Steven, Janet C. , Penny, Rebecca H. .
Sexual dimorphism in pollen grain size and flower number varies among populations in a cryptically dioecious plant, Thalictrum macrostylum (Ranunculaceae).
Thalictrum macrostylum (Ranunculaceae) is a rare herbaceous plant endemic to the southeastern United States. We visited four populations of this species in North Carolina to describe its breeding system, quantify secondary sexual dimorphism in flower number, and explore the ecological basis of dimorphism. Although flower morphology suggests that the species is androdioecious, field observations at four sites across North Carolina and subsequent scanning electron micrographs of pollen grains showed that "hermaphrodite" plants produce pollen lacking in pores, and the species is cryptically dioecious. We observed sexual dimorphism in pollen grain size in one population; females produced larger pollen grains than males. In addition, variation in pollen grain size was greater in females in this population. Sexual dimorphism in flower production was also variable among populations; flower number was equal in the two sexes at two study sites but males produced more flowers on average at the other two sites. The two populations with equal male and female flower number also had greater light levels, suggesting that light may be a limiting resource for females but not males. Within one of these populations, greater soil organic matter was positively correlated with flower production in males but not females, suggesting that nitrogen is more limiting in males. Thus, our study found preliminary evidence that sexual dimorphism in flower number in this species has an underlying ecological causation. Males and females may occupy slightly different ecological niches, resulting in dimorphism in some resource environments but not others. A planned common-garden study will investigate this pattern further.
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1 - Sweet Briar College, Department of Biology, Guion Science Center, Sweet Briar, VA, 24595, USA
Presentation Type: Oral Paper:Papers for Topics
Location: Lake Ontario/Hilton
Date: Tuesday, July 10th, 2007
Time: 4:15 PM