Phylogeny and Morphology of Seed Coat and Fibrillar Mucilage of Polemoniaceae

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Katherine Holt and Dr. Leigh Johnson, Botany and Range Sciences

The Phlox family (Polemoniaceae) has been the focus of many studies regarding plant speciation and diversification. Comprising just 379 species in 26 genera, considerable interest exists in understanding the phylogenetic relationships in the Phlox family to further increase its value as a model system for exploring diversification patterns (Johnson, 1996; Porter, 1996; Grant, 1998; Porter and Johnson, 2000; Prather, 2000). Although DNA-based phylogenetic hypotheses have been proposed, few morphological features have been described in sufficient detail to allow them to be incorporated into explicit phylogenetic analyses. Although a large number of character surveys in this family have been published, thorough analysis of seed morphology has been overlooked despite the use of this character in taxonomic keys and the significant differences observed among species in seed germination requirements. Some of the earliest classifications in this family distinguished species by whether or not their seeds appear mucilaginous upon wetting. In 1907, Brand elaborated on this characteristic and proposed four different categories of seeds: (from Grant, 1959)

More recently in 1978, Chuang surveyed external seed coat morphology in the genus Collomia with electron microscopy, but found little variation compared to large differences in pollen grain morphology. However in contemporary studies, pronounced differences in seed morphology are apparent between Collomia and species of Saltugilia (L. Johnson, personal communication). Furthermore, Schnopf and Deichgraber’s (1983) examination of the nature of the seed mucilage in Collomia grandiflora produced results that conflict with the categories proposed by Brand. Because an accurate description of seed morphology is needed before its taxonomic or ecological significance can be evaluated, I proposed to survey dry and hydrated seeds from a variety of Polemoniaceae species. This survey was planned to explicitly test the hypotheses of morphological categories proposed by Brand, and to provide the first family-wide survey of seed surface morphology useful for phylogenetic analysis.

For my Botany 530 course project, I began developing techniques for analysis of the differences in Polemoniaceae seeds upon wetting and the structure of mucilage or fibers that are produced. I used the preparation methods of Schnepf and Deichgraber (1983), which involved 2-48hr hydration, gluteraldehyde fixation, dehydration in an acetone series followed by critical point drying, and surveyed six species, mainly within Collomia. Some images were obtained but our results mainly illustrated the need for further development in methodology. So my full efforts were turned to the survey of dry seed coat morphology, in order to prepare a poster to present at the American Society of Plant Taxonomy (ASPT) meetings held in August 2001.

Examining dry seed surface morphology was straightforward, but extensive. Dry seeds were attached to aluminum stubs with double sided tape and sputter coated with gold for 2-3 minutes, before examining the seeds on Brigham Young University’s SEM. I examined multiple seeds from almost 40 species of representative genera throughout Polemoniaceae, at standard sizes including whole seed, 500x, 1000x, 2000x, 5000x, and 10,000x magnification, recording over 835 digital images. Analysis and comparison of macro, supercellular, primary, secondary, and tertiary sculpture of the seed coats proved complex and required the invention of new descriptive terminology of character states beyond the previous generalized character states which will be used for future data matrices in cladistic analyses of morphological variation within the Phlox family.

Collaboration with Dr. Johnson’s colleagues provided seeds and images of additional species. We compiled a portion of our data and were able to present a poster at the ASPT meetings in August, 2001 on the external seed coat morphology of Gilia (Polemoniaceae) and segregate genera, comparing and corresponding with phylogenetic relationships. In the fall, I will help prepare a manuscript of the data for publication in Systematic Botany or the American Journal of Botany. The experiences opportunities and skills gained from this research are invaluable for me and the work I accomplished will provide the first detailed survey of seed coat morphology in this family and the data may be subsequently useful as a framework for investigating correlations, if any, between external morphology and mucilage production vs. different cues required for seed germination.

References

  • Grant, Verne. 1959. Natural history of the Phlox family. The Hague, Netherlands. Pp. 33-34.
  • Grant, V. 1998. Primary classification and phylogeny of Polemoniaceae, with comments on molecular cladistics. Amer. J. Bot. 85: 741-752.
  • Hsiao, Yi-Chun, and T. I. Chaung. 1981. Seed-coat morphology and anatomy in Collomia (Polemoniaceae). Amer. J. Bot. 68(9): 1155-1164.
  • Johnson, L. A., and J. L. Schultz, D. E. Soltis, and P. S. Soltis. 1996. Monophyly and generic relationships of Polemoniaceae based on matK sequences. Amer. J. Bot. 83:1207-1224.
  • Prather, L. Alan, and Carolyn J. Ferguson, and Robert K. Jansen. 2000. Polemoniaceae phylogeny and classification: implications of sequence data form the chloroplast gene ndhF. Amer. J. Bot. 87(9): 1300-1308.
  • Porter, J. M. 1996. Phylogeny of Polemoniaceae based on nuclear ribosomal internal transcribed spacer DNA sequences. Aliso 15: 57-77.
  • Porter, J. Mark, and Leigh A. Johnson. 2000. A phylogenetic classification of Polemoniaceae. Aliso 19(1), pp. 55-91.
  • Schnopf, E., and G. Deichgraber. 1983. Structure and formation of fibrillar mucilages in seed epidermal cells. I. Collomia grandiflora (Polemoniaceae). Protoplasma 114: 210-221