Zoidogamy in Fossil Gymnosperms: the Centenary of a Concept, with Special Reference to Prepollen of Late Paleozoic Conifers RUUD J
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Proc. Natl. Acad. Sci. USA Vol. 93, pp. 11713-11717, October 1996 Evolution Zoidogamy in fossil gymnosperms: The centenary of a concept, with special reference to prepollen of late Paleozoic conifers RUUD J. POORT*, HENK VISSCHER*, AND DAVID L. DILCHERt *Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, 3584 CD, Utrecht, The Netherlands; and tPaleobotany Laboratory, Florida Museum of Natural History, University of Florida, Gainesville, FL 32611 Contributed by David L. Dilcher, August 2, 1996 ABSTRACT This year is the centenary of the surprising because it serves as a haustorial organ to digest the tissues of discovery in 1896 of zoidogamy in extant cycadophytes and the nucellus and provide nutrients for the developing motile Ginkgo. But by coincidence, also in the same year, the concept antherozoids. ofprepollen was introduced. The morphology ofprepollen was Similar to those in extant cycadophytes and Ginkgo, the considered justification for the probable production of motile common presence of pollen chambers in ovules of late Paleo- antherozoids in extinct gymnosperms. In this paper, the zoic gymnosperms already had been well documented (5). history of the prepollen concept is briefly outlined. It is Following Renault's observations, the study of the fossilized emphasized that, in addition to well-known examples in content of pollen chambers received considerable attention. pteridosperms and cordaitaleans, a prepollen condition also Supposedly multicellular structures in pollen grains and a occurred among late Paleozoic conifers. variety of small bodies were interpreted as fossil evidence for the production of antherozoids (6-8). Indirectly, but more Toward the end of the 19th century, it was generally assumed convincingly, zoidogamy was suggested on the basis of the that seed plants, both extant and extinct, were uniformly morphology of the pollen wall. Following studies by Kidston characterized by siphonogamy. This fertilization mechanism (9), it gradually was realized that some pteridosperms pro- involves formation of a pollen tube, a tubular outgrowth of the duced pollen with the overall morphological organization of microgametophyte through which immotile gamete cells or many isospores or microspores of pteridophytes. A common nuclei are delivered to the archegonia. The mechanism is feature is the presence of a proximal aperture. In modern fundamentally different from zoidogamy, the fertilization by heterosporous pteridophyte spores, this aperture splits open, means of motile antherozoids, which can be observed in permitting the protrusion of the antheridia-bearing gameto- spore-bearing plants. phyte. This is true also in heterosporous lycopsids, which As early as 1887, the possibility of zoidogamy in extinct release internally produced antherozoids through a proximal gymnosperms was mentioned by the French palaeobotanist aperture (10). Hence, functional interpretation of similar Renault (1), but this view attracted little attention. Then in apertures in fossil gymnospermous pollen suggested zoid- 1896, a century ago this year, the assumption of uniform ogamy (11) siphonogamy in gymnosperms was definitively shown to be a false concept. By remarkable coincidence, the presence of Prepollen zoidogamy was independently discovered in both recent and in fossil plants in the same year. At present, fossil gymnospermous pollen characterized by proximal apertures and absence of distal specializations indic- Zoidogamy in Gymnosperms ative of the production of a pollen tube is known as prepollen. The prepollen concept is intimately linked with zoidogamy. In 1896, the Japanese botanists Hirase and Ikeno published This year, we are at the centenary of the concept. In 1896, their classic notes on the discovery of the production of Renault coined the termprepollinies for large pteridospermous antherozoids in Ginkgo biloba (2) and Cycas revoluta (3). In the and cordaitalean pollen, characterized by a supposed multi- same year, Renault published the text of his monumental work cellular microgametophyte and assumed to be indicative of on Carboniferous and Permian plant fossils from France (4). zoidogamy (4). He considered this pollen to be intermediate By studying pollen grains present in pollen chambers of between pteridophyte spores and cycadophyte pollen, al- pteridospermous and cordaitalean ovules, Renault interpreted though he was unaware of the discoveries of Hirase and Ikeno. the supposedly internal cellular structure of the pollen as a The termprepollinies was used infrequently until it was revived multicellular microgametophyte. He hypothesized that these and anglicized by Schopf (12, 13). cells could have