Toll homolog expression in the beetle Tribolium suggests a different mode of dorsoventral patterning than in Drosophila embryos

The gene Toll (Tl) encodes a maternally supplied interleukin 1 receptor-related transmembrane protein, a key component required to establish dorsoventral polarity in the Drosophila embryo. We have isolated Tl homologs of a primitive dipteran, Clogmia albipunctata, and of the beetle Tribolium castaneum. Tribolium Tl protein (Tl) lacks sequences in the C-terminal portion of the cytoplasmic domains that are conserved in the dipteran homologs. The Tl homolog of Tribolium mediates the ventralizing activity when expressed as a gain-of-function variant in transgenic Drosophila, indicating that the sequences conserved in the Diptera are not essential for Tl signaling. In contrast to Drosophila where Tl gene expression occurs maternally and supplies uniformly distributed Tl in the egg membrane, Tl transcripts form a ventral-to-dorsal gradient in the Tribolium blastoderm stage embryo. This localized expression pattern of Tl transcripts, as compared with the strong maternal and ubiquitous expression in Drosophila and Clogmia embryos, suggests that dorsoventral patterning in long-germ band and short-germ band insects involves the same components but different modes of their action.