The maternal and zygotic roles of the gene Polycomb in embryonic determination in Drosophila melanogaster

Abstract

The mutation Polycomb ( Pc) is known to cause a variety of intersegmental transformations in homozygous and heterozygous individuals of Drosophila melanogaster; Pc + is thought to act as a negative regulator of genes of the bithorax complex. The function of this gene in the maternal germ line has been assessed by examining the variation in expression of these homoeotic phenotypes in individuals derived from a maternal germ line with a single or no dose of the Pc + allele. Mosaic individuals with a homozygous or heterozygous Pc germ line were produced by transplantation of pole cells, the embryonic precursors of the germ line. By employing an X-linked dominant female-sterile mutation, the identification of mosaic females and the study of progeny derived from the exogenous germ line were greatly simplified; the advantages of this system for the transplantation of pole cells for such analyses are described. In general, all thoracic and abdominal segments of homozygous Pc embryos differentiate characteristics of the eighth, most posterior, abdominal segment. The extent and uniformity of this transformation as well as other manifestations of the homozygous Pc genotype are described and shown to be correlated with the maternal germ line genotype; homozygous Pc embryos derived from a homozygous Pc maternal germ line show greater expression of these phenotypes than do genetically identical embryos derived from a heterozygous Pc maternal germ line. The expression of some homoeotic phenotypes typical of heterozygous Pc adults shows only a slight correlation with the maternal genotype, while no homoeotic transformations are clearly evident in heterozygous larvae of either origin. Thus, the maternal effect of Pc is rescuable. The results suggest that the Pc + gene is active in the maternal germ line but that the absence of the maternally derived Pc + product can be largely compensated by the introduction of a wild-type allele upon fertilization; this rescue indicates that the maternal activity of Pc + plays no major role in the normal process of embryonic segmental determination. The normal fertility of males and females with a homozygous Pc germ line and of their progeny suggests that Pc + plays no role in the determination or development of the germ line in either the maternal or zygotic genome.