How Big Is Your Y? A Genome Sequence-Based Estimate of the Size of the Male-Specific Region inMegaselia scalaris

The scuttle fly, Megaselia scalaris, is often cited as a model in which to study early sex chromosome evolution because of its homomorphic sex chromosomes, low but measurable molecular differentiation between sex chromosomes, and occasional transposition of the male-determining element to different chromosomes in laboratory cultures. Counterintuitively, natural isolates consistently show sex linkage to the second chromosome. Frequent natural transposition of the male-determining element should lead to the loss of male specificity of any nontransposed material on the previous sex-linked chromosome pair. Using next-generation sequencing data from a newly obtained natural isolate of M. scalaris, we show that even highly conservative estimates for the size of the male-specific genome are likely too large to be contained within a transposable element. This result strongly suggests that transposition of the male-determining region either is extremely rare or has not persisted recently in natural populations, allowing for differentiation of the sex chromosomes of this species.