Cypselae in Dahlia and Hidalgoa (Asteraceae: Coreopsideae): anatomical and morphological differences

Hidalgoa is a Neotropical genus in the tribe Coreopsideae (Asteraceae) comprising four species. Species of this genus were found to be embedded in a large supported clade within Dahlia. This genus includes forty species, most of which are endemic to Mexico. A number of morphological vegetative, floral, pollen, and fruit characters differ between the taxa of Dahlia and Hidalgoa. This study compares morphological and anatomical characters of the cypselae in Dahlia and Hidalgoa, identifying variations of potential taxonomic value to assess whether they should be treated as independent taxa. Cypselae of two species of Hidalgoa and fifteen species of Dahlia were analyzed using optical and scanning electron microscopy. In Hidalgoa, phytomelanin is positioned in the space between the outer and median mesocarp, forming a continuous smooth layer. A multiplicative pericarp was also identified in Hidalgoa, a rare feature in Asteraceae, in which the parenchyma cells of the mesocarp are thickened and sclerified at maturity. Vascular bundles in Hidalgoa are observed only in the ribs, and the largest awns of cypselae were identified in these species. In contrast, the cypselae in Dahlia displayed morphological uniformity, with the typical pericarp of the tribe Coreopsideae; theonly variation in Dahlia is that a few species possess exocarp cells with well-developed secondary walls. The phytomelanin layer in Dahlia is discontinuous and spiny. The anatomical attributes of cypselae in Hidalgoa are similar to those of taxa in other tribes or alliances in Asteraceae. These characters can be considered together with molecular and palynological data to further our understanding of whether Hidalgoa should be recognized as taxonomically distinct from Dahlia.