Evolutionary Relationships among <b><i>Boulengerella</i></b> Species (Ctenoluciidae, Characiformes): Genomic Organization of Repetitive DNAs and Highly Conserved Karyotypes

Abstract

Ctenoluciidae is a Neotropical freshwater fish family whose representatives are known as bicudas. The genus <i>Boulengerella </i>contains 5 species, and 4 of them (<i>B. cuvieri</i>, <i>B. lateristriga</i>, <i>B. lucius</i>, and <i>B. maculata</i>) were cytogenetically analyzed in the present study by conventional and molecular procedures. All 4 species have a very similar karyotype, with 2n = 36 chromosomes (14 metacentrics + 16 submetacentrics + 6 subtelocentrics; FN = 72). However, the heterochromatin distribution pattern is species-specific. In all 4 species, the nucleolus organizer region is located in pair 18, as also confirmed by cytogenetic mapping of 18S rDNA. In turn, 5S rRNA genes are present in 2 chromosome pairs: in pair 1 of all 4 species, and in pair 10 of <i>B. lateristriga</i>, <i>B. maculata, </i>and<i> B. cuvieri</i>, but in pair 4 of <i>B. lucius</i>. The telomeric probe highlighted terminal regions in all chromosomes, as well as an interstitial centromeric sequence in pair 3 of the 3 first-mentioned species. Notably, a conspicuous heteromorphic secondary constriction in chromosomes 18 was found only in the males of the 3 species, rendering one of the homologs much larger than the other one. This feature, associated with a large 18S rDNA block and accumulation of telomeric sequences, suggests the presence of an XX/XY sex chromosome system in the analyzed <i>Boulengerella </i>species.