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Balancer chromosomes are multiply inverted and rearranged chromosomes that are widely used in Drosophila genetics. First described nearly 100 years ago, balancers are used extensively in stock maintenance and complex crosses. Recently, the complete molecular structures of several commonly used balancers were determined by whole-genome sequencing. This revealed a surprising amount of variation among balancers derived from a common progenitor, identified genes directly affected by inversion breakpoints, and cataloged mutations shared by balancers. These studies emphasized that it is important to choose the optimal balancer, because different inversions suppress meiotic recombination in different chromosomal regions. In this review, we provide a brief history of balancers in Drosophila, discuss how they are used today, and provide examples of unexpected recombination events involving balancers that can lead to stock breakdown., Competing Interests: The authors have declared that no competing interests exist.

Heterochromatin-mediated repression is essential for controlling the expression of transposons and for coordinated cell type-specific gene regulation. The small ovary ( sov ) locus was identified in a screen for female-sterile mutations in Drosophila melanogaster , and mutants show dramatic ovarian morphogenesis defects. We show that the null sov phenotype is lethal and map the locus to the uncharacterized gene CG14438 , which encodes a nuclear zinc-finger protein that colocalizes with the essential Heterochromatin Protein 1 (HP1a). We demonstrate Sov functions to repress inappropriate gene expression in the ovary, silence transposons, and suppress position-effect variegation in the eye, suggesting a central role in heterochromatin stabilization.

Suppressors and enhancers of position effect variegation (PEV) have been linked to the establishment and maintenance of heterochromatin. The presence of centromeres and other inheritance elements in heterochromatic regions suggests that suppressors and enhancers of PEV, Su(var) s and E(var)s [collectively termed Mod(var)s], may be required for chromosome inheritance. In order to test this hypothesis, we screened 59 ethyl methanesulfonate-generated Drosophila Mod(var)s for dominant effects on the partially compromised inheritance of a minichromosome ( J21A) missing a portion of the genetically defined centromere. Nearly half of these Mod(var)s significantly increased or decreased the transmission of J21A. Analyses of homozygous mutant larval neuroblasts suggest that these mutations affect cell cycle progression and native chromosome morphology. Five out of six complementation groups tested displayed mitotic abnormalities, including phenotypes such as telomere fusions, overcondensed chromosomes, and low mitotic index. We conclude that Mod(var)s as a group are highly enriched for genes that encode essential inheritance functions. We propose that a primary function of Mod(var)s is to promote chromosome inheritance, and that the gene silencing phenotype associated with PEV may be a secondary consequence of the heterochromatic structures required to carry out these functions.

The demonstration by Zhang and Spradling (1) of efficient P element transposition into heterochromatic regions will aid ongoing studies of heterochromatin structure and function. P element insertions will provide entry points for further molecular analysis of heterochromatin and will allow the isolation of small and large heterochromatic deficiencies. The generation of heterochromatic P insertions also will aid the study of heterochromatic genes. Of the heterochromatic insertions isolated by Zhang and Spradling, five were homozygous lethal, and one of these defined a lethal locus not previously uncovered by heterochromatic deficiencies. P elements have previously been used to mutagenize and clone specific heterochromatic genes (14, 19, 26). New methods, like those described here (1, 32), should allow the efficient identification and molecular isolation of other single-copy heterochromatic genes. Furthermore, since position-effect suppression allowed the recovery of heterochromatic P insertions, it may also allow the recovery of insertions in euchromatic regions previously refractory to P mutagenesis. Studies of position-effect variegation show that genes normally found in heterochromatin require a heterochromatic context for normal expression and that heterochromatin is inhibitory to euchromatic gene expression (16). The physical basis of these related phenomena--chromatin assembly, nuclear positioning, and/or heterochromatin elimination--can be resolved only with a more thorough understanding of heterochromatin structure and functions. Analyzing heterochromatin also will help define the chromosomal components responsible for inheritance processes such as chromosome pairing, sister chromatid adhesion, and centromere function. These efforts will be facilitated by the effective use of P elements combined with other current molecular-genetic approaches.