Genetic variation in populations of Western Australian wild radish

Raphanus raphanistrum L. ( wild radish) is a major problematic weed worldwide. Random amplified polymorphic DNA ( RAPD) was used to estimate the degree of genetic diversity between and within 2 populations of wild radish (WARR 5 and WARR 6), found to exhibit multiple herbicide resistance compared with a susceptible population ( WARR 7). It is believed that weed species with high degrees of genetic variation show potential for developing resistance to herbicides. Of the 13 RAPD primers screened, 9 primers generated 97 polymorphic bands concomitant with a high level of polymorphism (82%) between the wild radish populations, characteristics of an outbreeding species. Analysis of molecular variance (AMOVA) showed a markedly higher proportion of diversity within populations (87%) as opposed to between populations (13%). Principal component analysis (PCA) further highlighted the large amount of variation between individuals within populations. Only one marker, OPC19-8, was found to be unique to the WARR 7 population but absent in WARR 5 and in most individuals of the WARR 6 populations. This marker may potentially be correlated with herbicide susceptibility. The 2 resistant wild radish populations were found to be closely related (0.7% dissimilar) to each other, whereas the susceptible population was genetically dissimilar to them by 2.3%. This higher level of dissimilarity between the susceptible and resistant populations may be explained by limited gene flow between them since the susceptible population is geographically located further away from the resistant populations. Hence, it may be concluded that the underlying genetic structure of the 3 wild radish populations seems to be similar despite WARR 6 and WARR 5 having been exposed to mixed herbicide usage for over 17 years.