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Once a heteromorphic system is established, the sex chromosome system may remain stable for an extended period. Here, we show the opposite case of sex chromosome evolution from a heteromorphic to a homomorphic system in the Japanese frog Glandirana rugosa. One geographic group, Neo-ZW, has ZZ-ZW type heteromorphic sex chromosomes. We found that its western edge populations, which are geographically close to another West-Japan group with homomorphic sex chromosomes of XX-XY type, showed homozygous genotypes of sex-linked genes in both sexes. Karyologically, no heteromorphic sex chromosomes were identified. Sex-reversal experiments revealed that the males were heterogametic in sex determination. In addition, we identified another similar population around at the southwestern edge of the Neo-ZW group in the Kii Peninsula: the frogs had homomorphic sex chromosomes under male heterogamety, while shared mitochondrial haplotypes with the XY group, which is located in the east and bears heteromorphic sex chromosomes. In conclusion, our study revealed that the heteromorphic sex chromosome systems independently reversed back to or turned over to a homomorphic system around each of the western and southwestern edges of the Neo-ZW group through hybridization with the West-Japan group bearing homomorphic sex chromosomes.<\/jats:p>\n                  <jats:p>This article is part of the theme issue \u2018Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)\u2019.<\/jats:p>","DOI":"10.1098\/rstb.2020.0105","type":"journal-article","created":{"date-parts":[[2021,7,25]],"date-time":"2021-07-25T19:05:32Z","timestamp":1627239932000},"update-policy":"https:\/\/doi.org\/10.1098\/crossmark-policy","source":"Crossref","is-referenced-by-count":20,"title":["Sex chromosome evolution from a heteromorphic to a homomorphic system by inter-population hybridization in a frog"],"prefix":"10.1098","volume":"376","author":[{"given":"Mitsuaki","family":"Ogata","sequence":"first","affiliation":[{"name":"Preservation and Research Center, City of Yokohama 1 ,"},{"name":"155-1 Asahi Ward, Yokohama 241-0804, Japan 1 ,"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Kazuo","family":"Suzuki","sequence":"additional","affiliation":[{"name":"Hikiiwa Park Center 2 ,"},{"name":"1629 Inari-cho, Tanabe 646-0051, Japan 2 ,"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Yoshiaki","family":"Yuasa","sequence":"additional","affiliation":[{"name":"Himeji City Aquarium 3 ,"},{"name":"440 Nishinobusue, 670-0971 Himeji, Japan 3 ,"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3564-2882","authenticated-orcid":true,"given":"Ikuo","family":"Miura","sequence":"additional","affiliation":[{"name":"Amphibian Research Center 4 , ,"},{"name":"Hiroshima University 4 , ,"},{"name":"1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan 4 , ,"},{"name":"Institute for Applied Ecology 5 , ,"},{"name":"University of Canberra 5 , ,"},{"name":"Canberra, ACT 2601, Australia 5 , ,"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"175","published-online":{"date-parts":[[2021,7,26]]},"reference":[{"key":"2026010320213138700_RSTB20200105C1","doi-asserted-by":"crossref","first-page":"R736","DOI":"10.1016\/j.cub.2006.08.021","article-title":"Relationships between vertebrate ZW and XY sex chromosome systems","volume":"16","author":"Ezaz","year":"2006","journal-title":"Curr. 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