Seed Plant Phylogeny and the Relationships of Gnetales

Abstract

Most phylogenetic analyses of morphological data agree that Gnetales are a monophyletic group related to angiosperms and Bennettitales. However, they disagree on whether these groups (anthophytes) are related to coniferopsids or to Mesozoic seed ferns, and thus on whether the flowers of Gnetales are primitively simple or reduced. Molecular analyses indicate that both Gnetales and angiosperms are monophyletic but disagree on their relationship. The conclusion of Nixon et al. (1994) that Gnetales are paraphyletic, with angiosperms nested within them, is weakly supported; when several questionable embryological characters are redefined in a neutral manner, Gnetales are inferred to be monophyletic Jurassic reproductive structures associated with linear leaves and ephedroid pollen (Piroconites) consist of a bract and a scalelike sporophyll covered with Welwitschia-like microsynangia or ovules, recalling the bractsporophyll complex of glossopterids. An analysis of seed plants incorporating these fossils and other new data links Gnetales with Piroconites, angiosperms with Caytonia, and both groups (plus Bennettitales and Pentoxylon) with glossopterids, making up a clade called the glossophytes. These results imply that glossophytes originally had glossopterid-like leaves and bract-sporophyll complexes, which were transformed into carpels with bitegmic ovules in angiosperms, but reduced to single, terminal ovules in Gnetales; flowers arose independently in the two lines. The common ancestor of angiosperms and Gnetales may be as old as Permian, and some of their shared advances, such as double fertilization (without endosperm formation), may have arisen as adaptations to seasonal temperate climates in Gondwana.