Karyotypic stability in the paleopolyploid genus Ceiba Mill. (Bombacoideae, Malvaceae)

The genus Ceiba Mill. belongs to the subfamily Bombacoideae (Malvaceae), a paleopolyploid lineage characterized by numerous small chromosomes, which has frequently been reported to have variable intra- and interspecific chromosome numbers. The genus is of Miocene origin, representing a suitable model for studying the relationships between chromosome variability and paleopolyploidy. A comparative cytogenetic analysis of five Ceiba species was undertaken to determine their karyotype variability. New chromosome number counts, chromosome morphological observations, CMA/DAPI double staining, and in situ hybridization (FISH) with 5S and 45S rDNA were performed for five Ceiba species, which represent three out of five main lineages of the genus. Karyotypic data were discussed in the light of molecular phylogenies available for the group. All species showed 2n = 86 and similar karyotypes, composed predominantly of metacentric chromosomes. Two pairs of CMA+/DAPI− bands were located on the short arms of the metacentric chromosomes. The CMA+ bands colocated with 45S rDNA sites, while the 5S rDNA sites were situated in the interstitial regions of other chromosome pairs. Contrary to the intra- and interspecific chromosome number diversity reported for Ceiba species in the literature, our findings suggest chromosome number stability in four of the five lineages within the genus for which data are available. Moreover, our data suggest that karyotypes are evolutionary conserved in the three lineages for which we generated new karyotypic data. Our data, as well as recent cytogenetic reviews of other Bombacoideae genera, indicate numerical stability for these taxa suggesting that counting errors, especially in earlier research, may have overestimated karyotype variability. Chromosome count errors can be attributed to technical difficulties associated with high chromosome numbers, and/or the reduced size of Bombacoideae chromosomes.