4th chromosome balancers

As explained in An Introduction to Balancers, X, second and third chromosomes must have both multiple inversions and recessive lethal or sterile mutations to work effectively as balancers. Since meiotic crossovers normally do not occur between fourth chromosomes, inversions are not necessary for a fourth chromosome to act as a balancer. Any fourth chromosome with a recessive lethal or sterile mutation can serve as a balancer for another fourth chromosome.

The recessive lethal, fourth chromosome mutations that work best for balancing other mutations are those that are also associated with easily scored phenotypes. Here we have listed a few convenient classes of recessive lethal mutations.

  • Dominant visible mutations

    ci[D] and ey[D] have dominant wing vein and eye phenotypes, respectively. They are the two mutations used most often for balancing other fourth chromosome mutations. Their phenotypes can be easy to score in some stocks, but they tend to get suppressed upon long-term culture. It is often necessary to outcross and rebuild stocks to recover strong ci[D] and ey[D] phenotypes.

    ci[Ce-2] gives obvious ocellar and ocellar bristle phenotypes and is less suppressible, but, for no apparent reason, it has not been used much for making stocks.

    T(3;4)Antp[W] is associated with dominant deformed eye and antennal phenotypes with excellent penetrance and expressivity and could be used as a fourth chromosome balancer.

    T(1;4)B[S] is associated with a severe Bar-eyed (B[S])  phenotype and can be used in crosses to follow the inheritance of fourth chromosomes. It is not, however, recessive lethal or sterile, so it cannot be used for making stable stocks. It’s not particularly useful as a balancer, but it’s convenient in some crossing schemes.

  • Dominant mutations that aren’t as useful as you might think

    sv[spa-Cat]bt[D] and Scn[1] are dominant, but their phenotypes usually have weak penetrance and expressivity, so they are less useful than other mutations.

    Mutations in RpS3A or deletions removing it are associated with moderate-to-strong, dominant Minute phenotypes, but heterozygotes have erratic viability and sterility.

  • Mutations associated with GFP expression

    P{ActGFP}unc-13[GJ] is a ubiquitously expressed GFP marker that is nice for making stocks where balancer-bearing flies need to be identified in preadult stages. The construct also carries miniwhite, which allows balancers to be followed in w[-] backgrounds.

    All Mi{ET1} insertions carry the 3XP3-GFP marker, which results in green eyes and ocelli. We have several lethal insertions.

    The attP target insertion M{3xP3-RFP.attP}ZH-102D expresses RFP in eyes and ocelli and phiC31-mediated insertions into it also show fluorescence. It is homozygous viable and fertile, so it is useful for following fourth chromosomes in crosses, but not for creating stable, balanced stocks.

  • w[+] and y[+] markers

    Lethal or sterile w[+]- or y[+]-marked transposon insertions can be useful for balancing mutations in w[-] or y[-] mutant backgrounds.

    For w[+]-marked insertions, consider PBac{5HPw[+]}P{GSV1}P{GawB} and P{lacW} stocks. Some fourth chromosome deletions and homologous replacement alleles are also associated with miniwhite insertions.

    For y[+]-marked insertions, consider PBac{3HPy[+]}Mi{MIC} and PBac{HpaI-GFP.A}  insertions.

    For w[+]- and y[+]-marked insertions, consider P{SUPor-P} and P{EPgy2} insertions.

    For w[+]- and GFP-marked insertions, consider P{Act-GFP}unc-13[GJ] (see Mutations associated with GFP expression above).

  • Cautionary notes about fourth chromosome crosses

    Flies with three fourth chromosomes are viable, fertile, healthy and visibly indistinguishable from flies with the usual two chromosomes. Consequently, diplo-4 gametes are not uncommon in crosses and can lead one to misinterpret complementation results.

    Likewise, flies with a single fourth chromosome are viable. Because the fourth chromosome carries the Minute gene RpS3A, a fly with a single fourth chromosome will show Minute phenotypes. They will have macrochaetae one-half to two-thirds the normal length and thinner than usual, their bodies will be slightly smaller than usual, and they will be less viable and fertile. Haplo-4 progeny typically appear at a rate of ~1 in 5,000 due to spontaneous meiotic nondisjunction.

Related links  Balancers   All Blue Balancers