• achlorophyllous taxa;
  • Burmanniaceae;
  • Dioscoreaceae;
  • floral morphology;
  • microsporogenesis;
  • Nartheciaceae;
  • seed coat;
  • septal nectaries;
  • yams

Relationships of Dioscoreales are examined by combined analysis of three molecular data sets (plastid rbcL, atpB and nuclear 18S rDNA genes) and a morphological data set. The combined analysis corroborates a narrow circumscription of Dioscoreales, which includes Burmanniaceae and Thismiaceae in the order, and also indicates that Nartheciaceae are closely related. Dioscoreales thus comprise three distinct clades: a well-supported ‘core Dioscoreales’ clade (Dioscorea, Trichopus, Avetra, Tacca and Stenomeris), a Burmanniaceae–Thismiaceae clade and Nartheciaceae. The improvement in resolution and high bootstrap percentages found by the total evidence analysis relative to analyses of separate data sets indicates that both morphological and molecular data are crucial in resolving the relationships of Dioscoreales. Combined analysis of morphological and molecular characters is instructive in interpreting the evolution of morphological characters such as microsporogenesis and revealing characters that were previously not regarded as significant in the higher-level systematics of this group, for example stamen and hypanthium morphology. Other morphological synapomorphies in Dioscoreales include tuberous underground parts, glandular hairs, seed coat anatomy, and calcium oxalate crystals. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society, 138, 123–144.