The Multiply Inverted Balancer Chromosome FM7

(A) Schematic diagram of the generation of chromosomal inversions from X to FM7. Color represents the regions that are involved in inversions, and arrows are used to indicate the orientation of each region with respect to the centromere. Circles represent the centromeres. The numbers below each structure represent positions of the standard polytene map.

(B) A hypothetical structure displaying the pairing relations of the two homologs in an FM7/X heterozygote.

(C) A picture of DAPI-stained chromosome at meiotic metaphase I in FM7/X oocytes (courtesy of W. Gilliland). The two X and 4th chromosomes are positioned between the two autosomal bivalents and the poles, with the dot-like 4th chromosomes located closer to the poles. The FM7 chromosome (denoted by the brighter DAPI staining at its tip) is located between the autosomes and the upper pole, while the normal sequence X chromosome lies in the lower half of the spindle. Note that the X and FM7 chromosomes are not connected by chiasmata.