Genetic diversity of Poa annua in western Oregon grass seed crops

genetic diversity of Poa annua L. populations collected from western Oregon grass-seed fields was surveyed using 18 randomly amplified polymorphic DNA (RAPD) markers. Markers from 1357 individual plants from 47 populations co llected at three sampling dates (fall, winter, and spring) for 16 sites wer e used to measure genetic diversity within and among populations. Site hist ories varied from low to high herbicide selection pressure, and some sites were subdivided by 3 years of differing post-harvest residue management. Ge ne diversity statistics, simple frequency of haplotype occurrence, and anal ysis of molecular variance (AMOVA) revealed the presence of significant var iability in P. annua among sites, among collection dates within sites, and within collection dates. Nei gene-diversity statistics and population-diffe renriation parameters indicated that P. annua populations were highly diver se. Mean Nei gene diversity (h) for all 47 populations was 0.241 and total diversity (HT) was 0.245. A greater proportion of this diversity, however, was within (H-S=0.209) rather than among (G(sr)=0.146) populations. When po pulations were grouped by season of collection, within-group diversity was H-S=0.241, while among-group diversity was G(sr)=0.017. When populations we re grouped by site, within-group diversity was Hs=0.224, while among-group diversity was G(sr)=0.087. The diversity among populations within season fo r fall, winter, and spring collections was G(sr)=0.121, 0.142, and 0.133, r espectively. Populations collected from fields with histories of high herbi cide selection pressure showed low differentiation among collection dates, with G(sr) as low as 0.016, whereas those collected from fields with low he rbicide selection pressure showed greater differentiation among collection dates, with GS, as high as 0.125. At high selection-pressure sites, populat ions were also lower in gene diversity las low as h=0.155), while at low se lection-pressure sites there was higher gene diversity las high as h=0.286) . The site to site variability was greater For the high selection-pressure sites (G(sr)=0.107 or 69% of the total among-population variance), while th e season of germination variability was greater at sites of low herbicide-s election pressure (G(sr)=0.067, or 70% of the total among-population varian ce). High initial diversity coupled with a long-term re-supply of genotypes from the seed bank must have been factors in maintaining the genetic diver sity of this weed despite the intensive use of herbicides. Knowledge of the genetic diversity of Willamette Valley P. annua should help in formulating more effective strategies for managing this weed.