Received: 6 August, 2008. Accepted: 7 August, 2009. Invited Review International Journal of Plant Developmental Biology ©2010 Global Science Books Male Gametophyte Development and Evolution in Extant Gymnosperms Danilo D. Fernando 1* • Christina R. Quinn 1 • Eric D. Brenner 2 • John N. Owens 3 1 Department of Environmental and Forest Biology, State University of New York College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse, NY 13210, USA 2 The New York Botanical Garden, 200 th St. and Kazimiroff, Bronx, NY 10458, USA 3 69/35 Sukhumvit Rd., Dtumbon Na Jomtein, Amphur Sattahip, Chonburi 20250, Thailand Corresponding author: * dfernando@esf.edu ABSTRACT The male gametophytes of gymnosperms are characterized by the diversities in pollen morphology, cellular composition and pattern of cell division, pollen tube morphology, sperm delivery, growth pattern through the ovule and nucellus, and pollen tube wall composition both within and among the four living orders, i.e., Cycadales, Ginkgoales, Coniferales, and Gnetales. At dehiscence, gymnosperm pollen grains contain a variable number of cells yet none have sperm at this stage. Pollen germination in the ovule usually occurs within a few hours or days in gnetophytes, about a week or so in conifers and Ginkgo, or after several months in cycads. Complete development of the male gametophytes typically involves two to five mitotic divisions. Evolution of the male gametophyte appears to have involved a reduction of its component cells with prothallial cells being among those reduced or eliminated. There is a shift in the site of sperm discharge from a proximal position in pollen grains of cycads and Ginkgo to distal in conifers and gnetophytes. Two methods of sperm delivery occur in gymnosperms: zooidogamy, defined by pollen tubes with motile sperm as exhibited in cycads and Ginkgo, and siphonogamy, defined by pollen tubes with non-motile sperm which are directly delivered into the egg as exhibited in conifers and gnetophytes. Different pollen tube morphologies occur in the nucellus, i.e., branched and haustorial in cycads and Ginkgo, and unbranched and non-haustorial in conifers and gnetophytes. Pollen tubes form heterotrophic relationships with the nucellus, but it is only in cycads that intracellular penetration results in significant destruction of the nucellus. Pollen tube walls of gymnosperms contain cellulose and arabinogalactan proteins; however, pectins are prevalent in cycads and mixed -glucan in Ginkgo. A standard terminology to describe the cellular composition of the male gametophytes in gymnosperms is proposed. _____________________________________________________________________________________________________________ Keywords: pollen grain, pollen tube, siphonogamy, sperm, sexual reproduction, zooidogamy CONTENTS INTRODUCTION........................................................................................................................................................................................ 47 EVOLUTION AND CLASSIFICATION OF GYMNOSPERMS................................................................................................................ 48 MORPHOLOGY AND POLARITY OF POLLEN GRAINS ...................................................................................................................... 49 CELLULAR COMPOSITION OF POLLEN GRAINS ............................................................................................................................... 50 POLLEN GERMINATION AND SPERM FORMATION........................................................................................................................... 55 GROWTH OF POLLEN TUBES IN THE OVULE .................................................................................................................................... 57 COMPOSITION OF INTINE AND POLLEN TUBE WALL...................................................................................................................... 58 MADS-BOX GENES AND REPRODUCTIVE DEVELOPMENT ............................................................................................................ 59 DIRECTION OF GYMNOSPERM MALE GAMETOPHYTE RESEARCH ............................................................................................. 60 ACKNOWLEDGEMENTS ......................................................................................................................................................................... 60 REFERENCES............................................................................................................................................................................................. 60 _____________________________________________________________________________________________________________ INTRODUCTION The appearance of pollen grains is a key innovation in seed plant evolution. Before pollen grains, fertilization was restricted to a wet environment that was required for the sperm to swim to the archegonia containing the egg cell. This mode of fertilization still occurs in land plants such as bryophytes, lycophytes and pteridophytes. This innovation of the male gametophyte increased the success of fertiliza- tion in seed plants by eliminating the chances and hazards associated with the aquatic transfer of sperm (Gifford and Foster 1989). Within the gymnosperms, and seed plants in general, two methods of sperm delivery occur: zooidogamy and siphonogamy. In zooidogamous seed plants such as the cycads and Ginkgo, the pollen tubes function as haustoria, i.e., multiple branches emerge at their distal end that penet- rate the nucellus to absorb water and nutrients for the deve- lopment of the sperm. At their proximal end, the pollen tube releases flagellated sperm into a specialized chamber in the ovule from which the sperm subsequently swim to the egg cell (Friedman 1993; Poort et al. 1996; Rudall and Bateman 2007). The fossil record also provides several examples of zooidogamy in extinct gymnosperms (Poort et al. 1996; Nishida et al. 2003, 2004; Leslie 2008). The discovery of the zooidogamous mode of fertilization in Ginkgo biloba (Hirase 1896) and Cycas revoluta (Ikeno 1896) linked gymnosperms with pteridophytes, which is considered as one of the seminal biological discoveries in the 19 th century. In conifers and gnetophytes, pollen tubes elongate from the nucellus to an archegonium (or female gametophyte in Gnetum and Welwitschia) and deliver nonflagellated sperm directly into the egg (siphonogamy). The shift in the posi- ®