Figure 1.—

Schematic of D. virilis, D. americana, and F₁ hybrid chromosomes. For D. virilis and D. americana females, X chromosomes and autosomes are labeled (order is the same for males and hybrids shown below). The Y and dot chromosomes are represented by hooked bars and small squares, respectively. The X–4 and 2–3 chromosomal fusions of D. americana are represented by connected bars (each fusion forms a backward “L”). AV refers to an F₁ hybrid with D. americana as the maternal parent, whereas VA refers to an F₁ hybrid with D. virilis as the maternal parent. Note that D. americana males carry one unfused chromosome 4. Only in one direction of the interspecific cross (VA) does the F₁ male inherit two unfused copies of chromosome 4, which allows the independent assortment of this chromosome in backcrosses.