First cytogenetic characterization of Pseudoloricaria laeviuscula (Valenciennes, 1840): a monotypic genus of Loricariidae (Loricariinae)

ABSTRACT This study presents the first cytogenetic characterization of Pseudoloricaria laeviuscula, a species widely distributed in the Amazon and Tocantins-Araguaia basins. Individuals collected from the Negro River, Amazonas, Brazil, exhibited a diploid chromosome number (2n) of 54, with a karyotypic formula of 14m + 10sm + 4st + 26a and a fundamental number of 82. The nucleolus organizer region (NOR) was interstitially located on a pair of subtelocentric chromosomes, confirmed via FISH using the 18S rDNA probe. Constitutive heterochromatin is preferentially located in centromeric regions, and 5S rDNA was mapped pericentromerically in a single acrocentric pair. The telomeric probe exclusively mapped the terminal regions of the chromosomes. The cytogenetic study revealed plesiomorphic characteristics of the Loricariidae family, such as the diploid number, NOR location and heterochromatin distribution. However, the localization of 18S and 5S rDNA sites on distinct chromosomes represents a derived feature. Comparative cytogenetic analysis of species in the Loricariichthys group indicates the occurrence of non-Robertsonian chromosomal rearrangements. Thus, the data from the present study expand our knowledge of the genus Pseudoloricaria and can serve as cytotaxonomic markers for a better understanding of the Loricariichthys group and its relationships within the subfamily Loricariinae.