The wide array of genetic tools available for genetic dissection of fruit fly Drosophila melanogaster makes its undoubtedly the most preferred model organism when it comes to in vivo experiments. Balancer chromosomes forms a very important part of this repertoire. Balancer chromosome strains provide key advances for mutagenesis screens, for stock maintenance and for tracking quantitative traits. It was in 1918 Muller identified the first balanced mutation and later Sturtevant proposed that differences in the gene order in this region between different species of flies were due to an inversion in gene order.
Balancer chromosome has the following properties which make them very valuable in performing genetic crosses:
(i) an inversion or inversions to suppress the recovery of viable recombination products over the length of the chromosome.
(ii) a dominant phenotype that enables the inheritance of the chromosome to be tracked easily in subsequent crosses and
(iii) a recessive lethal mutation that eliminates the homozygous balancer from the population of breeding flies.
Prevents Crossing over and recombination:
Lethal recessive gene is difficult to maintain and likely to be lost from a population, because it confers a disadvantage on the progeny of heterozygotes. So the question comes how can we maintain the lethal allele?
Here’s what happens when we try to breed two heterozygotes: If we have mutant allele gene “Y” and so the genotype will be + / Y. The population does not breed true, because in the next generation the wild-type (+/+) appears. In subsequent generations, this will dominate the population, as 100% of the progeny of a wild-type fly survives, whereas only 75% of the progeny of a Y/+ fly survives. So our only option for keeping this line alive is to select for mutant heterozygotes (assuming we can recognize them!) and perform virgin crosses in each generation. This is so labor intensive as to be impracticable if we have lot of mutant lines.
But we have one more reliable option and here where balancer chromosomes comes to our rescue. A balancer chromosome is the one with multiple inversions, one or more dominant markers , usually 2-4 recessive markers and hence are lethal in homozygous condition.
An inversion is a chromosome rearrangement in which the chromosome has been broken twice and the medial fragment has reattached in inverted order. A chromosome sequence denoted a b c d e f might undergo an inversion to become a e d c b f.
Now if we cross our lethal recessive gene to the balancers they breed true and we don’t have to go through the laborious path of selecting heterozygotes always.
Easily detectable genetic markers :
The second desirable property of the balancer is that it should carry a clearly visible dominant marker, that allows us to follow the fate of the balancer chromosome in any cross we should subsequently choose to do. In the case of TM3, this is Sb (Stubble), which conspicuously makes the hairs on the back of the fly short and stubbly. Believe it or not, this is a relatively easy marker to score!
Drosophila has balancers for all chromosomes except fourth ,which is very small and believed to be devoid of any homologous recombination. But one can make use of marker like “eyD” and some other alleles of eyeless gene (lethal recessive ones) to keep a track of the mutation or insertion on fourth chromosome.Eyeless gene plays a very important role in eye formation of Drosophila and mutant alleles of these gene usually show a clear eye phenotype and these can be used to track fourth chromosome as balancers are not available. One can order these alleles from Bloomington stock centre.
The phenomenon of lack of recombination in male Drosophila can be used in certain genetic crosses without worrying about losing the gene of interest while in the unbalanced state.
Books for further study of Balancer chromosomes and basic Drosophila genetics:
Genome of Drosophila Melanogaster by Zimm and Lindslay
Introduction to Genetics by Sturtevant AH & Beadle GW