Actin messenger in maternal RNP particles from an insect embryo (Smittia spec., Chironomidae, Diptera)

An early role of maternal mRNA in establishing the dorsoventral pattern in pelle mutant Drosophila embryos

Mutations of the maternal effect locus pelle (pll) cause dorsalized Drosophila embryos. In extreme mutants, the embryo develops into a long hollow tube of dorsal cuticular structures with no sign of ventral pattern elements. Injection of wild-type cytoplasm or poly(A)+RNA into mutant pll embryos partially restores the normal pattern. Rescuing activity is present in the wild-type cytoplasm until the late blastoderm stage, but is already absent from the poly(A)+RNA fraction by the time of pole cell formation. At the same time, pll embryos fail to respond to injected biologically active poly(A)+RNA. This indicates that pll+ mRNA is lost early from the pool of maternal RNA and that there is a non-RNA component of rescue. This component, most likely the pll+ protein, appears to be unequally distributed in wild-type embryos.

Molecular genetics of Krüppel, a gene required for segmentation of the Drosophila embryo

Krüppel is a member of the 'gap' class of segmentation genes of Drosophila melanogaster, mutations of which cause contiguous groups of segments of the fruitfly embryo to fail to develop. In the case of Krüppel mutant embryos, thoracic and anterior abdominal segments are deleted. The molecular cloning of the Krüppel locus will lead to an understanding of the crucial role that gap genes seem to have in early embryonic development. It has already allowed the identification of a blastoderm-specific Krüppel transcript and the phenotypic rescue of mutant embryos by injected cloned DNA.

An analysis of dopa decarboxylase expression during embryogenesis in Drosophila melanogaster

Dopa decarboxylase (DDC) activity appears near the end of embryogenesis in Drosophila. High titers of 20-OH-ecdysone on the other hand are found at midembryogenesis. Several explanations for this lag were investigated, since this hormone has been shown to induce a rapid increase in DDC activity at pupariation (G. P. Kraminsky, W. C. Clark, M. A. Estelle, R. D. Gietz, B. A. Sage, J. D. O'Connor, and R. B. Hodgetts, 1980, Proc. Natl. Acad. Sci. USA 77, 4175-4179). Using immunological and genetical criteria, it was shown that the same structural gene encodes DDC in embryos, mature larvae, and young adults. This rules out the existence of a distinct embryonic DDC gene unresponsive to 20-OH-ecdysone. Second, no evidence was found to support the hypothesis that a delay in the translation of DDC transcripts, produced in response to the elevated titer of 20-OH-ecdysone at midembryogenesis, caused the lag. Northern analysis of the RNA molecules homologous to cloned genomic sequences revealed that DDC transcripts were present at two different times during embryogenesis. A transcript was found in both ovaries and 0- to 2-hr embryos. However, this species disappeared by 4 hr and DDC transcript levels remained low until late in embryogenesis, when a significant increase occurred. This increase was presumably responsible for the appearance of the enzyme at this time. The northern blotting revealed nine DDC transcript species were present during embryogenesis and hybridization to intron-specific probes indicated that five of these contained at least part of one (or both) of the two introns. Three putative mature mRNA species were identified by their small size, relative abundance, apparent lack of intron sequence, and their presence on polysomes. The two mature species found during the late stages were postulated to differ in the length of their poly(A)+ tails. The third mature species was found only in ovaries and very young embryos and may well be of maternal origin. Data are examined in light of the possibility that this species is derived from a precursor initiated at a novel promotor.

Photoreversible inhibition by ultraviolet light of germ line development in Smittia sp. (Chironomidae, Diptera)

Pole cell formation in embryos of the parthenogenetic midge, Smittia sp., can be delayed or inhibited by irradiation of the posterior egg pole with ultraviolet light (uv). This leaves the schedule of nuclear divisions and chromosome eliminations virtually unaffected. However, uv irradiation delays the precocious migration to the posterior pole of one nucleus, which normally becomes included in the first pole cell. This effect is photoreversible, i.e., mitigated by application of blue light after uv. Photoreversibility indicates that a nucleic acid component is involved as an effective target. During normal development of Smittia a number of chromosomes are eliminated during mitosis V, not only from somatic nuclei but also in the germ line. In the latter, this mitosis takes place during the first gonial division in the larva. After uv irradiation, the first pole cell nucleus has undergone supernumerary mitoses before pole cell formation and, as a result, is driven into mitosis V precociously as the pole cell divides. This is frequently associated with chromosome elimination from pole cells, which in turn is correlated with subsequent disappearance of already formed pole cells. Adults derived from embryos without pole cells do not form ovaries. Pole cell formation, pole cell preservation, and ovary development are separately inhibited by uv, and inhibition of each step is photoreversible. The results are discussed in the context of germ cell determination, protection against chromosome elimination, and the role of chromosomes limited to the germ line.