Leonard G. Robbins

Molecular genetic analysis of Drosophila rDNA arrays

Large repeated DNA arrays are a major component of the eukaryotic genome, but we know little about their internal organization. Understanding their architecture, however, is critical for describing genome structure and for inferring the mechanisms that shape it. One repeated family that is yielding a picture of how structure, function and recombination mechanisms come together is the ribosomal DNA (rDNA) of Drosophila melanogaster.

Ribosomal DNA Organization Before and After Magnification in Drosophila melanogaster

In all eukaryotes, the ribosomal RNA genes are stably inherited redundant elements. In Drosophila melanogaster, the presence of a Ybb− chromosome in males, or the maternal presence of the Ribosomal exchange (Rex) element, induces magnification: a heritable increase of rDNA copy number. To date, several alternative classes of mechanisms have been proposed for magnification: in situ replication or extra-chromosomal replication, either of which might act on short or extended strings of rDNA units, or unequal sister chromatid exchange. To eliminate some of these hypotheses, none of which has been clearly proven, we examined molecular-variant composition and compared genetic maps of the rDNA in the bb2 mutant and in some magnified bb+ alleles. The genetic markers used are molecular-length variants of IGS sequences and of R1 and R2 mobile elements present in many 28S sequences. Direct comparison of PCR products does not reveal any particularly intensified electrophoretic bands in magnified alleles compared to the nonmagnified bb2 allele. Hence, the increase of rDNA copy number is diluted among multiple variants. We can therefore reject mechanisms of magnification based on multiple rounds of replication of short strings. Moreover, we find no changes of marker order when pre- and postmagnification maps are compared. Thus, we can further restrict the possible mechanisms to two: replication in situ of an extended string of rDNA units or unequal exchange between sister chromatids.

The use of maternally coded gene products in Drosophila

Both maternal and zygotic expression of many essential genes are required for normal development. For some of these genes, absence of maternal function yields striking embryonic defects. The experiments reported here examine two questions about such genes: (1) Are embryonic effects of maternal deficits a common property of maternally-and-zygotically active genes? and (2) Is use of the maternal products of these genes restricted to early embryogenesis? A comparison of times of lethality of mutant sons of normal and mutant-heterozygous mothers has been made for six mutations in the zeste-white region of the Drosophila X chromosome. Four of the mutations are defective in single cistrons and two are deficiencies that between them remove thirteen essential loci. All of these mutations had previously been shown to have both maternal and zygotic effects, and all of them had been tested, using homozygous germ-line clones, for the effects of complete maternal defects. For several of them, homozygous germ-line clones cause embryonic defects. Of the six, only one, Df(l)K95, shows a shift from larval to embryonic lethality when the mothers are heterozygous, and even in that case lethality occurs at the very end of embryogenesis. These results have two implications: (1) maternally-derived transcripts do not always serve a solely embryonic role; and (2) an embryonic effect of a complete maternal deficit does not by itself demonstrate an embryo-restricted function for the maternal transcript.

The meiotic behavior of some single-cistron mutants in the zeste-white region of the Drosophila melanogaster X chromosome

SummaryThere are two dosage sensitive sites in the zeste-white region of the Drosophila melanogaster X chromosome that affect meiotic chromosome behavior. Single-cistron mutants at essential and female fertility loci in the two segments have been tested for meiotic effects similar to those of deficiencies. None of the mutants have detectable meiotic effects. A de novo search for meiotic mutants in the region has not uncovered any, but the results suggest that a deficiency for the zeste-white region would be useful for detecting meiotic mutants elsewhere in the genome. Tests for interactions between the deficiency and known meiotic mutants support this. Though tentative, these results suggest that non-essential regions need not be devoid of function.