On the Cover

The genus Metasequoia is ancient, widely distributed in the Northern Hemisphere from the late Cretaceous, 100 million years ago (Ma) through the Pleistocene period (11,500 years ago). During the Paleocene-Eocene Thermal Maximum (55 Ma), extensive forests of Metasequoia occurred as far north as 80° N latitude. The genus was first described and named from the fossil record by Miki in 1941.1,2 Shortly thereafter, scattered stands of a large deciduous conifer were discovered in eastern Szechuan Province, growing solitarily in rice paddies and floodplains, and extending up ravines.3 Cheng and Hu proposed a new species, Metasequoia glyptostroboides, as the “living fossil” remnant of three other Metasequoia species identified only in fossil form.2,3 The common name dawn redwood was suggested by Chaney to emphasize its early fossil record.1 Seeds derived from about three trees in China were disseminated throughout the United States and Europe. While rapid growing, the dawn redwoods demonstrated diminished fertility, due to the very low genetic diversity. New genetic stock provided by Chinese botanists in the 1990s appears to be solving this problem. Dawn redwoods are rapid growing, reaching over 35– 40 m within 60 years. Trunk is straight with numerous branches with narrow conical crown. Bark is reddish brown when young, darker, fissuring and peeling in strips when older. Needles are 15 mm long, opposite, in a flattened, feathery pinnate or bipinnate compound leaf; bright green above and pale green below; deciduous, turning reddish-brown. Male florescence is light yellow brown in narrow clusters up to 30 cm. Box-like cones are 1–2.5 cm on long stalks.1 Pollination is in late winter to early spring. Subfamily members have been incriminated in allergic rhinitis and asthma.4 – 6 Two specimens on the Oregon State University campus have been identified as point sources for allergic rhinoconjunctivitis (Jacobson, personal communication). Taxodiaceae and Cupressaceae were long considered separate families based on differences in leaf structure. However, morphologic similarities, DNA sequences, rbcL sequences, and immunologic studies support merging the families.7,8 The former is now classified as a subfamily of Cupressaceae, Taxodioideae, though evidence suggests the remainder of Cupressaceae arose out of Taxodiodeae.7 Metasequoia, Sequoia, and Sequoiadendron are closely related genera within the subfamily, other members being bald cypress, Taxodium, and Japanese cedar, Cryptomeria. Cupressaceae members are strongly crossreactive, with consistent cross-inhibition with animal antisera and human IgE antibodies.9 Schwietz and colleagues reported extraordinary cross-reactivity between a large number of family members, which Midoro-Horiuti and co-workers showed was due to group 1 allergen homology.10,11 Metasequoia and Sequoia pollen grains are similar: spheroid, 24 –32 ␮m in diameter, and are similar to Juniperus grains other than the exine being slightly rougher and having a single germ pore.12,13 This exit papilla is conical, similar to Taxodium, but less prominent and bent than Cryptomeria. The extine is 1␮m. The intine is thick, up to 6 ␮m, and the inner border is buckled, giving a stellate starburst appearance, but less angular than Juniperus. Surface sculpturing is microverrucate.

Dawn Redwood Cover, Table of Contents, and Pollen Photography: Richard W. Weber, MD, and David R. Weldon, MD

REFERENCES 1. Dawn Redwood. http://oregonstate.edu/dept/Idplants

2. Metasequoia. http://en.wikipedia.org/wiki/Metasequoia 3. Chaney RW. The bearing of the living metasequoia on problems of Tertiary paleobotany. Proc Nat Acad Sci 1948; 34:503–515. 4. Lewis WH, Vinay P, Zenger VE. Airborne and Allergenic Pollen of North America. Baltimore, Johns Hopkins University Press, 1983:11. 5. Bucholtz GA, Lockey RF, Serbousek D. Bald cypress tree (Taxodium distichum) pollen, an allergen. Ann Allergy 1985;55:805– 810. 6. Midoro-Horiuti T, Nouno S, Seino Y. Skin tests of pollen grains of taxodiaceae and cupressaceae in children with bronchial asthma. Acta Paediatr Jpn 1992;34:501–504. 7. Judd WS, Campbell CS, Kellogg EA, Stevens PF. Plant Systematics: A Phylogenetic Approach. Sunderland, MA, Sinauer Associates, 1999:152–158. 8. Gadek PA, Alpers DL, Heslewood MM, Quinn CJ. Relationships within Cupressaceae sensu lato: a combined morphological and molecular approach. Am J Botany 2000;87: 1044 –1057. 9. Weber RW: Cross-reactivity of plant and animal allergens. Clin Rev Allergy Immunol 2001;21:153–202. 10. Schwietz LA, Goetz DW, Whisman BA, Reid MJ. Crossreactivity among conifer pollens. Ann Allergy Asthma Immunol 2000;84:87–93. 11. Midoro-Horiuti T, Goldblum RM, Kurosky A et al. Molecular cloning of the mountain cedar (Juniperus ashei) pollen major allergen, Jun a 1. J Allergy Clin Immunol 1999;104: 613– 617. 12. Wodehouse RP. Pollen Grains. New York, McGraw-Hill, 1935:268 –271. 13. Davis OK. Pollen morphology Metasequoia. December 1999. http://www.geo.arizona.edu/palynology/pid00010 Request for reprints should be addressed to: Richard W. Weber, MD National Jewish Medical & Research Center 1400 Jackson Street Room J326 Denver, CO 80206

RICHARD W. WEBER, MD

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