Proteins foretelling head or abdomen development in the embryo of Smittia spec. (Chironomidae,Diptera)

The development of the segment pattern in Smittia embryos can be manipulated experimentally. Centrifugation during intravitelline cleavage leads to a mirror image duplication of most of the head in the absence of abdominal segments (“double cephalons”). Conversely, mirror image duplications of abdominal segments in the absence of head and thorax (“double abdomens”) can be generated by UV-irradiation of the anterior pole before blastoderm formation. By subsequent exposure to blue light, UV-irradiated embryos can be reprogrammed for normal development (photoreversal). We have characterized an “anterior indicator” protein (designated AI₁; M_r ? 35,000; IEP ? 4.9). Its synthesis was restricted to anterior fragments of embryos during a late blastoderm stage (Bl_VI). This protein was synthesized, however, in both anterior and posterior fragments of prospective double cephalons. Conversely, this protein was synthesized neither in anterior nor in posterior fragments of UV-induced double abdomens. Upon photoreversal, the protein was synthesized again in anterior fragments. Thus, synthesis of this protein in a given fragment always indicated development of head and thorax there. Likewise, we have characterized a “posterior indicator protein” (designated PI₁, M_r ? 50,000, IEP ? 5.5). Its synthesis during early blastoderm stages (Bl_I and Bl_II) was restricted to posterior fragments but not to pole cells in normal embryos. In UV-induced double abdomens, PI_I was synthesized in both anterior and posterior fragments at stage Bl_II. Photoreversal again led to restriction of PI_I synthesis to posterior fragments. Thus, the synthesis of PI_I in a given fragment at stage Bl_II always foreshadowed the formation of an abdomen several hours before this can be discerned morphologically. The synthesis of two other proteins (designated a₁ and p₁) was also restricted, during certain blastoderm stages, to anterior or posterior fragments, respectively. However, UV-irradiation or centrifugation had little or no effect on the synthesis of these proteins. Conversely, programming embryos for double abdomen development by UV-irradiation caused a set of reproducible, and mostly photoreversible, changes in the pattern of proteins synthesized in anterior embryonic fragments. However, the synthesis of most of the affected proteins was not region-specific in normal embryos.