A Sri Lankan indica rice (Oryza sativa L.) cultivar Rathu Heenati was found to be resistant to all the four biotypes of the brown planthopper (BPH) (Nilaparvata lugens Stål). In the present study, we constructed a linkage map to identify the locus (loci) for the BPH resistance genes, using an F 2 population from a cross between Rathu Heenati and a susceptible cultivar 02428. Insect resistance was evaluated using 156 F 2:3 lines and the genotype of each F 2 plant was inferred from the phenotype of the corresponding F 2:3 lines. Three loci detected by QTL (quantitative trait locus) analysis, were assigned to chromosomes 3, 4 and 10. The phenotypic variance of the three QTLs indicated that the QTL on chromosome 4 is a major BPH resistance gene in Rathu Heenati. Through linkage analysis, it was found that this BPH resistance gene was located between two SSR markers RM8213 and RM5953 on the short arm of chromosome 4, with map distances of 3.6 cM and 3.2 cM, respectively. This gene, tentatively designated as Bph17, should be useful for the breeding of varieties resistant to BPH in a marker-assisted selection (MAS) program.

The genetic mechanism underlying six palatability properties of cooked rice and three physico-chemical traits was dissected in 66 BC(3)F(2) chromosome segment substitution lines (CSSLs), using a complete linkage map in three successive years. The CSSLs showed transgressive segregation for all traits studied. Significant correlation was detected among most palatability traits. A total of 25 QTLs for the nine traits were identified on nine chromosomes, and many QTLs affecting different quality traits were mapped in the same regions. Six QTLs--qLT-8 for luster, qTD-6 and qTD-8 for tenderness, qIVOE-6 and qIVOE-8 for integrated value of organoleptic evaluation, and qAC-8 for amylose content--were repeatedly detected across the 3 years. Phenotypic values were significantly different between the recurrent parent, cultivar Asominori, and the CSSLs harboring any of the six QTL alleles across the three environments, indicating that these six QTLs were non-environment-specific and could be used for marker-assisted selection in rice quality improvement.

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Some insect chitinases are required to degrade chitin and ensure successful metamorphosis. Although chitinase genes have been well characterized in several model insects, no reports exist for the rice striped stem borer, Chilo suppressalis, a highly destructive pest that causes huge yield losses in rice production. Here, we conducted a genome-level analysis of chitinase genes in C. suppressalis. After amplification of full-length transcripts with rapid amplification of cDNA ends, we identified 12 chitinase genes in C. suppressalis. All these genes had the conserved domains and motifs of glycoside hydrolase family 18 and grouped phylogenetically into five subgroups. C. suppressalis chitinase 1 (CsCht1) was highly expressed in late pupae, whereas CsCht3 was abundant in early pupae. Both CsCht2 and CsCht4 were highly expressed in larvae. CsCht2 was abundant specifically in the third-instar larvae and CsCht4 showed periodic high expression in 2- to 5-day-old larvae in each instar. Tissue specific expression analysis indicated that CsCht1 and CsCht3 were highly expressed in epidermis whereas CsCht2 and CsCht4 were specifically abundant in the midgut. Knockdown of CsCht1 resulted in adults with curled wings, indicating that CsCht1 might have an important role in wing expansion. Silencing of CsCht2 or CsCht4 arrested moulting, suggesting essential roles in larval development. When the expression of CsCht3 was interfered, defects in pupation occurred. Overall, we provide here the first catalogue of chitinase genes in the rice striped stem borer and have elucidated the functions of four chitinases in metamorphosis.

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