Patterns of chromosomal evolution in the florally diverse Andean clade Iochrominae (Solanaceae)

Abstract

Iochrominae is a largely Andean clade known for its remarkable diversity of floral forms and colors. Although knowledge of chromosomal changes can provide insights into the processes underlying speciation, such data in Iochrominae are scant. We performed cytogenetic analyses to characterize chromosome number and morphology, CMA/DAPI heterochromatic bands, and distribution of rDNA sites in Iochrominae. Ancestral karyotypes were reconstructed on a newly-estimated molecular phylogeny in order to test congruence between karyotype evolution and clade differentiation. We found that, compared with its closest relatives, Iochrominae comprises species with highly symmetrical karyotypes, with no changes in base chromosome number. The common ancestor of Iochrominae was inferred to be a diploid with 2n = 24, with a karyotype with 0–2 submetacentric chromosomes and the rest metacentric, an arm ratio ca. 1.30, one locus of 45S or NORs, and one locus of 5S. Using phylogenetic comparative methods, we estimated the number of changes for these chromosomal traits, and found the highest for 5S loci. Patterns of character change are largely homoplastic, although combinations of traits can be useful to identify groups within Iochrominae. Asymmetry was the only character that allow us to differentiate this clade among its relatives. Overall, our study suggests that the diversification of Iochrominae has not been accompanied by the formation of strong chromosomal barriers, which may help to explain the crossability of many species and even genera within the group.