Evolutionary biology of the parasitic angiosperm Arceuthobium americanum (Viscaceae) as determined by population genetic analysis and infectivity experiments

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Jerome, Cheryl Ann
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In this study, a multidisciplinary approach incorporating population genetic analysis and infectivity experiments was used to explore the evolutionary biology of 'Arceuthobium americanum' Nutt. ex Engelm. (Viscaceae). This study represents one of the few comprehensive investigations into the evolutionary, geographical, biological, ecological, and historical factors influencing a parasitic plant. 'Arceuthobium americanum' infects three principal hosts and has the most extensive geographic range of any N. American dwarf mistletoe. Based on the lack of apparent morphological and phenological differences between populations of 'A. americanum', past researchers have found no evidence for recognizing subspecific taxa. In the present study, molecular analysis using amplified fragment length polymorphism analysis (AFLP) indicated that 'A. americanum' is divided into three distinct genetic races, each associated with a different host taxon in regions of allopatry: (1) 'Pinus banksiana' in western Canada; (2) ' Pinuscontorta' var. 'murrayana' in the Sierra Nevada and Cascade Mountain ranges in the western U.S.A.; and (3) 'Pinus contorta' var. 'latifolia' in the western U.S.A. and Canada. These observations suggested that host identity, geographic isolation, and environmental factors have all contributed to race formation in ' A. americanum'. Molecular analysis using AFLP indicated that hosts are divided into only two genetic groups: (1) 'P. banksiana' and hybrids; and (2) 'P. contorta' var. 'latifolia' and var. 'murrayana'. This observation suggests that host identity is not the primary factor leading to race formation in 'A. americanum '. Findings from infection experiments also question the role played by host identity since there is no indication of host X parasite interactions. Nonetheless, the role of host cannot be completely ruled out since the genetic races of 'A. americanum' can clearly be defined by this parameter. The lack of fine-scale patterning within 'A. americanum' races was attributed to random dispersal of seeds over long distances by animal vectors, as well as to adaptation of parasite populations to non-geographically patterned host genotypes and local environmental conditions. Historical factors such as glaciations and founder events were also found to impact structuring and genetic diversity in A. americanum' populations. Despite this lack of fine-scale patterning, the existence of three distinct genetic races of 'A. americanum' provides insight into the evolutionary potential of this taxon. Given sufficient time, it is possible that these races will become reproductively isolated, and undergo speciation.