Mezzasalma M., F. Andreone, F. Glaw, F.M. Guarino, G. Odierna, A. Petraccioli & O. Picariello

Changes in heterochromatin content and ancient chromosome fusion in the endemic Malagasy boid snakes Sanzinia and Acrantophis (Squamata: Serpentes). pp. 140-144.

Abstract. Chromosome changes may follow or precede genetic divergence, promoting lineage diversification at different taxonomic levels. The taxonomy (either as species or subspecies) of the Malagasy tree boas, Sanzinia madagascariensis and S. volontany,  is still debated. In order to detect and evaluate the evolutionary significance of possible karyological differences between them  as well as between Sanzinia and the closely related Malagasy ground boas of the genus Acrantophis, we conducted a comparative chromosomal analysis, using standard and C-banding methods. The two Sanzinia species have similar karyotypes of 2n = 34 chromosomes, with 12 biarmed and six uniarmed macrochromosomes and 16 microchromosomes. A. dumerili likewise had 2n = 34 chromosomes, but different numbers of macrochromosomes (M) and microchromosomes (m) (18M+16m and 16M+16m, respectively). Heterochromatin was scarce in A. dumerili, while the two species of Sanzinia exhibited fully heterochromatic arms of macrochromosome pairs 5, 6 and 9. Our results highlight that the genetic diversification between the two Sanzinia species occurred without any evident karyological modification and allow us to hypothesize an evolutionary scenario of the chromosome diversification in Malagasy boas. We demonstrate that the karyotype of A. dumerili probably originated from the hypothesized ancestral snake karyotype (2n = 36, 18M+18m) through a translocation of a microchromosome to a macrochromosome one. We also suggest that karyological changes may have had an impact on the evolutionary history of Malagasy tree and ground boas, possibly promoting intergeneric lineage diversification. In fact, the karyotypes of Sanzinia likely derived from a karyotype like that of A. dumerili by means of heterochromatin addition to a microchromosome pair and the euchromatin-heterochromatin transformation in the macrochromosome pairs 5 and 6.

Key words. Chromosome, heterochromatin transformation, karyotype, C-banding, snakes, Madagascar.