Genetica 121: 75–80, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. 75 A novel ZZ/ZW sex chromosome system for the genus Leporinus (Pisces, Anostomidae, Characiformes) Paulo Cesar Venere 1 , Irani Alves Ferreira 1 , Cesar Martins 2 & Pedro Manoel Galetti Jr. 3 1 Instituto de Ciências e Letras do M´ edio Araguaia, Universidade Federal de Mato Grosso, 78698-000 Pontal do Araguaia, MT (Phone/Fax: +55-66-4011470; E-mail: pvenere@uol.com.br); 2 Departamento de Morfologia, Instituto de Biociências, Universidade Estadual Paulista, 18618-000 Botucatu, SP; 3 Departamento de Gen´ etica e Evolução, Universidade Federal de São Carlos, 13565-905 São Carlos, SP, Brazil Received 14 April 2003 Accepted 23 August 2003 Key words: chromosome, fish, Leporinus, sex chromosomes, ZZ/ZW system Abstract A wide range of sex chromosome mechanisms, including simple and multiple chromosome systems is character- istic of fishes. The Leporinus genus represent a good model to study sex chromosome mechanisms, because an unambiguous ZZ/ZW sex chromosome system was previously described for seven species, while the remaining studied species of the genus do not show differentiated sex chromosomes. The occurrence of sex chromosomes in Leporinus trifasciatus and Leporinus sp2 from the Araguaia river, Amazon basin, Brazil, was here investigated. ZZ/ZW sex chromosomes were detected for both species. The Z and W chromosome morphology of L. trifasciatus is the same as described for other species of the genus Leporinus. However, the Z and W chromosomes of L. sp2 were quite different in their morphology and banding pattern suggesting that the ZW system of this species have originated independently from the ZW system previously described for other Leporinus. Introduction Cytological advances over the last few years have improved the information on fish chromosomes. Con- sidering sex differentiation, fish are characterized by remarkable variability of sex-determination mech- anisms, including polyfactorial systems, male and female heterogamety, hermaphroditism, and environ- mental sex determination (Yamamoto, 1969; Conover, Van Voorhes & Ehtisham, 1992). While XX/XY and ZZ/ZW sex chromosome systems are characteristic for mammals and birds, respectively, eight types of sex chromosome systems have already been described for fishes (Moreira-Filho, Bertollo & Galetti, 1993; Almeida Toledo & Foresti, 2001). Extensive karyotype diversity has been reported in the neotropical fish fauna and several studies have demonstrated the occurrence of sex chromosomes in different groups, especially in freshwater spe- cies. Different male or female heterogamety can be found among related species (Centofante, Bertollo & Moreira-Filho, 2002). A XX/XY sex chromosome system has been described for four neotropical fresh- water fish species, representing four different orders (Almeida Toledo & Foresti, 2001). On the other hand, a ZZ/ZW system has been described in several families of: (i) Characiformes, such as Anostom- idae (Galetti et al., 1981b; Galetti, Lima & Venere, 1995), Characidae (Bertollo & Cavallaro, 1992), Prochilodontidae (Feldberg et al., 1987) and Parodon- tidae (Moreira Filho, Bertollo & Galetti, 1993); (ii) Siluriformes, such as Loricariidae (Andreatta et al., 1993; Artoni, Venere & Bertollo, 1998); (iii) Cyp- rinodontiformes, such as Poecilidae (Haaf & Schmid, 1984). Also, three different multiple sex chromosome mechanisms occur for several species of neotropical fish fauna, including X 1 X 1 X 2 X 2 /X 1 X 2 Y, XX/XY 1 Y 2 and ZZ/ZW 1 W 2 systems (Centofante, Bertollo & Moreira-Filho, 2002). Although several sex chromo- some systems appear in fish, most species do not show differentiated sex chromosomes. The diversity of mechanisms that appears in fish, even within some