Developmental Evidence for the Convergence ofSassafras(Laurales) andHeliconia(Zingiberales) Pollen

Open Access
Publisher Website
Google Scholar
Abstract
Sassafras, a dicotyledon, and Heliconia, a monocotyledon, both have inaperturate pollen grains with a highly reduced exine and thick channeled intine. This general pollen type has apparently evolved independently in at least 18 orders of angiosperms. The ontogenetic basis for this widespread convergence in pollen structure is simply a reduction or deletion of exine and a correlated elaboration of intine deposition. In Sassafras and Heliconia several developmental differences are superimposed on the overall ontogenetic similarities. The ektexine, which stains with auramine O and fluoresces under UV, is limited to the formation of spinules in both genera, but the details of spinule development differ. Spinule development is initiated in the tetrad stage, but not completed until the early free spore stage in Sassafras, whereas Heliconia spinules do not change appreciably from the form taken in the tetrad stage. The presence of gemmae in Sassafras and their absence in Heliconia is another distinctive sporoderm difference that is indicative of independent origins. The minute gemmae dotting the interspinule surface of Sassafras are endexinous in origin and develop during the free spore stage; Heliconia, with a pattern typical of monocotyledons, fails to elaborate endexine and the pollen surface between spinules is essentially smooth. The intine in both genera is similar. The thick exintine develops radially-oriented channels and the relatively thin endintine is compact. An hypothesis is presented that views thinning of the exine as the most important aspect of the pollen wall evolution and intine thickening as a compensatory specialization.