The Search for Flower Homeotic Gene Homologs in Basal Angiosperms and Gnetales: A Potential New Source of Data on the Evolutionary Origin of Flowers

Abstract

The evolutionary origin of flowering plants has long been contentious. The large morphological gap between flowering plants and their potential gymnosperm relatives makes homology difficult to assess. Uncertainty at the base of the angiosperm clade prevents firm reconstruction of plesiomorphic flower characters. The recent discovery of homeotic genes that specify flowers and flower organs raises the possibility of a new class of evidence bearing on flower origins. Homeotic genes may give strong evidence on homology. Sequence changes or events related to morphological evolution may help resolve the base of the flowering plant tree. This article reports the creation of resources to facilitate isolation of homeotic and other genes from taxa critical to flowering plant origins: we have made 16 genomic DNA libraries of 15 species, including Gnetales (Welwitschia [two libraries], Gnetum [two species], and Ephedra) and basal angiosperms (Nymphaea, Peperomia, Magnolia, Illicium, Drimys, Cinnamomum, Trochodendron, and Platanus), as well as an advanced monocot (Juncus) and two species of the advanced dicot Heliotropium. Sequences of the first genes cloned from these libraries, a LEAFY homolog from Welwitschia and one from Gnetum, along with recently released pine sequences, demonstrate that a paralogous duplication of LEAFY predated the divergence of Coniferales and Gnetales. LEAFY is not always single copy in diploids but has persisted as paralogs for a long interval.