Rice molecular markers and genetic mapping:Current status and prospects

Dramatic changes in climatic conditions that supplement the biotic and abiotic stresses pose severe threat to the sustainable rice production and have made it a difficult task for rice molecular breeders to enhance production and productivity under these stress factors. The main focus of rice molecular breeders is to understand the fundamentals of molecular pathways involved in complex agronomic traits to increase the yield. The availability of complete rice genome sequence and recent improvements in rice genomics research has made it possible to detect and map accurately a large number of genes by using linkage to DNA markers. Linkage mapping is an effective approach to identify the genetic markers which are co-segregating with target traits within the family. The ideas of genetic diversity, quantitative trait locus（QTL） mapping, and marker-assisted selection（MAS） are evolving into more efficient concepts of linkage disequilibrium（LD） also called association mapping and genomic selection（GS）, respectively. The use of cost-effective DNA markers derived from the fine mapped position of the genes for important agronomic traits will provide opportunities for breeders to develop high-yielding, stress-resistant, and better quality rice cultivars. Here we focus on the progress of molecular marker technologies, their application in genetic mapping and evolution of association mapping techniques in rice.

Constructing Molecular Marker Linkage Maps of Chromosome 14Sh and 22Sh and QTL Mapping for Major Traits by Use of Substitution Lines of Gossypium hirsutum L.

CSB14Sh,which is isogenic for its recurrent parent TM-1 except for chromosome 14 short arm,was crossed with TM-1,and the F2 population was produced.A total of 3800 SSR primer pairs covering the whole genome were used to screen polymorphism among two parents,TM-1 and CSB14Sh,

Identification and Molecular Mapping of the Rs Dm R Locus Conferring Resistance to Downy Mildew at Seedling Stage in Radish(Raphanus sativus L.)

Downy mildew （DM）, caused by the fungus Peronospora parasitica, is a destructive disease of radish （Raphanus sativus L.） worldwide. Host resistance has been considered as an attractive and environmentally friendly approach to control the disease. However, the genetic mechanisms of resistance in radish to the pathogen remain unknown. To determine the inheritance of resistance to DM, F1, F2 and BC1F1 populations derived from reciprocal crosses between a resistant line NAU-dhp08 and a susceptible line NAU-qtbjq-06 were evaluated for their responses to DM at seedling stage. All F1 hybrid plants showed high resistance to DM and maternal effect was not detected. The segregation for resistant to susceptible individuals statistically iftted a 3：1 ratio in two F2 populations （F2（SR） and F2（RS））, and 1：1 ratio in two BC1F1 populations, indicating that resistance to DM at seedling stage in radish was controlled by a single dominant locus designated as RsDmR. A total of 1 972 primer pairs （1 036 SRAP, 628 RAPD, 126 RGA, 110 EST-SSR and 72 ISSR） were screened, and 36 were polymorphic between the resistant and susceptible bulks, and consequently used for genotyping individuals in the F2 population. Three markers （Em9/ga24370, NAUISSR826700 and Me7/em10400） linked to the RsDmR locus within a 10.0 cM distance were identiifed using bulked segregant analysis （BSA）. The SRAP marker Em9/ga24370 was the most tightly linked one with a distance of 2.3 cM to RsDmR. These markers tightly linked to the RsDmR locus would facilitate marker-assisted selection and resistance gene pyramiding in radish breeding programs.

Characterization and Fine Mapping of a Necrotic Leaf Mutant in Maize(Zea mays L.)

Maize（Zea mays L.） is a commercially important crop.Its yield can be reduced by mutations in biosynthetic and degradative pathways that cause death.In this paper,we describe the necrotic leaf（nec-t） mutant,which was obtained from an inbred line,81647.The nec-t mutant plants had yellow leaves with necrotic spots,reduced chlorophyll content,and the etiolated seedlings died under normal growth conditions.Transmission electron microscopy revealed scattered thylakoids,and reduced numbers of grana lamellae and chloroplasts per cell.Histochemical staining suggested that spot formation of nec-t leaves might be due to cell death.Genetic analysis showed that necrosis was caused by the mutation of a recessive locus.Using simple sequence repeat markers,the Nec-t gene was mapped between mmc0111 and bnlg2277 on the short arm of chromosome 2.A total of 1287 individuals with the mutant phenotype from a F_2 population were used for physical mapping.The Nec-t gene was located between markers T31 and H8 within a physical region of 131.7 kb.