Pollen Diversity and Exine Evolution in Viburnum and the Caprifoliaceae Sensu Lato

Abstract

Studies of pollen development and function (Heslop-Harrison, 1971), along wiih correlations between pollen morphology, incompatibility system, and mode of pollination (DeNettancoun, 1977; Lee, 1978; Plitmann & Levin, 1983), have prompted speculation about the adaptive significance of pollen characteristics (Heslop-Harrison, 1976, 1979; Lewis, 1977). At the same time considerable attention has been devoted to the related but logically sepárate task of determining the actual course of pollen evolution, with special emphasis on the exine (Erdtman, 1966; Walker & Doyle, 1975; Ferguson & Muller, 1976; Nowicke & Skvarla, 1979). A particular hypothesis about pollen evolution is usually established by considering pollen diversily in the context of presumed relationships. Pollen characters are mapped onto a classification (which is usually based on many other kinds of characters), and the most plausible sequence of evolutionary events for the pollen is established in this context. This procedure is basically sound, but clearly the results obtained can be no better than the hypothesis of relationships employed. Unfortunately, relation- ships have not yet been rigorously established for most plant groups, and present classifications do not always reflect these accurately or unambiguously. Thus, although considerable progress has been made in tracing the course of pollen evolution, the level of resolution has not always been very satisfactory.