Males carrying Dsim\C(1;Y)A13 and no free Y chromosome are fertile. Study of the viability of hybrids between D.melanogaster and D.simulans demonstrates the absence of the D.simulans X chromosome in the hybrid genome, and not the presence of the D.simulans Y chromosome, is a determinant of the hybrid inviability.
Compound-XY chromosome that consists of almost the entire X and Y chromosomes.