Chromosome-level assemblies of the Pieris mannii butterfly genome suggest Z-origin and rapid evolution of the W chromosome

The insect order Lepidoptera (butterflies and moths) represents the
largest group of organisms with ZW/ZZ sex determination. While the origin
of the Z chromosome predates the evolution of the Lepidoptera, the W
chromosomes are considered younger, but their origin is debated. To shed
light on the origin of the lepidopteran W, we here produce
chromosome-level genome assemblies for the butterfly Pieris
mannii, and compare the sex chromosomes within and between P. mannii and
its sister species P. rapae. Our analyses clearly indicate a common origin
of the W chromosomes of the two Pieris species, and reveal similarity
between the Z and W in chromosome sequence and structure. This supports
the view that the W in these species originates from Z-autosome fusion
rather than from a redundant B chromosome. We further demonstrate the
extremely rapid evolution of the W relative to the other chromosomes and
argue that this may preclude reliable conclusions about the origins of W
chromosomes based on comparisons among distantly related Lepidoptera.
Finally, we find that sequence similarity between the Z and W chromosomes
is greatest toward the chromosome ends, perhaps reflecting selection for
the maintenance of recognition sites essential to chromosome segregation.
Our study highlights the utility of long-read sequencing technology for
illuminating chromosome evolution.