American Journal of Botany
Pyrrhopappus geiseri and P. carolinianus are partially cross-compatible (20-52% akene set in ar-tificial crosses). Four barriers to gene exchange prevent or reduce hybridization. Pyrrhopappus geiseri flowers in the early spring, whereas P. carolinianus flowers from midspring to summer with peak flowering in May and June. Overlap in anthesis between the two species is therefore limited. The two species typically occupy different soil-types, P. geiseri on clay soils and P. carolinia,ws on sa nds. The diurnal periods of stigmatic receptivity are distinct. Pyrrhopappus geiseri is pollen receptive in the late morning and P. carolinianus at dawn. As a result of the prior three barriers P. geiseri is pollinated by bees of the Andrena verecunda complex, primarily A . crawfordi. and P. carolinianus by Hemihalictus lustrans. Pyrrhopappus geiseri and P. carolinianus are almost completely reproductively and ecologically isolated and are not members of one hologamodeme.
Barber, Susan C. and Estes, James R., "Comparative Pollination Ecology of Pyrrhopappus Geiseri and Phyrrhopappus Carolinianus" (1978). An. Paper 210.
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