A Tetrahedral Megaspore Arrangement in a Seed Fern Ovule of Pennsylvanian Age

Schabilion, J. T.; Brotzman, N. C.

doi:10.2307/2442419

Cited by 7 publications

(2 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Kremp, however, commented only on variation in the relationship of nutrition to aperture form in energy-storage spores. To Schabilion and Brotzman (1979), in agreement with Taylor (1965), evidence for a tetrahedral arrangement in P. vera is inconclusive, but they reported a tetrahedral tetrad for the Pennsylvanian pteridosperm, Conostoma anglogermanicum.…”

Section: Origin Of the Ovule: A New Conceptsupporting

confidence: 70%

The Origin of the Ovule

Herr

1995

American Journal of Botany

View full text Add to dashboard Cite

Of several theories for the origin of the ovule advanced in this century and based largely on fossil evidence, the telomic concept is widely considered the most plausible. Its principal tenet is the evolution of the integument through fusion of sterile branches or teiomes around a terminal megasporangium. The only point of agreement in these theories is that the entire nucellus is a megasporangium that retains a single megaspore and the endosporic female gametophyte. Their differences center on the origin of the integument. A new concept offered here on evidence from ovules of both Paleozoic and extant seed plants significantly alters the telomic theory. It proposes that the nucellus is a sporangiophore of stem origin that bears a terminal megasporangium; that at least some of the fused integumentary telomes were fertile; and that among all features cited to characterize ovules, the unique nature of the retained megaspore alone defines the seed habit. Changes in the seed plant megaspore that extended the period of nutrient absorption over the whole course offemale gametophyte development, along with complex physiological changes in the nucellus, were probably achieved along a single phylogenetic line beginning in a Late Devonian population of progymnosperms. For such a combination of events to have occurred more than once is highly unlikely, and, therefore, a monophyletic origin for seed plants is proposed. Several primitive features in ovule structure, some not evidenced since the Lower Carboniferous Period, occur in a mutant form of Arabidopsis thaliana isolated from genetically transformed plants. Their recurrence provides additional support for the proposed concept of ovule origin and also suggests that the genetic mechanisms for expression of primitive features in advanced taxa could be initiated in each case by mutation of a single homeotic gene.The ovule, a salient feature of gymnosperms and angiosperms, is traditionally described as a megasporangium or nucellus surrounded and protected by one or two integuments. Usually, the megasporangium retains a single megaspore, which through endosporic germination produces a female gametophyte. With syngamy, the mature ovule becomes an immature seed, and the seed matures as the zygote produces the embryo. Seeds thus represent the endpoint ofovule ontogeny. Previous studies on ovule origin and the beginning ofthe seed habit, focused largely on evidence from fossil gymnosperms and arborescent lycopods, usually apply the terms ovule and seed interchangeably, since a clear distinction is seldom revealed. Considerable variation in the interpretation of fossil evidence has led to several theories for ovule origin, but an unquestioned advocacy for anyone over all others has not appeared. This report will recount past theories for ovule origin, examine the presumptions for some interpretations ofthe fossil record, and present evidence from developmental studies of extant seed plants previously not considered for this issue. The preponderance of evidence presented supports a...

show abstract

Section: Origin Of the Ovule: A New Conceptsupporting

confidence: 70%

The Origin of the Ovule

Herr

1995

American Journal of Botany

View full text Add to dashboard Cite

show abstract

“…In all heterosporous pteridophytes meiosis of the megaspore mother-cell results in a tetrahedral tetrad of megaspores, whereas in most seed plants the megaspores form a linear tetrad. The only evidence for tetrahedral tetrads in seed plants occurs in Devonian taxa (e.g., Archaeospena, Pettitt and Beck, 1968) and lyginopterid seed ferns (Pettitt, 1969;Schabilion and Brotzman, 1979). On this basis Lyginopteris is scored as plesiomorphic for this character.…”

Section: Linear Tetrad Ofmegasporesmentioning

confidence: 99%

Phylogenetic Relationships in Seed Plants

Crane

1985

Cladistics

View full text Add to dashboard Cite

Abstract— The phylogenetic relationships of nineteen extant and fossil seed plants are considered. Analysis of 31 characters produced ten topologically similar and equally parsimonious cladograms. A strict consensus tree derived from these cladograms places Lyginopteris as the sister taxon to the other seed plants included. Within this clade all the taxa considered, except medullosans and cycads, form a single monophyletic group defined by the presence of flattened seeds and saccate pollen (“platy‐sperms”). Relationships between medullosans, cycads, and “platysperms” were not resolved, but within the “platysperm” clade conifers and cordaites (Cordaixylon, Mesoxylon) + Ginkgo form a monophyletic group (“coniferophytes”). The “higher platysperms” (glossopterids, Caytonia, corystosperms, Bennettitales, Pentoxylon, Gnetales, and angiosperms) are also monophyletic, but their relationship to “coniferophytes,” peltasperms, and Callistophyton is unresolved. Pentoxylon is placed as sister taxon to the Bennettitales, and together they form the sister group to a clade in which Gnetales and angiosperm are sister taxa. The Bennettitales + Pentoxylon + Gnetales + angiosperms (“anthophytes”) form a monophyletic sister group to the corystosperms. This analysis is compared with current classifications of seed plants. It does not support a close relationship between Bennettitales and cycads, it provides no evidence for seed plant polyphyly, and it strongly suggests that the current concept of seed ferns has little value in a phylogenetic context.

show abstract

Morphologic variability of Pennsylvanian lyginopterid seed ferns

Taylor

Millay

1981

Review of Palaeobotany and Palynology

View full text Add to dashboard Cite

A Tetrahedral Megaspore Arrangement in a Seed Fern Ovule of Pennsylvanian Age

Cited by 7 publications

References 0 publications

The Origin of the Ovule

The Origin of the Ovule

Phylogenetic Relationships in Seed Plants

Morphologic variability of Pennsylvanian lyginopterid seed ferns

Contact Info

Product

Resources

About