Glyphosate-resistant Palmer amaranth is a serious problem in southern
cropping systems. Much phenotypic variation is observed in Palmer amaranth
populations with respect to plant growth and development and susceptibility
to herbicides. This may be related to levels of genetic diversity existing
in populations. Knowledge of genetic diversity in populations of Palmer
amaranth may be useful in understanding distribution and development of
herbicide resistance. Research was conducted to assess genetic diversity
among and within eight Palmer amaranth populations collected from North
Carolina and Georgia using amplified fragment length polymorphism (AFLP)
markers. Pair-wise genetic similarity (GS) values were found to be
relatively low, averaging 0.34. The highest and the lowest GS between
populations were 0.49 and 0.24, respectively, while the highest and the
lowest GS within populations were 0.56 and 0.36, respectively. Cluster and
principal coordinate (PCO) analyses grouped individuals mostly by population
(localized geographic region) irrespective of response to glyphosate or
gender of individuals. Analysis of molecular variance (AMOVA) results when
populations were nested within states revealed significant variation among
and within populations within states while variation among states was not
significant. Variation among and within populations within state accounted
for 19 and 77% of the total variation, respectively, while variation among
states accounted for only 3% of the total variation. The within population
contribution towards total variation was always higher than among states and
among populations within states irrespective of response to glyphosate or
gender of individuals. These results are significant in terms of efficacy of
similar management approaches both in terms of chemical and biological
control in different areas infested with Palmer amaranth.