Micromorphology and Character Evolution of Nutlets in Tribe Mentheae (Nepetoideae, Lamiaceae)

Abstract

Mentheae is an economically important tribe in Lamiaceae with many ornamentals and culinary herbs. Although its monophyly is well supported by molecular data, the subtribal delimitation and the intergeneric relationships are still poorly understood. The nutlet morphology (SEM) of the tribe and the presence/absence of myxocarpy is documented here for 57 out of 65 genera in order to assess the systematic value of nutlet characters and to evaluate the existing molecular phylogenies for this group. Significant variation was found in nutlet shape, morphology of the abscission scar, distribution of trichomes, and surface sculpture. Five different surface types were recognized in Mentheae based on differences in cell outline, anticlinal walls, cell boundary relief, and curvature of the outer periclinal wall. Representative nutlet features proved to be phylogenetically informative in Mentheae mainly at the generic level. Cladistic analyses were performed based on 15 morphological nutlet characters and/or available molecular data (nuclear ribosomal ITS and chloroplast trnL-trnF spacer sequences). The resulting topologies indicate that the addition of nutlet characters to the DNA-data matrix improved the overall resolution and also increased bootstrap support values in comparison with exclusively molecular phylogenetic analyses. The evolution of major nutlet characters within the tribe Mentheae is discussed.