Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: February 1, 2010
Publication Date: February 22, 2010
Citation: Pinson, S.R., Jia, Y., Gibbons, J. 2010. Early generation selection for rice fissure resistance proves effective and indicates a fissure resistance gene on chromsome 1. Rice Technical Working Group Meeting Proceedings. CDROM. Technical Abstract: Whole rice kernels have two to three times more market value than brokens, which means that any reduction in milling yield results in financial losses for both rice producers and millers. One of the leading causes of reduced milling yield is exposure of the rice kernels to severe moisture changes before or after harvest which causes them to fissure. ‘Cypress’, a southern U.S. variety released in 1993, is known for its resistance to kernel fissuring, but is not grown widely today, having been replaced with cultivars having higher yield potential and disease resistance. While breeders would like to incorporate Cypress’ fissure resistance into improved cultivars, their efforts have been limited due to a lack of methods for identifying and selecting for fissure-resistance in early breeding generations. A laboratory method wherein small samples of seed are evaluated for fissure rates after controlled rewetting has proven to reliably identify fissure resistance among pure-breeding material grown in several replicated environments. The present study was conducted to determine if this laboratory evaluation method could be used to accomplish early generation breeding selections, which would be limited to small amounts of heterogeneous seed obtained from unreplicated F2 and F3 progeny plants. To measure the efficacy of this laboratory method as a selection tool, we used it to conduct divergent selection for fissure resistance (FR) and susceptibility (FS) among F2 and F3 plants, then evaluated the success of the selections using F3 and F4 progeny testing. The amount of phenotypic change accomplished with a round of selection is known as “Realized Heritability”. Laboratory Selection Method: Seed samples of 50 dried kernels each were evaluated for rates of kernel fissuring after exposure to controlled levels of fissure-inducing humidity. The relative humidity of the air around the seed samples was controlled by using a growth chamber to provide a 45 (+/- 1) degrees C air temperature outside of a closed-box system containing seed samples suspended over a layer of water. Dried seed samples were first held in a controlled environment for 14 days to allow their grain moistures to equilibrate, then were pretreated with 45 (+/- 1) degrees C dry heat for 0 to 4 hours before placing them in the high-humidity boxes. The humidity within the sealed boxes was documented to reach 100% RH within 60 minutes after sealing. Forty seed samples at a time were placed in each sealed box, and two boxes were run in synchrony. Critical to the success of this evaluation method is the use of seed that is fully mature but not overmature so as to ensure that it is not already field-fissured. We accomplished this by hand harvesting mature seed from the upper portions of panicles when they were completely straw colored, and had dried to the point that the seed were seen to pull away from the hulls, giving them a dry, papery appearance. The widely-spaced F2 plants tillered profusely and exhibited a wide range in heading time among panicles per plant. The ideally mature seed was hand-harvested from the tips of the earliest panicles, without waiting for later panicles to mature. Seed was dried gently using forced unheated air, to prevent causing post-harvest fissures. The response of rice kernels to the laboratory fissure-induction system was known to vary depending on the source or growing conditions of the seed to be studied. The lengths of the dry 45 degrees C heat pre-treatments ranged from 0 to 4 hours, and length of the humidity exposure periods ranged from 8 to 16 hours, as they were adjusted for each field replication. Additional amounts of seed of the Cypress and LaGrue check varieties were collected from each field study in order to determine these optimum lab-fissuring conditions for each set of seed studied. In 2006, seed was harvested from 312 Cypress (FisR) x LaGrue (FisS) F2 plants grown in the field and interspersed with multiple replicates of parental single-plant plots. Progeny from the 10% most FisR and FisS F2s were planted as replicated F3 families in TX and AR in 2007. FisS F2:3 progeny fissured twice as much as FisR progeny, with an average response to selection of 13.5%. Response to F3 selection was smaller, averaging 2.6%. Broad-sense heritability averaged 0.38 +/- 0.13 over all generations. Narrow-sense heritabilities (h2) were 0.47 and 0.54 for the FisR and FisS F2 selections, respectively, and smaller thereafter due to the reduced response to F3 selection. The laboratory fissure-evaluation system proved to be a successful breeding selection tool in that the FisR and FisS parents identified in one generation proved produced progeny that retained these fissuring differences. This study documented, for the first time ever, successful early-generation selection for FisR, opening new opportunity for breeders to develop rice cultivars improved for this important agronomic trait. During the course of selecting for FisR and FisS F3 families and individuals, an association between FisR and Cypress’ sd1 allele was detected. All of the most FisR F2 and F3 plants proved to be homozygous sd1, while the progeny of the FisS visibly segregated Sd1Sd1, sd1sd1, and Sd1sd1. This indicates that sd1 on chromosome 1 is linked to a FisR gene is one of multiple genes required for rice to be as highly FisR as Cypress. To map additional FisR genes, molecular characterization of the divergent FisR and FisS progeny populations is being pursued.