Pollen pool heterogeneity in jack pine (Pinus banksiana Lamb.): a problem for estimating outcrossing rates?

Pollen pool heterogeneity, which violates an assumption of the mixed-mating model, is a major potential problem in measuring plant mating systems. In this study, isozyme markers were used to examine pollen pool heterogeneity in two natural populations of jack pine, Pinus banksiana Lamb., in northwestern Ontario, Canada. Population multilocus estimates of outcrossing rate ranged from 0.83 to 0.95 and differed significantly between populations. Single-tree multilocus outcrossing rates were found to be homogeneous among trees in both populations. Computer simulation studies indicated that a consanguineous pollen pool (pollen gametes related to the mother tree) was capable of biasing population outcrossing estimates downward. Random pollen pool heterogeneity (uncorrelated with maternal genotypes) did not appear to affect population outcrossing estimates in the simulations. Heterogeneity G-tests and Spearman rank tests showed that pollen pool heterogeneity existed in the two natural populations examined; however, it did not have a major effect on population outcrossing estimates, since the consanguineous pollen pool detected was probably a relatively minor component of the outcross pollen pool in both populations. In addition, heterogeneity G-tests were found to be not sensitive in detecting pollen pool heterogeneity caused by consanguineous pollen pool.