The Dryas iulia genome supports multiple gains of a W chromosome from a B chromosome in butterflies

James J Lewis*, Francesco Cicconardi*, Simon H Martin, Robert D Reed, Charles G Danko, Stephen H Montgomery*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

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Abstract

In butterflies and moths, which exhibit highly variable sex determination mechanisms, the homogametic Z chromosome is deeply conserved and is featured in many genome assemblies. The evolution and origin of the female W sex chromosome, however, re-mains mostly unknown. Previous studies have proposed that a ZZ/Z0 sex determina-tion system is ancestral to Lepidoptera, and that W chromosomes may originate from sex-linked B chromosomes. Here, we sequence and assemble the female Dryas iulia genome into 32 highly contiguous ordered and oriented chromosomes, including the Z and W sex chromosomes. We then use sex-specific Hi-C, ATAC-seq, PRO-seq, and whole genome DNA sequence datasets to test if features of the D. iulia W chromo-some are consistent with a hypothesized B chromosome origin. We show that the pu-tative W chromosome displays female-associated DNA sequence, gene expression, and chromatin accessibility to confirm the sex-linked function of the W sequence. In contrast with expectations from studies of homologous sex chromosomes, highly re-petitive DNA content on the W chromosome, the sole presence of domesticated repeti-tive elements in functional DNA, and lack of sequence homology with the Z chromo-some or autosomes is most consistent with a B chromosome origin for the W, although it remains challenging to rule out extensive sequence divergence. Synteny analysis of the D. iulia W chromosome with other female lepidopteran genome assemblies shows no homology between W chromosomes and suggests multiple, independent origins of the W chromosome from a B chromosome likely occurred in butterflies.
Original languageEnglish
JournalGenome Biology and Evolution
Volume13
Issue number7
Early online date12 Jun 2021
DOIs
Publication statusPublished - 6 Jul 2021

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Publisher Copyright:
© The Author(s) 2021. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution.

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