EXPRESSION PATTERNS OF TWIST AND SNAIL IN TRIBOLIUM (COLEOPTERA) SUGGEST A HOMOLOGOUS FORMATION OF MESODERM IN LONG AND SHORT GERM BAND INSECTS

The mesodermal region in Drosophila is determined by a maternally deri ved morphogenetic gradient system which specifies the different cell f ates along the dorsoventral axis, including the prospective mesodermal cells at the ventral side of the embryo. There are at least two zygot ic target genes, twist and snail, which are required for mesoderm form ation in Drosophila. To analyze whether a similar mode of mesoderm spe cification might also apply to short germband insect embryos, we have cloned twist and snail-related gene fragments from the flour beetle Tr ibolium and have analyzed their expression pattern. Both genes are exp ressed in a ventral stripe at early blastoderm stage, which is restric ted to the region of the developing germ rudiment. The cells expressin g the two genes are those that invaginate during gastrulation, indicat ing that the early stages of mesoderm specification are indeed very si milar between the two species. Interestingly, both genes are also expr essed during germband extension in a subregion of the growth zone of t he embryo which forms the mesodermal cells. This suggests that the exp ression of the two genes is required for mesoderm formation both at ea rly blastoderm stage and during germband elongation until the end of t he segmental growth process. (C) 1994 Wiley-Liss, Inc.