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United States Department of Agriculture

Agricultural Research Service

Title: Ssr Marker Confirmation of Reciprocal Outcrossing Rates Between Rice and Red Rice Lines in Arkansas over a Five-Year Period

Authors
item Gealy, David
item Estorninos, JR., L - UNIV OF AR RREC

Submitted to: Proceedings of Southern Weed Science Society
Publication Type: Proceedings
Publication Acceptance Date: December 1, 2006
Publication Date: January 22, 2004
Citation: Gealy, D.R., Estorninos, Jr., L.E. 2004. SSR marker confirmation of reciprocal outcrossing rates between rice and red rice lines in Arkansas over a five-year period. [abstract] Proceedings of Southern Weed Science Society. Vol. 60

Technical Abstract: Outcrossing between rice and red rice can negatively impact the rice industry, especially when herbicide-resistant rice cultivars are grown. Previous research has indicated that outcrossing between rice and red rice in Arkansas farm fields can occur with either plant type serving as the pollen donor. These earlier reports relied on observations of key phenotypic traits only. Depending on the particular rice cultivar and red rice line involved, these traits included leaf pubescence, tall plant type, purple/red colored stems or awns, delayed flowering, or resistance to imidazolinone herbicides. Those reports suggested that outcrossing rates can vary greatly within the same year as well as from year to year, and that outcrossing often occurs preferentially when red rice serves as the pollen donor. However, reliance on these methods can lead to inaccuracies and often, an overestimation of true outcrossing rates. Thus, in the present study, we used molecular (SSR) marker analysis to genetically confirm natural hybrid formation between a number of commercial rice cultivars and specific red rice lines over a five-year period. The markers (RM5, RM232, RM234, RM253, and RM488) were chosen because of their ability to distinguish commercial rice cultivars from a number of Arkansas red rice lines. Seed for these analyses were generated from previous experiments in 2000 to 2004 in drill-seeded field plots (2 to 4 m wide by 5 m long and three replications) near Stuttgart, AR. Each year, three to seven combinations of a particular rice cultivar and red rice line were included due to their expected overlapping flowering periods. Rice and red rice plants were grown in adjacent rows (18 cm apart) to ensure maximum outcrossing. Seed from these rice and red rice plants was harvested separately, and ~3000 to 5000 seedlings per replication were evaluated in the field in 2005 to determine reciprocal outcrossing rates. DNA samples were obtained from leaves of all plants exhibiting phenotypic traits indicative of putative rice-red rice hybrids, were subject to PCR, and then analyzed on an ABI 3700 sequencer. Of these plants, only those with SSR marker profiles consistent with the rice cultivars and red rice lines present in the original plots were considered to be true hybrids for the purposes of calculating outcrossing rate. Outcrossing rates varied greatly from year to year. For example, outcrossing between Kaybonnet and #8 (awned) red rice as pollen donor ranged from 0.039% in 2001 to 0.868% in 2004 (more than twenty-fold difference), and the other rice-red rice combinations in 2001 and 2004 also exhibited similar large differences in outcrossing. Likewise, the outcrossing rate between imidazolinone-resistant CFX18 (i.e. CL161) rice and #8 red rice was 0.095% in 2003 and 0.829% in 2004. When rice served as the pollen donor, outcrossing rates were usually much lower than in the reverse situation. Considering all of the rice-red rice combinations and years evaluated in this study, outcrossing averaged 0.0584% with rice as the pollen donor, and nearly 30% of these registered zero (i.e. undetectable). By contrast, outcrossing rates averaged 0.264% with red rice as the pollen donor, and outcrossing was detectable in all cases. Outcrossing rates were negatively correlated with differences between red rice and rice heading dates, when either rice (r = -0.41) or red rice (r = -0.23) served as pollen donor. Outcrossing rates (with red rice as pollen donor) were also negatively correlated (r = -0.28) with the height differential between rice and red rice panicles. These observations are consistent with the notion that outcrossing between rice and red rice will tend to decrease with increasing temporal separation between flowering periods and increasing spatial separation between panicles. Results from this multi-year, multi-variety study, suggest that a large proportion of the rice-red rice hybrids formed under natural conditions are likely to be borne on rice plants (with red rice as pollen donor) and removed from fields with the harvested crop. This phenomenon may help simplify management of rice-red rice hybrids in Arkansas rice fields and reduce the negative impacts of outcrossing in imidazolinone-resistant rice systems

Last Modified: 12/17/2014