Cloning and characterization of Bmrunt from the silkworm Bombyx mori during embryonic development

Abstract

Pair-rule genes (genes that are expressed only in alternate segments, odd or even) play an important role in translating the broad gradients of upstream genes into dual segment periodicity for body plan patterning in Drosophila. However, homologues of pair-rule genes show a remarkable diversity of expression patterns and functions in other insects. We cloned the homologue of runt in the silkworm Bombyx mori, an intermediate germband-type insect. Whole-mount in situ hybridization revealed three stripes arose one by one before gastrulation at the blastoderm stage. Five additional stripes were then generated sequentially as the growth zone elongated. Eight stripes appeared in a pair-rule manner with two-segment periodicity, each of which was confined to the posterior of an odd-numbered parasegment. The weaker segmental secondary stripes emerged de novo in even-numbered parasegments. The Bmrunt transcript vanished before blastokinesis and was then expressed again in the whole embryo. RNA interference for Bmrunt caused severely truncated, almost completely asegmental defects. This cadual-like phenotype suggests that Bmrunt does not function as a pair-rule gene in silkworm segmentation. Bmrunt is required for formation of most body segments and axis elongation in B. mori.