In butterflies, sex is determined by chromosome differences between males and females. But unlike in humans with the familiar X and Y, in butterflies, it is the females that determine the sex of offspring.
They do so by either passing along either their Z (male) or W (female) chromosomes. Males are ZZ, while females are ZW. This ZW pattern is also prevalent in birds, some fish, and insects like butterflies. Similar to XY pairs, ZW pairs are different from each other in their shape and gene content: The Z chromosome is larger and has many genes, while the W consists mainly of repetitive DNA.
Sex chromosomes typically evolve from a pair of autosomes. After acquiring sex determining genes, W-chromosomes often degenerate and lose genes (just like the male Y in humans). Beatriz Vicoso and her team are interested in how females compensate for the loss of genetic information, in a broad biological phenomenon called dosage compensation. In this case, either the genes get up-regulated in females (since they have only one Z) or down-regulated in ZZ males.
But to date, prior studies have painted an incomplete picture, with some species having complete compensation of the Z while others seemingly less so.
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