State of the art of the widely distributed B chromosomes of the tree frog Boana albopunctata (Anura, Hylidae)

FERRO, JUAN MARTÍN; PRETTO GATTO, KALEB; NETTO, FLAVIA; RESQUÍN, JUAN JOSÉ; COSTA, WILLIAM; MARTI, D. A.; LOURENÇO, LUCIANA; BALDO, DIEGO

Congreso; 4th B Chromosome Conference; 2019

BackgroundBoana albopunctata is a Neotropical frog that is widely distributed in the central and eastern region of South America. In this frog, variations in the standard chromosome number of 2n = 22 were previously described due to the presence of B chromosomes. The main goal of this work was to summarize our current knowledge about the B chromosomes of B. albopunctata. In order to gain information about the supernumeraries of this particular frog, and about anuranB chromosome in general, we studied these elements from different approaches.Materials and MethodsWe have cytogenetically analyzed 350 specimens of Boana albopunctata collected in different localities of Argentina, Brazil, and Paraguay. We characterized the meiosis and mitosis of B chromosomes, describing in a preliminary way their behavior, banding patterns, and population prevalence. With the aim of evaluating the possible origin of these elements, chromosome painting was carried out with B probes obtained from microdissected chromosomesResultsMost specimens of this species had a standard karyotype with 2n = 22, and near 29% showed 1 to 3 B chromosomes. Supernumeraries were present in almost all populations with a clinal pattern of variation in their prevalence, with frequencies ranging from 5 to 50%. In the sampled geographical range (ca. 1000 km), almost all Bs were similar in morphology (metacentric), size (smaller than the smaller pair), and in the DAPI fluorochrome pattern. Chromosome paintingwith a B probe showed exclusive hybridization to Bs, confirming apparent homology between elements of different populations, although subtle differences in size and C-bands among Bs of particular populations were detected. During the early stages of meiosis, these elements were differentiable to standard or A chromosomes due to their pyknotic pattern, being positive in prophase I and negative in metaphase I. Moreover, these elements segregate early without dividing their sister chromatids, and without generating gross negative effects on the endophenotype of the cell carrying them.ConclusionsOur results suggest the occurrence of a common morph of a B chromosome in Boana albopunctata that is widely distributed among populations, representing thus far the most broadly ranged B chromosome among anurans. The chromosome microdissection and painting confirmed further homology between Bs and a common origin for these elements. Likewise, the remarkable differences observed in their population prevalence, reveal our current ignorance about their biology, remaining to uncover the possible causes of that promoted this feature.