In recent years, phylogenetic studies based on morphological and molecular data have resulted in modifications in the systematic and phylogenetic relationships of Serpentes (Zaher, 1999; Vidal et al., 2000, 2010; Zaher et al., 2009; Grazziotin et al., 2012; Pyron et al., 2013). The monophyletic tribe Xenodontini (Vidal et al., 2000, 2010; Grazziotin et al., 2012) is one of the South American snake radiations comprising about 70 species (Uetz and Jirí, 2013). According to the classification considered the clade is included in the subfamily Xenodontinae into Dipsadidae (Zaher et al., 2009) or in the subfamily Dipsadinae into Colubridae (Pyron et al., 2013).
The Xenodontini (sensu Vidal et al., 2010 and Grazziotin et al., 2012) is composed by the genera Lygophis Fitzinger 1843, Erythrolamprus Boie 1826 and Xenodon Boie 1826, clade also recover in other phylogenetic analysis (Zaher et al., 2009; Pyron et al., 2013; see nomenclatural discussion in Curcio et al., 2009). The genus Lygophis was resurrected by Zaher et al. (2009), and confirmed by other studies (Vidal et al., 2010; Grazziotin et al., 2012; Pyron et al., 2013). Lygophis comprises eight species grouped in the ‘anomalus’ and the ‘lineatus’ morphological groups; with three and five species, respectively (Dixon, 1985, Michaud and Dixon, 1987).
The cytogenetics of the Xenodontini is poorly known. Chromosomal data are restricted to six Erythrolamprus species and four Xenodon species (Beçak, 1968; Beçak and Beçak, 1969; Beçak et al., 1971, 1975; Gutiérrez et al., 1984). Karyological information has been obtained using conventional cytological staining protocols. Localization of the nucleolar organizer regions (NORs) was only carried out on E. poecilogyrus schotti (Trajtengertz et al., 1995).
Although information is scarce, the Xenodontini appear to be a karyologically diverse tribe. In fact, four diploid numbers (2n = 28, 30, 32, 34), eight karyotype formulas, the ZW sex determination system, and a remarkable intra- and inter-generic karyotypic variability were described by previous studies (Beçak, 1968; Beçak and Beçak, 1969; Beçak et al., 1971, 1975; Gutiérrez et al., 1984). In Erythrolamprus, one karyotype 2n = 32 and five karyotypes 2n = 28 which differ in micro- and macro-chromosome number and macrochromosomes morphology have been found (Beçak and Beçak, 1969; Beçak et al., 1971, 1975; Gutiérrez et al., 1984). Three species of Xenodon have the same diploid number of 2n = 30 including 14 or 16 macrochromosomes and microchromosomes, while X. rabdocephalus (2n = 34) exhibits 22 macro-chromosomes and a reduced micro-chromosome complement of 12 (Beçak, 1968; Beçak and Beçak, 1969; Beçak et al., 1971; Gutiérrez et al., 1984).
Cytogenetic characters can be used to infer evolutionary relationships if they are analysed together with other independent characters (morphological, molecular, immunological, isozyme) (Sites and Reed, 1994). Indeed, such an approach has been demonstrated to be important to understand the diversity and evolution in snakes (Oguiura et al., 2009; Mezzasalma et al., 2014).
Hitherto no cytological characters were known for the genus Lygophis. To fill this information vacuum, we documented the karyotype and the location of Ag-NORs in four species: L. dilepis Cope, 1862, L. meridionalis (Schenkel, 1902) and L. flavifrenatus Cope, 1862 (all belonging to the ‘lineatus’ group sensu Michaud and Dixon, 1987) and L. anomalus (Günther, 1858) (belonging to the ‘anomalus’ group sensu Dixon, 1985).