A natural hybrid, an autopolyploid, or a new species? An integrative case study of a distinctive <i>Costus</i> species (Costaceae) from the Atlantic Forest of Brazil

Sixteen species of <i>Costus s.s.</i> are cited to Brazil, all of which can be found in the Brazilian Amazon, while only three also grow in the Atlantic Forest: <i>C. arabicus</i>, <i>C. scaber</i>, and <i>C. spiralis</i>. Studies of the genus from the Brazilian Atlantic Forest evidenced a distinctive fourth taxon, similar overall to <i>C. spiralis</i>, which is an ornithophilous species, however the flower morphology of this new taxon indicates it to be melittophilous. Hybridization and polyploidy (allopolyploidy or autopolyploidy) may play a role as an important source for evolutionary events, especially in recent speciose lineages such as Neotropical <i>Costus</i>. Here we investigated the evolutionary origin of this interesting <i>Costus</i> species using an integrative approach. This approach comprised a morphological analysis, cytogenetic analyses (chromosome counts, CMA/DAPI staining, and fluorescent <i>in situ</i> hybridization), genome size estimations, and a molecular phylogenetic reconstruction (using the plastid regions <i>rps16</i> and <i>trnL-F</i>, and the nuclear ITS and ETS), aiming to answer the following questions: is it a hybrid?; is it an autopolyploid of <i>C. spiralis</i>?; or, if it is a different species, is it even phylogenetically related to <i>C. spiralis</i> or <i>C. arabicus</i>? The studies performed did not indicate any evidence of polyploidy, however, a chromosomal polymorphism was observed between the populations of the taxon studied. The phylogenetic position of this peculiar taxon with <i>C. erythrophyllus</i>, <i>C. spiralis</i>, and <i>C. varzearum</i> also excludes the putative partial heritage of <i>C. arabicus</i> which had been previously hypothesized. Our results confirm that it is a new, undescribed species, which is confirmed by our morphological observations of floral features. A complete taxonomic treatment of the new species is provided, including a key for the species of the genus in the Atlantic Forest.