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Abstract Detail



Developmental and Structural Section

FRIEDMAN , WILLIAM E [1].

Fertilization in Welwitschia mirabilis: it Doesn't Get Any Weirder (or Apomorphic)!.

Almost twenty-five years ago, a regular process of “double fertilization” that yielded two diploid zygotes was discovered and documented in species of Ephedra and Gnetum (Gnetales). In Ephedra, double fertilization involves the two nuclei from a binucleate sperm cell in a pollen tube and the egg and ventral canal nucleus within a single archegonium. In Gnetum, which lacks archegonial structure in its female gametophyte, double fertlization involves the two sperm nuclei from a binucleate sperm cell in a pollen tube and a pair of nuclei from the female gametophyte during the syncytial stage of development. The question of whether a similar process of gnetalean double fertilization occurs in Welwitschia remained unanswered. Female and male gametophyte development, the fertilization process, and early embryo growth and differentiation were examined in Welwitschia mirabilis, the monotypic genus that is sister to Gnetum. In Welwitschia, fertilization occurs 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 (tetrasporic) female gametophyte that grow upwards through the nucellus. Entry of a binucleate sperm cell into the highly vacuolate prothallial tube appears to stimulate the rapid coagulation of cytoplasm around a single female nucleus, which then serves as an egg cell. One sperm nucleus enters the female gamete, while the second sperm nucleus and tube nucleus remain just outside the female gamete and ultimately degenerate. Thus, Welwitschia lacks the regular gnetalean pattern of double fertilization found in Ephedra and Gnetum. Double fertilization events in Ephedra and Gnetum are almost certainly evolutionarily homologous, having been inherited from a common ancestor of the Gnetales. A phylogenetically-based analysis of character evolution within the Gnetales indicates that the absence of regular double fertilization events in Welwitschia is the result of a loss of the gnetalean pattern of double fertilization in this highly derived taxon. The initiation of embryos within the nucellus and distant from the embryo-nourishing tissues of the female gametophyte may, in turn, provide a suggestion as to why the production of two embryos from each pollen tube (gnetalean double fertilization) may have been lost in Welwitschia.


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1 - Harvard University, Organismic and Evolutionary Biology, Arnold Arboretum of Harvard University, 1300 Centre Street, Boston, Massachusetts, 02131, United States

Keywords:
none specified

Presentation Type: Oral Paper:Papers for Sections
Session: 25
Location: Pines North/Boise Centre
Date: Tuesday, July 29th, 2014
Time: 11:30 AM
Number: 25006
Abstract ID:469
Candidate for Awards:None


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