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ORIGINS OF HETEROSPORY AND THE SEED HABIT: THE ROLE OF HETEROCHRONY

ORIGINS OF HETEROSPORY AND THE SEED HABIT: THE ROLE OF HETEROCHRONY,William A. DiMichele,Jerrold I. Davis,Richard G. Olmstead

ORIGINS OF HETEROSPORY AND THE SEED HABIT: THE ROLE OF HETEROCHRONY   (Citations: 12)
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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 endo- spory may have arisen paedomorphically from homosporous ancestors, specifically by progenesis in the gametophyte generation. Heterospory and gametophytic unisexuality are not necessarily evolu- tionary 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.
Published in 1989.
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