Allozyme diversity in the apomictic vine Bryonia alba (Cucurbitaceae): potential consequences of multiple introductions
American Journal of Botany
Bryonia alba (Cucurbitaceae) is a Eurasian herbaceous vine that spreads vegetatively through the production of many stems from a large, tuberous root. The only known U.S. populations of this aggressive apomict are in Idaho, Montana, Utah, and Washington and likely stem from deliberate (and subsequent accidental) introductions. To assess levels and patterns of genetic diversity of B, alba across its introduced range, 23 populations were analyzed for allozyme variation using 12 enzyme systems. Onaverage, 14.9% of loci are polymorphic per population (1.19 alleles per locus)-low values compared to other vascular plant taxa. Mean percent polymorphic loci differed among regions, with the highest values in the Washington and northern Idaho region(20.2%) and Montana (19.0%) and lower values in populations from the Utah and southern Idaho region (7.4%). Observed heterozygosity exceeded that expected at Hardy-Weinberg equilibrium in six of 23 populations, and a statistically significant excessof heterozygosity was detected at Pgm-1 in 18 of 23 populations. The level of population differentiation is high (G(ST) = 0.544); however, the level of differentiation among populations within regions is much lower. These results are consistent withthe genetic variation and structure expected for an apomict. Based on the level of genetic differentiation among populations, the current disjunct distribution of B. alba in its new range results from two, and possibly three, separate introductionsin the western United States. These introductions may stem, in part, from the vine's 19th century use as a medicinal and ornamental plant.