produced motile antherozoids rather than In the 1970s, the prepollen concept was elaborated by developing pollen tubes. Chaloner (14), and further comments were contributed by The discoveries of Hirase and Ikeno surprised the interna- Jonker (15, 16). More importantly, basic morphological and tional botanical community and had an immediate and tre- ultrastructural information became available, confirming the mendous impact on the comparative analysis of reproductive presence offunctional proximal sutures in prepollen, including biology in gymnosperms. Fertilization by means of motile the huge (up to 0.5-mm-long) pollen types of medullosalean antherozoids soon became universally recognized as the fun- pteridosperms for which Renault had used the term prepolli- damental reproductive strategy of all extant cycadophyte nies (17). As a result, prepollen is currently defined as "the genera, as well for Ginkgo. Zoidogamy provided a functional microspores of certain extinct seed plants characterized by explanation for the presence of the pollen chamber, long since proximal apertures and presumed proximal germination, known in the ovules of these taxa. The function of the pollen rather than the distal, equatorial or other typical apertures of tube in these taxa had to be regarded as exclusively nutritive, seed plant pollen grains" (18). Prepollen thus represents pollen that had not yet developed the capacity to produce The publication costs of this article were defrayed in part by page charge pollen tubes (Fig. la). payment. This article must therefore be hereby marked "advertisement" in In the definition of prepollen, emphasis is given to the accordance with 18 U.S.C. §1734 solely to indicate this fact. functional interpretation ofthe proximal aperture in the pollen 11713 Downloaded by guest on September 29, 2021 11714 Evolution: Poort et al. Proc. Natl. Acad. Sci. USA 93 (1996) a b zoids are released by wall decay, in the fossil pollen zoidogamy is supported morphologically by the presence of a preformed suture in the proximal wall (refs. 23 and 24; Fig. lb). c.... A more difficult problem to solve is the functional inter- **; pretation of fossil pollen characterized by the presence of a leptoma but without any indication of a proximal aperture. X...... Such pollen grains are likely to have produced pollen tubes, but it remains impossible to decide whether they represent zoid- ogamous or siphonogamous plants. Even the unique discovery of a fossil pollen grain with a well-preserved branched pollen tube (25) remains inconclusive. This particular grain is regu- larly cited to be indicative for siphonogamy in the late Paleo- zoic. However, other criteria, such as the presence of a pollen chamber in the corresponding ovules, strongly suggests zoid- ogamy. It should be realized that the origin of pollen tubes and siphonogamy are separate evolutionary steps (24, 26). An additional problem in the interpretation of apertures in fossil gymnosperm pollen. is the possibility of the presence of distinctive apertures (proximal, distal, or equatorial) that are FIG. 1. Scheme of zoidogamous (a and b) and siphonogamous (c) not related to pollen tube formation or the release of anthero- fertilization strategies of conifer pollen; outer pollen wall omitted. (a) zoids, but to harmomegathy, i.e,. the process by which pollen Prepollen condition; release of motile antherozoids through proximal grains change in shape to accommodate variations in the aperture; no outgrowth of pollen tube; relatively large size (late Paleozoic). (b) Pollen grains with proximal release of motile anthero- volume of the cytoplasm caused by changing hydration. zoids; distal outgrowth of a haustorial pollen tube with an exclusively As far as prepollen is concerned, a wealth of morphological nutritive function (late Paleozoic and early Mesozoic). (c) Pollen and ultrastructural data is now available for a variety of pollen grains with distal outgrowth of a pollen tube functioning as a carrier types, found in situ in polleniferous organs of lyginopteridalean for immotile gamete cells or nuclei to migrate to the archegonium and medullosalean pteridosperms (17, 27-42). Although not (Mesozoic to Recent). always explicitly cited as prepollen, these studies firmly con- firm a prepollen condition in the Lyginopteridales and Medul- wall rather than to multicellular structures. At present, there losales. Pollen of other pteridosperms is frequently character- is a strong and justified reluctance to accept early records of ized by only a distal aperture. This also applies to pollen of a multicellular microgametophytes in some (pre)pollen. Notably number of cordaitalean taxa. However, a prepollen condition in permineralized material, effects of fossilization are a likely is also evident in some cordaitalean pollen types, such as the explanation for pseudocellular