Red rice has always occurred in Uruguayan rice fields at low population levels. Over the last 10 years, the intercropping period was reduced due to more intensive rice production. At the same time, red rice populations and the number of infested rice fields have increased becoming a serious rice production problem.

• Panicles of different red rice biotypes were collected from rice fields across Uruguay (Table 1, Figure 1), individual plants of each type were transplanted in the field and records of morphological traits used as variety descriptors were taken on a single plant basis.

• Seeds were harvested from these plants and grown in the tropical greenhouse of NIAR, Tsukuba, Japan for DNA extraction.

• Total genomic DNA was extracted from young leaves of 20 days old plants using the CTAB method (Saghai- Maroof et al. 1984).

• The genetic diversity was analyzed in 34 samples of red and black rice, and 6 cultivars using AFLP methodology with silver stainned gels.

• Asociations among the accesions were analyzed by the cluster analysis UPGMA (using the NTSYS program); and NJ ("Neighboring joining method") using AFLPDIST software.

There was a clear relationship between AFLP groups and morphological characters. Three diferentiated clusters appeared in both dendrograms (Figure 2, Figure 3, Figure 4). Wild type traits, such as, black hull, purple apex and long awn were found in most of the samples in group A, whereas traits found in cultivated rice were found in the weedy rice of group B and C (straw hull and apex and short or no awn). Cluster B included all the cultivars analyzed and some weedy accesions (Figure 5).

• The AFLP technique using silver staining to visualize bands has been successfully applied to the molecular characterization of Uruguayan weedy and cultivated rice and to evaluate diversity among red rice populations.

• A hypothesis to explain the evolution of weedy rice in Uruguay is presented here. It is suggested that at different times there has been selection for shattering and subsequently disruptive selection for either wild or domestication traits, (Figure 6).

• It appears that weedy rice has two strategies for survival: adapting to natural (Group A) or cultivated environment (Groups B, C), (Figure 6).

• Weedy rice is still evolving through hybridization- differentiation cycles with cultivars which resulted in intemediate types, (Figure 5).

• A clear relationship was found between AFLP groups and morphological characters while no relation was found between geographical origin of samples and genetic diversity, (Figure 5).

• The first group with black hull and apex and long awns can be easily controled because it is more easily visualized (cluster A).

• The second and third groups would be particulary more difficult to control due to their similarity with the cultivars (clusters B and C).

• The control of weedy rice should consider cultural practices to eliminate the soil seed bank of weedy rice such as repeted cultivation prior to planting and excelent water control, as well as good cleanning of seeds to discard any black hulled awned seed.

Saghai-Maroof, K.; Soliman, M.; Jorgensen, R.A. and Allard, R.W. (1984). Ribosomal DNA Spacer-Length Polymorphisms in Barley: Mendelian Inheritance, Chromosomal Location, and Population Dynamics. Proceedings of the National Academy of Sciences of the United States of America 81:8014-8018.