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Developmental and Structural Section

Buzgo, Matyas [2], Soltis, Pamela S. [1], Soltis, Douglas E. [2], Soltis, Douglas E. [2].

Floral developmental morphology of Amborella (Amborellaceae), the sister to other extant angiosperms.

Floral development in the monotypic genus Amborella, the sister to all other extant angiosperms, involves a series of gradual transitions among adjacent floral organs, obscuring clear domains of floral organ identities. Even the transition from bracts to tepals is gradual, making it difficult to determine exactly where a flower begins in this species. However, bracts can clearly be distinguished from floral organs by their axillary meristem. The transition from tepals to stamens or carpels in the unisexual flowers is likewise gradual, and features shared by tepals, stamens, and carpels identify their structural homology. Although flowers of Amborella are described as having a spiral phyllotaxy, developmental studies indicate that phyllotaxy is sometimes ambiguous. In the outer tepals phyllotaxy actually approaches a whorl. This observation, coupled with observations of spiral and whorled phyllotaxy in other basal angiosperms (e.g., Drimys, Ceratophyllum, Nuphar), further demonstrates the flexibility of early floral development in basal angiosperms, with some taxa not fully committed to either spiral or whorled phyllotaxy. These results for Amborella suggest that some floral characters should be coded as continuous characters rather than discrete in studies of floral evolution in basal angiosperms. Furthermore, morphology suggests a gradual transition of B-class gene function in a centripetal direction throughout the entire flower, in agreement with patterns of B-class gene expression in flowers of Amborella. Extending the concept of “shifting boundaries” for B-class genes to other genes and gynoecium, patterns of gradual transitions among floral organs have prompted a “fading borders” view of gene expression, in which each organ identity gene is broadly expressed across the floral meristem but o­nly weakly at the outer and inner limits of its expression. This model applies to Amborella and perhaps other basal angiosperms that also exhibit gradual transitions among floral organs.

Related Links:
Trends in Plant Sciences
International Journal of Plant Sciences
Floral Genome Project

1 - University of Florida, Department of Natural History, Florida Museum of Natural History, P.O. Box 117800, Gainesville, Florida, 32611-7800, USA
2 - University of Florida, Department of Botany, 220 Bartram Hall, P.O. Box 118526, Gainesville, Florida, 32611-8526, USA

floral development
floral evolution
Floral morphology
Floral o­ntogeny
transcription factors

Presentation Type: Paper
Session: 1-2
Location: Alpine A (Snowbird Center)
Date: Monday, August 2nd, 2004
Time: 8:15 AM
Abstract ID:461

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