The joy of balancers

Abstract

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.

Fig 1. Common types of inversions.

Paracentric inversions (A) do not encompass the centromere whereas pericentric inversions (B) do. Recombination between either type of inversion and a structurally wild-type homolog produces aneuploid chromosomes, which cause embryonic lethality.

Fig 2. Commonly used balancer chromosomes in Drosophila showing approximate sizes of inverted segments.

X chromosome balancers invert an uncertain amount of pericentric heterochromatin (gray). Second and third chromosome balancer inversions do not bisect centric heterochromatin, and thus heterochromatin is not represented here. The large duplication present on SM5 is indicated (orange box).