Oplegnathus fasciatus and O. punctatus (Teleostei: Centrarchiformes: Oplegnathidae), are commercially important rocky reef fishes, endemic to East Asia. Both species present an X1X2Y sex chromosome system. Here, we investigated the evolutionary forces behind the origin and differentiation of these sex chromosomes, with the aim to elucidate whether they had a single or convergent origin. To achieve this, conventional and molecular cytogenetic protocols, involving the mapping of repetitive DNA markers, comparative genomic hybridization (CGH), and whole chromosome painting (WCP) were applied. Both species presented similar 2n, karyotype structure and hybridization patterns of repetitive DNA classes. 5S rDNA loci, besides being placed on the autosomal pair 22, resided in the terminal region of the long arms of both X1 chromosomes in females, and on the X1 and Y chromosomes in males. Furthermore, WCP experiments with a probe derived from the Y chromosome of O. fasciatus (OFAS-Y) entirely painted the X1 and X2 chromosomes in females and the X1, X2, and Y chromosomes in males of both species. CGH failed to reveal any sign of sequence differentiation on the Y chromosome in both species, thereby suggesting the shared early stage of neo-Y chromosome differentiation. Altogether, the present findings confirmed the origin of the X1X2Y sex chromosomes via Y-autosome centric fusion and strongly suggested their common origin.

Departamento de Genética e Evolução Universidade Federal de São Carlos Rodovia Washington Luiz Km 235 C P 676 São Carlos SP 13565 905 Brazil Secretaria de Estado de Educação de Mato Grosso SEDUC MT Cuiabá MT 78049 909 Brazil

Departamento de Genética e Evolução Universidade Federal de São Carlos Rodovia Washington Luiz Km 235 C P 676 São Carlos SP 13565 905 Brazil University Clinic Jena Institute of Human Genetics 07747 Jena Germany

Key Lab of Mariculture and Enhancement of Zhejiang Province Marine Fishery Institute of Zhejiang Province Zhoushan 316100 China

Key Lab of Mariculture and Enhancement of Zhejiang Province Marine Fishery Institute of Zhejiang Province Zhoushan 316100 China College of Fisheries Zhejiang Ocean University Zhoushan 316100 China

Laboratory for Marine Biology and Biotechnology Institute of Oceanology Chinese Academy of Sciences 7 Nanhai Road Qingdao 266071 China

Laboratory of Fish Genetics Institute of Animal Physiology and Genetics Czech Academy of Sciences Rumburská 89 277 21 Liběchov Czech Republic

University Clinic Jena Institute of Human Genetics 07747 Jena Germany

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