Origins of heterospory and the seed habit: the role of heterochrony

The origin of the seed-plant life history, and subsequent diversification of seed morphology, should be considered two independent, fundamentally distinct evolutionary problems. Gametophytic endospory may have arisen paedomorphically from homosporous ancestors, specifically by progenesis in the game...

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Bibliographic Details
Published in:	Taxon 1989-02, Vol.38 (1), p.1-11
Main Authors:	Dimichele, W.A, Davis, J.I, Olmstead, R.G
Format:	Article
Language:	eng
Subjects:	Biological and medical sciences Ecological life histories Evolution Evolutionary theories Fundamental and applied biological sciences. Psychology Gametophytes heterospory history life cycle (organisms) Megaspores origin Ovules phenotype Plant cytology, morphology, systematics, chorology and evolution Plant evolution Plant morphology Plants seed morphology seed plant Sporophytes Vascular plants
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Summary:	The origin of the seed-plant life history, and subsequent diversification of seed morphology, should be considered two independent, fundamentally distinct evolutionary problems. Gametophytic endospory may have arisen paedomorphically from homosporous ancestors, specifically by progenesis in the gametophyte generation. Heterospory and gametophytic unisexuality are not necessarily evolutionary antecedents of endospory; rather, they may have arisen as developmental consequences of endospory. Once seeds existed, their early pattern of diversification was not one of gradual change in form, but of a proliferation of integumentary types differing widely from one another. Subsequently, the morphological spectrum narrowed, with closed integuments eventually prevailing. This two-stage pattern of diversity suggests an initial phenotypic breakthrough into a largely "empty" adaptive zone, followed by intensified biotic competition as the adaptive zone filled.
ISSN:	0040-0262 1996-8175