Systematics/Phytogeography / Taxonomie/ Section
Dickinson, Timothy A. .
An informal infraspecific taxonomy for Crataegus section Douglasianae (Rosaceae tribe Pyreae) in the Pacific Northwest.
Phenetic analyses of western North American black-fruited hawthorns, together with results from studies of ploidy level and DNA sequence variation, suggest that a modest revision of the taxonomy of this group is called for. Section Douglasianae in the Pacific Northwest comprises two easily distinguished species, C. suksdorfii (flowers with about 20 stamens each) and C. douglasii (flowers with about 10 stamens each). Over the past decade one segregate species has been described with 20-stamen flowers, and six more that have 10-stamen ones. Crataegus douglasii and at least three of the 10-stamen segregate species are largely apomictic tetraploids. 20-stamen Crataegus suksdorfii and C. shuswapensis, on the other hand, comprise diploids, auto- and allotriploids, and allotetraploids, similarly apomictic in the case of the polyploids. Phenetic clustering, whether agglomerative or divisive, fails to form single-species clusters, especially in the case of the 10-stamen taxa. In a subsample for which both morphological and microsatellite data are available, clusters of individuals sharing microsatellite alleles are dismembered when clustering is based on morphological data – except when these clusters represent randomly sampled individuals from the same topodeme. These results suggest that C. douglasii and the 10-stamen segregate species likely represent more or less distinguishable lines in which agamospermy predominates. Some of these lines may have arisen from hybrids between C. douglasii and red-fruited taxa in Crataegus section Coccineae and perhaps could be recognized as nothospecies. Rather than synonymizing or reducing the rank at which entities for which a hybrid origin is unlikely, their status as microspecies can be emphasized in floristic and taxonomic treatments by using quoted names as informal epithets. This would also be the best way in which to recognize additional, newly discovered entities unless they can be shown to be either sexual diploids, or substantially differentiated in morphology and distribution.
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North American Black-Fruited Hawthorns
1 - Royal Ontario Museum - Green Plant Herbarium (TRT), Department of Natural History, 100 Queen's Park, Toronto, Ontario, M5S 2C6, CANADA
Presentation Type: Oral Paper:Papers for Sections
Date: Tuesday, July 29th, 2008
Time: 1:45 PM