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

FERRO JM; RESQUÍN JJ; LOURENÇO LB; NETTO F; MARTÍ D; PRETTO GATTO K; COSTA W; BALDO D

Botucatú

Congreso; 4th B chromosome Conference; 2019

BackgroundBoana albopunctata is a Neotropical frog that is widely distributed inthe 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 maingoal of this work was to summarize our current knowledge aboutthe B chromosomes of B. albopunctata. In order to gain informationabout the supernumeraries of this particular frog, and about anuranB chromosomes in general, we studied these elements from differentapproaches.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, andpopulation prevalence. With the aim of evaluating the possible originof these elements, chromosome painting was carried out with Bprobes obtained from microdissected chromosomes.ResultsMost specimens of this species had a standard karyotype with 2n =22, and near 29% showed 1 to 3 B chromosomes. Supernumerarieswere present in almost all populations with a clinal pattern of variation in their prevalence, with frequencies ranging from 5 to 50%. Inthe sampled geographical range (ca. 1000 km), almost all Bs weresimilar in morphology (metacentric), size (smaller than the smallerpair), 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 particularpopulations were detected. During the early stages of meiosis, theseelements were differentiable to standard or A chromosomes due totheir pyknotic pattern, being positive in prophase I and negative inmetaphase I. Moreover, these elements segregate early without dividing their sister chromatids, and without generating gross negativeeffects 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 chromosomeamong 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 populationprevalence, reveal our current ignorance about their biology, remainingto uncover the possible causes of that promoted this feature