Davidsonia

Stewart, C.D., and Ross, C.M. 2006. Embryological and Phenological Comparison between Arceuthobium americanum (the lodgepole pine dwarf mistletoe) growing on Pinus contorta var. latifolia in British Columbia and on P. Banksia in Manitoba. Davidsonia 17(4):107-115

The genus Arceuthobium comprises angiosperms that are aerial parasites on Pinaceae and Cupressaceae. These parasites are serious forest pests in North America, where they damage timber trees. In Canada, Arceuthobium americanum (the lodgepole pine dwarf mistletoe) is principally found on Pinus contorta var. latifolia (lodgepole pine) in British Columbia (BC) and Alberta as well as on Pinus banksiana (jack pine) in Alberta, Saskatchewan and Manitoba. Although widespread, A. americanum seems morphologically uniform in its range. However, the taxon has been divided into two distinct genetic races based on the variation observed in amplified fragment length polymorphism (AFLP) data. This led us to predict that there would be some variation in anatomical, as opposed to external morphological form. Therefore, the objective of this study was to determine if the embryological anatomy of A. americanum growing on P. contorta var. latifolia in BC differed from that growing on P. banksiana in Manitoba. Samples of female flowers were collected from both BC and Manitoba throughout the growing season and prepared for routine light microscopy. Key embryological stages were observed, and the relative timing of these stages were compared between the BC and Manitoba collections. Our results were in direct contrast with our prediction, which had to be falsified: hundreds of prepared sections revealed that major embryological events, including double fertilization and zygotic division, are indistinguishable and occur at essentially the same time. This first study on emrbyology of two plants growing on the two pine speces at the extreme geographic ranges shows that the development is similar. Despite climate, host, ecology, and geography, which remain variables of undetermined relative significance, the mechanism for embryological development is likely highly conserved for A. americanum across Canada.