Homeobox Genes DataBase

Drosophila Genes in Development: Dorsal-ventral polarity, gastrulation-mesoderm

Snail

Function

Gastrulation is a process of tissue involution that creates three germ layers from one. In Drosophila, mesoderm is created as a result of ventral furrow formation, one of the three invagination events taking place during gastrulation. Invagination is initiated by two regulatory genes, twist (twi) and snail (sna). sna is sufficient to initiate the invagination of the ventral-most embryonic cells in the absence of twist gene activity, but without twist the invaginated cells fail to express mesodermal genes under the control of twist, such as tinman and bagpipe. Low levels of SNA that fail to repress neuroectoderm determinants in the presumptive mesoderm are nonetheless able to promote invagination (Ip Y. T., 1994a).

snail is a transcriptional repressor. It acts to restrict neuroectoderm and neural fate in the invaginating mesoderm. snail mutants display a massive derepression of mesectodermal genes such as single-minded and rhomboid (Kasai Y., 1992), (Ip Y.T., 1992).

snail, in its capacity as a transcriptional repressor, assures the extent and integrity of the mesodermal domain.

snail has a second function as a regulator of neurogenesis. Its role here is independent of daughterless/achaete-scute, and it acts in both the central and peripheral nervous systems. The nature of snail's neural actions are uncharacterized. It may act to repress non neural fates. If so, then its role in the nervous system is the reverse of its role in the mesoderm (Ip Y. T., 1994b).

snail is also involved is the specification of the wing disc. In this role, snail is expressed in the ectoderm. escargot and snail are required for the maintenance of vestigial expression in the wing disc (Fuse N., 1996).

Protein

There are five C terminal zinc fingers (Boulay J. L., 1987).

Subcellular location

Nuclear

Expression Patterns

The snail gene encodes a protein with a zinc-finger motif, and here we report that the snail gene product is a sequence-specific DNA binding protein. The snail protein recognizes a 14-base-pair consensus sequence that is found nine times in a 2.8-kilobase sim regulatory region. These results provide evidence for the direct control of sim transcription by snail. (Kasai Y., 1992).

sna is expressed ventrally in the blastoderm, encodes transcription factors and promotes the invagination of the ventral furrow by activating or repressing appropriate target genes. Anteriorly hkb together with sna determines endodermal fate. (Reuter R., 1994).

sna transcripts are first detected during late syncytial blastoderm (stage 4). By stage 5, SNA mRNA is detected along the ventral surface of the embryo, extending to the anterior and posterior poles. In the early blastoderm, the level of SNA mRNA transcripts in the posterior region begins to decrease. In early germ band elongation, SNA mRNA is seen in the anterior midgut rudiment as well as on the dorsal surface of the invaginating amnioproctodeum (Alberga J., 1991).

sna expression during neurogenesis evolves from segmentally repeated neuroectodermal domains to a pan-neural pattern (Ip Y. T., 1994b). Formation of SNA protein in the neurogenic region is predominantely cytoplasmic (Alberga J., 1991).