We show that the geophilomorph centipede Strigamia maritima possesses an XX/XY system of sex chromosomes, with males being the heterogametic sex. This is, to our knowledge, the first report of sex chromosomes in any geophilomorph centipede. Using the recently assembled Strigamia genome sequence, we identified a set of scaffolds differentially represented in male and female DNA sequence. Using quantitative real-time PCR, we confirmed that three candidate X chromosome-derived scaffolds are present at approximately twice the copy number in females as in males. Furthermore, we confirmed that six candidate Y chromosome-derived scaffolds contain male-specific sequences. Finally, using this molecular information, we designed an X chromosome-specific DNA probe and performed fluorescent in situ hybridization against mitotic and meiotic chromosome spreads to identify the Strigamia XY sex-chromosome pair cytologically. We found that the X and Y chromosomes are recognizably different in size during the early pachytene stage of meiosis, and exhibit incomplete and delayed pairing.This work was in part funded by Wellcome Trust (wellcome.ac.uk) Ph.D. studentship WT089615MA to JEG. Cytological experiments by MD and FM were funded by Grant IAA600960925 of the Grant Agency of The Czech Academy of Sciences (until 2013; gaav.cz) and by Grant 14- 22765S of the Czech Science Foundation (since 2014; gacr.cz). KS was supported by JSPS Excellent Young Researchers Overseas Visit Program (21– 7147; jsps.go.jp).This is the final version of the article. It first appeared from the Public Library of Science (PLOS) via https://doi.org/10.1371/journal.pone.015029
