Genetic diversity of red rice in Arkansas and the role of gene flow in red rice diversification

Red rice (Oryza sativa L.) is a noxious weed in rice production systems throughout the world. In the southern United States red rice has been a challenging weed since the beginning of rice production in 1685. Because it is of the same genus and species as cultivated rice, red rice can not be controlled with herbicides in conventional rice fields. Red rice infestation can decrease cultivated rice yield up to 80%. Lately, red rice can be selectively controlled using the imazethapyr-resistant Clearfield(TM) (CL) rice technology. To manage red rice better, information is needed regarding: (1) yield losses due to different red rice biotypes; (2) the reciprocal outcrossing frequencies between cultivated rice and red rice biotypes; (3) impact of plant and environmental factors on outcrossing rate" and (5) the extent of genetic introgression into cultivated rice from red rice in the last century.;Field and greenhouse experiments were conducted to examine (1) the variability in maximum outcrossing rate between 12 red rice biotypes and 'CL161' rice during their peak flowering overlap in the field and (2) the genetic compatibility of red rice biotypes with CL161 rice. In field experiments, the maximum outcrossing rate between red rice biotypes and CL161 ranged from 0.03 to 0.25%. In manual crosses between red rice biotypes and CL rice, seed set, which indicates genetic compatibility, ranged from 49 to 94%. The majority of red rice biotypes had similar compatibility with CL161 rice.;Field experiments were conducted to determine the impact of planting date, CL cultivars and red rice biotypes on the outcrossing rate. To further understand rice and red rice effects on outcrossing rate, we also aimed to determine the relative contribution of flowering time of CL rice and red rice biotypes and air temperature and relative humidity on outcrossing rate. Lower outcrossing rates were observed in late plantings compared with earlier planting. In general, a higher outcrossing rate (1.26%) was observed between red rice biotypes and CLXL8 hybrid rice than with CL161 rice (0.21%). The outcrossing rate of red rice biotypes with CL161 ranged from 0 to 0.21% and that with CLXL8 hybrid rice from 0 to 1.26%. Among the plant and environmental factors, red rice type is the main factor that influences the outcrossing rate with CL 161 rice. On the other hand, with CLXL8, the minimum relative humidity during the flowering influences outcrossing rate the most.;Experiments were also conducted to determine the gene flow rate from red rice biotypes to cultivated rice and to evaluate the morphology, phenology, and fecundity of resulting outcrossing. The gene flow rate from weedy rice to cultivated rice ranged from 0.01 to 0.2% and differed by red rice biotype. F1 crosses between CL161 rice x red rice were 138 to 150 cm tall and flowered 1 to 5 d later than CL161 rice. Seeds of all crosses had red pericarp, were pubescent, shattered at maturity, and germinated >90%.;Field experiments were conducted to evaluate the emergence pattern of red rice biotypes at different planting dates and evaluate the effect of red rice biotype, CL rice cultivar, and planting date on cultivated rice yield loss. The emergence rate of red rice biotypes increased with later planting dates. In general, CL rice yield losses due to red rice biotypes increased in later planting dates, up to 49%. CL rice yield losses from different red rice biotypes ranged from 14 to 45% and 6 to 35% in CL161 and CLXL8 hybrid rice, respectively. Cultivated rice becomes less competitive with red rice in later plantings, resulting in higher yield losses.;The genetic diversity in red rice populations in Arkansas and introgression of genetic material in these red rice populations from cultivated rice in the last 100 yrs was investigated. One hundred thirty-seven red rice accessions from four ecological zones in Arkansas and 36 rice cultivars that have been grown in Arkansas in the past century were fingerprinted using 27 rice microsatellite markers. All rice cultivars grown in Arkansas formed a cluster separate from red rice accessions. Generally, red rice accessions were separated in 4 groups: awnless strawhull, blackhull, brownhull, and awned strawhull, and some accessions between red rice biotypes and rice cultivars. Awnless strawhull accessions were genetically distant from blackhull (GD = 0.55) and brownhull (GD = 0.60). Also blackhull and brownhull types are genetically distant (GD = 0.96) to each other than awnless strawhull types. The blackhull and strawhull red rice diversity is affected by their zone of origin. Twenty-five percent of red rice accessions in Arkansas had some genetic material introgression from cultivated rice in the last century.