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Seed Production and Pollen Vectors in Several Nectarless Plants

Michael N. Melampy and Anita M. Hayworth
Evolution
Vol. 34, No. 6 (Nov., 1980), pp. 1144-1154
DOI: 10.2307/2408295
Stable URL: http://www.jstor.org/stable/2408295
Page Count: 11
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Seed Production and Pollen Vectors in Several Nectarless Plants
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Abstract

Evidence is presented to show that the breeding systems of Thalictrum dioicum, T. polygamum, and T. clavatum represent two different adaptive radiations from an ancestral system similar to that found in Isopyrum biternatum. Isopyrum biternatum is an entomophilous, self-compatible species in which outcrossing is encouraged by protogyny. As a food resource, I. biternatum is not preferred by insect pollen vectors, probably because it lacks nectar. Consequently, in years when late springs shorten the flowering season, seed production may be significantly reduced. The three Thalictrum species rely less on insect pollinators and do not experience the low and unpredictable seed set experienced by I. biternatum. One alternative to insect-pollination is wind-pollination, which occurs in T. dioicum and T. polygamum. For wind-pollination to evolve, however, several problems must be overcome. Wind-pollination is most effective in unobstructed habitats, although T. polygamum does produce seed in heavily obstructed areas by the additional use of insect pollinators. Thalictrum dioicum, which is exclusively wind-pollinated, may compensate for obstructions by producing very large quantities of pollen. Another reliable mechanism for ensuring pollination is autogamy. However, this tactic sacrifices the genetic effects of outbreeding. The autogamous T. clavatum is a habitat specialist, which may be associated with the loss of the genetic diversity necessary to exploit a variety of environments. Individual genotypes have apparently become fine-tuned to specific conditions, which is adaptive if subsequent generations can find similar conditions for establishment and growth. Thalictrum clavatum, due to its proximity to streams, maximizes its seed output and, hence, its chances for colonizing suitable habitats by producing many small seeds that can be dispersed by water.

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