title: Development and evolution of the female gametophyte and fertilization process in Welwitschia mirabilis (Welwitschiaceae).
abstract: OBJECTIVE
  The female gametophyte of Welwitschia has long been viewed as highly divergent from other members of the Gnetales and, indeed, all other seed plants. However, the formation of female gametes and the process of fertilization have never been observed.
  
  
  METHODS
  Standard histological techniques were applied to study gametophyte development and the fertilization process in Welwitschia.
  
  
  RESULTS
  In Welwitschia, fertilization events occur when pollen tubes with binucleate sperm cells grow down through the nucellus and encounter prothallial tubes, free nuclear tubular extensions of the micropylar end of the female gametophyte that grow up through the nucellus. Entry of a binucleate sperm cell into a vacuolate prothallial tube appears to stimulate the rapid coagulation of cytoplasm around a single female nucleus, which differentiates into an egg cell. One sperm nucleus enters the female gamete, while the second sperm nucleus remains outside and ultimately degenerates. Only a single fertilization event occurs per mating pair of pollen tube and prothallial tube.
  
  
  CONCLUSIONS
  Welwitschia lacks the gnetalean pattern of regular double fertilization, as found in Ephedra and Gnetum, involving sperm from a single pollen tube to yield two zygotes. Moreover, an analysis of character evolution indicates that the female gametophyte of Welwitschia is highly apomorphic both among seed plants, and specifically within Gnetales, but also shares several key synapomorphies with its sister taxon Gnetum. Finally, the biological role of prothallial tubes in Welwitschia is examined from the perspectives of gamete competition and kin conflict.
authors: Department of Organismic and Evolutionary Biology, 26 Oxford Street, Harvard University, Cambridge, Massachusetts 02138 USA; and Arnold Arboretum of Harvard University, 1300 Centre Street, Boston, Massachusetts 02131 USA.
authors: William E
authors: WE
authors: Friedman
mesh_terms: D005075:Biological Evolution; D005306:Fertilization; D055993:Germ Cells, Plant / Q000254:growth & development*; D035682:Gnetophyta / Q000235:genetics*; D010802:Phylogeny; D063245:Plant Development; D053205:Pollen Tube; D054817:Pollination
keywords: Gnetales; Welwitschia; Welwitschiaceae; double fertilization; embryo; evolutionary history; gametophyte; seed plants