterns of sna
transcript and protein are similar early in embryogenesis, there are some subtle differences. At mid-cellularization, the sna
transcript and protein expression boundaries sharply delimit the presumptive mesoderm. At the same time, twi
transcript and protein is expressed in a gradient that extends slightly past the sna
expression zone into the presumptive ectoderm. The twi
transcript and protein expression pattern gets sharper later, during gastrulation. twi
protein is expressed in the mesoderm throughout germ band extension, whereas the sna
protein product disappears from the mesoderm partway through germ band extension, and appears in neurectodermal cells which might be neuroblasts.
High levels of sna
transcript are detected in northern blots of embryos 2-4 hours after egg-laying. The levels decrease dramatically, but are still detectable, 4-6 hours after egg-laying.