Changes in the apparent localization of anterior determinants during early embryogenesis (Smittia spec., Chironomidae, Diptera).

InSmittia embryos, anterior UV irradiation causes the formation of double abdomens in which head and thorax are replaced by a mirror-image duplication of abdominal segments. The transformation is ascribed to the inactivation of cytoplasmic ribonucleoprotein particles acting as anterior determinants. During the first 5 h after egg deposition (h.a.d.), high yields of double abdomens are produced by microbeam irradiation of a small target area (diameter 20 μm) behind the anterior pole. This localization coincides with a yolk-free cytoplasmic cone observed behind the anterior pole of newly depositedSmittia eggs (Zissler and Sander 1973). Irradiation of this area becomes less efficient between 5 and 7 h.a.d., while double abdomen yields increase after irradiation of adjacent target areas. A dramatic increase in the double abdomen yield is also observed after irradiation of the entire anterior surface. Under these conditions, the UV fluence required for a given yield decreases by a factor of 3.9 between 5.5. and 7 h.a.d. This can be explained quantitatively by the assumption that the effective targets move an average distance of 8 μ from the strongly absorbing endoplasm towards the egg surface. During the period of apparent target deshielding, the distance of nuclei from the egg surface actually decreases from 11 to 3 μm. However, the timing and direction of the nuclear migration and of the apparent deshielding differ, indicating that the shift of effective targets may be triggered by, but does not exactly follow, the nuclear movement. The data suggest that a major fraction of anterior determinants shifts from a small volume behind the anterior pole to a spread-out distribution over the surface of the anterior half.