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. 2005 Apr 19;102(16):5892-7.
doi: 10.1073/pnas.0501985102. Epub 2005 Apr 4.

Life history biology of early land plants: deciphering the gametophyte phase

Thomas N Taylor  1 , Hans Kerp, Hagen Hass
Affiliations

Life history biology of early land plants: deciphering the gametophyte phase

Thomas N Taylor et al. Proc Natl Acad Sci U S A. .

Abstract

The ca. 400-million-year-old Rhynie chert biota represents a benchmark for studies of early terrestrial ecosystems. The exquisite preservation of the organisms documents an ancient biodiversity that also includes various levels of biological interaction. Absent from the picture until recently has been detailed information about the development of the gametophyte phase and the alternation of generations of the macroplants in this ecosystem. Here, we trace the development of the gametophyte phase of Aglaophyton, an early land plant with an unusual complement of structural and morphological characters. Mature gametophytes consist of a fleshy protocorm attached to the substrate by basal rhizoids; arising from the upper surface are one to several upright gametangiophores bearing multiple gametangia. Stomata are present on the upper surface of the protocorm and gametangiophore, and endomycorrhizal fungi extend throughout the gametophyte. Gametophytes are unisexual, producing either antheridiophores or archegoniophores. There is no evidence that gametophytes later become hermaphroditic. The sexual dimorphism of the Rhynie chert gametophytes is inconsistent with theoretical ideas about the haploid phase of early land plants. The gametophyte phase of early land plants can now be considered within an ecological and evolutionary framework that, in turn, can be used to develop hypotheses about some aspects of the population dynamics and growth of these early land plants.

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Figures

Fig. 1.
Fig. 1.
Life history of A. major/L. rhyniensis showing stages in the development of the dimorphic gametophytes. The mature sporophyte (lower left) bears sporangia with spores of two types. Blue spores develop into mature antheridiophores; orange spores develop into archegoniophores.
Fig. 2.
Fig. 2.
Rhynie chert plants. (A–L) Lyonophyton. (P) Horneophyton. (O) Remyophyton.(Q) Aglaophyton.(A) Spore before germination with conspicuous trilete mark. (Scale bar, 15 μm.) (B) Early stage in gametophyte development showing initial transverse division (arrow). (Scale bar, 15 μm.) (C) Two-celled stage showing formation of a cross wall (arrow). (Scale bar, 13 μm.) (D) Multicellular gametophyte (arrow) protruding from broken spore coat. (Scale bar, 15 μm.) (E) Polar view of multicellular gametophyte showing arrangement of cells just beneath apical cell. (Scale bar, 15 μm.) (F) Gametophyte showing apical cell (upper arrow) and spore coat remnants (lower arrow). (Scale bar, 15 μm.) (G) Stoma on lower surface of gametangiophore. (Scale bar, 40 μm.) (H) Cortical cells with endomycorrhizal hyphae in intercellular spaces (arrows). (Scale bar, 50 μm.) (I) Cortical cells of gametangiophore containing arbuscules. (Scale bar, 12 μm.) (J) Longitudinal section of several conducting elements. (Scale bar, 7 μm.) (K) Longitudinal section of distal end of antheridiophore with two antheridia (arrows). (Scale bar, 2 mm.) (L) Longitudinal section of antheridium. (Scale bar, 100 μm.) (M) Section of antheridium showing sperm packets. (Scale bar, 5 μm.) (N) Longitudinal section of archegonium showing neck canal (upper arrow) and probable egg chamber (lower arrow). (Scale bar, 40 μm.) (O) Dense cluster of gametophytes showing size of antheridiophores and archegoniophores. (Scale bar, 6.5 mm.) (P) Spore aggregate. (Scale bar, 1.0 mm.) (Q) Bulbil (arrow) on vegetative axis. (Scale bar, 1.0 mm.)
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