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Abstract

Chromosome evolution presents an enigma in the mega-diverse Lepidoptera. Most species exhibit constrained
chromosome evolution with nearly identical haploid chromosome counts and chromosome-level gene collinearity
among species more than 140 million years divergent. However, a few species possess radically inflated chromosomal
counts due to extensive fission and fusion events. To address this enigma of constraint in the face of an
exceptional ability to change, we investigated an unprecedented reorganization of the standard lepidopteran
chromosome structure in the green-veined white butterfly (Pieris napi). We find that gene content in P. napi has
been extensively rearranged in large collinear blocks, which until now have been masked by a haploid chromosome
number close to the lepidopteran average. We observe that ancient chromosome ends have been maintained and
collinear blocks are enriched for functionally related genes suggesting both a mechanism and a possible role for selection in determining the boundaries of these genome-wide rearrangements.