Evidence for long-distance pollen dispersal in milkweeds (Asclepias exaltata)

We investigated pollen-mediated gene flow and interspecific matings in natural populations of poke milkweed, Asclepias exaltata. Genetic data from 16 polymorphic isozymes allowed unambiguous identification of the diploid paternal genotype for 225 singly sired fruits collected from six populations in Shenandoah National Park, Virginia. Paternity analysis of progeny arrays revealed that 29%-50% (39.2%, mean) of the seeds produced in these populations were sired by plants located outside each target population. Variation among populations appears to be related to isolation distance (Kendall's τ = -0.69, N = 6, P > 0.1), although the negative correlation was statistically significant only when the results from a previous study of A. exaltata were also included in the analysis (Kendall's τ = -0.78, N = 7, P < 0.05). The coupling of standard paternity exclusion analysis on individual seeds (12 seeds/fruit) with estimation of cryptic gene flow from Monte Carlo simulations accounted for only 65% of the gene flow detected using the progeny-array analysis. These results suggest that standard paternity exclusion, incorporating cryptic gene flow, will inherently underestimate actual gene flow rates when the number of paternal parents is smaller than the number of seeds per fruit (i.e., matings are correlated). Indirect estimates of the number of migrants per generation (Nm = 2.3) obtained from Nei's genetic diversity statistic (GST) are much smaller than realized gene flow measured in 1990 (Nm = 9.4, progeny-array method). In addition, interspecific hybridization with common milkweed, Asclepias syriaca, via long-distance pollen dispersal was detected in two fruits (0.8%). Thus, pollen-mediated gene flow and infrequent interspecific matings provide mechanisms to lower genetic differentiation among populations to maintain novel, low-frequency alleles within populations of A. exaltata.