Rice (Oryza sativa L.) is one of the world’s most important crops providing a staple food for nearly half of the global population (FAO 2004). Alongside adverse environmental conditions, rice arthropod pests are considered the major contributor to losses in rice production around the world (Pathak and Khan 1994). Total yield losses from insect pests may total 90% (Seni and Naik 2017a) although more often around 20% (Matteson 2000). Rice arthropod pests are highly diverse with nearly 300 species hailing from numerous insect orders, and 23 known species causing severe economic damage (Pasalu and Katti 2006). In Thailand, the whitebacked planthopper (WBPH) Sogatella furcifera (Horváth) (Homoptera: Delphacidae) (Ramesh et al 2014), brown plant hopper (BPH), Nilaparvata lugens (Stål) (Seni and Naik 2017b) and Asian rice gall midge, Orseolia oryzae (Wood-Mason) (Kumar et al 2011) are the major causes of economic crop losses. Devising breeding programs contingent on genomic determinants of insect resistance is a primary strategy recommended for the development of a sustainable global agriculture capable of feeding humanity’s burgeoning nutritional demands (Horgan 2017).
Among these insect pests, BPH is the most problematic in South-East Asia and has affected rice production with increasing frequency over recent years (Hu et al 2011). The primary control method of BPH is through the application of commercially available chemical pesticides (Wu et al 2020). However, the extensive use of chemical pesticides can have several detrimental consequences including selection for insect resistance in target species, injurious impact on target species’ natural enemies (Matteson 2000), increases in rice production costs, and negative human health and environmental hazards including pollution and residue persistence (Nagata 1984; Liu et al 2003; Jin et al 2008; Kontsedalov et al 2012). There is also evidence that increased pesticide use may increase BPH damage due to exogenous chemical disruption of host-plant transcriptomes involved in plant-insect defence mechanisms (Kenmore 1991; Cheng et al 2012; Ali et al 2014), while there is also evidence that synthetic pesticides kill natural enemies and may promote the emergence of new BPH biotypes (Tanaka et al 2000). There have been at least 29 cases of evolved insecticide resistance to major classes of insecticides including organophosphates, carbamates, pyrethroids, neonicotinoids, insect growth regulators, and phenylpyrazoles in the BPH alone (Wu et al 2018). Notably, different BPH biotypes are known to undermine specific resistance genotypes indicating genomic variation in BPH resistance breakdown (Jing et al 2017).
BPH cause critical damage to rice by attacking the xylem and phloem tissues, resulting in tissue damage caused by "hopper burn" with concomitant yield losses. Indirect damage from BPH involves transmission of rice grassy stunt virus and ragged stunt virus, that again strongly impact yields (Balachiranjeevi et al 2019). Despite its notable successes (Pingali 2012), the adoption of Green Revolution agricultural practices (Matteson 2000) appears at least partially responsible for ongoing rice crop insect pest outbreaks. Modern agriculture, including heavy fertiliser use, the use of a limited number of high yield varieties (i.e., with concomitant reduction in genetic diversity potentially imposing compromised defences), and year-round multi-cropping, have favoured the build-up of rice pest populations (Way and Heong 1994). BPH can adapt to commonly used rice varieties by evolving resistant genes (Bottrell and Schoenly 2012).
Recently, 40 candidate loci for BPH resistance genes have been identified largely through marker-assisted selection (MAS) or QTL mapping. There are four prominent gene clusters that contain many BPH resistance genes. Cluster A on chromosome 12 including BPH1, BPH2, BPH7, BPH9, BPH10, BPH18, BPH21, and BPH26. Cluster B on chromosome 4S, including BPH12, BPH15, BPH17, BPH17-ptb, BPH20, BPH22(t), BPH30. Five genes on chromosome 6 (cluster C), including BPH3, BPH4, BPH25, BPH29, and BPH32. Cluster D on chromosome 4L includes BPH6, BPH18(t) and BPH27, BPH27(t), and BPH34. Seven have been map using large scale populations. Four genes were identified by linkage mapping, and a further seven were cloned and characterized for BPH resistance (BPH9, BPH14, BPH17, BPH18, BPH26, BPH29, and BPH32) (Nguyen et al 2019).
In order to inform breeding programs via the identification of genomic determinants of insect resistance and understand the impact of key functional variants genome-wide association studies (GWAS) methodologies have become a commonplace tool (Korte and Farlow 2013), yet they suffer from some inherent drawbacks. These include the influence of rare variants associated with extreme phenotypes (Wray et al 2013) and low-informativeness of SNP markers (Collard et al 2005). Moreover, they do not account for epistatic interactions that may synergistically modify functional effectiveness (Clark 2004; Bardel et al 2005). Thus, haplotype mining tools offer potentially valuable technologies in genomics-assisted breeding (Bhat et al 2021). Haplotype identification may increase informativeness (Hamblin and Jannink 2011), while incorporating epistatic relationships within identified genomic regions.
Here we conduct bioinformatic analyses of BPH resistance across a large whole-genome sequenced rice crop panel held at the Rice Gene Discovery Unit in Thailand. Using phenotypic assay data evaluating four separate BPH populations against a single susceptible rice variety, our analyses identified functional gene haplotypes (FGHs) associating with BPH resistance. To do this we built on the previously published riceExplorer resource (Darwell et al 2022) to devise a novel bioinformatics pipeline, haploAnnotator, that incorporates both WGS and phenotypic data sources to produce annotated gene maps for 27 chromosomal regions (i.e., genes) of interest for which we identified definite chromosome number and base pair start-end positions. Additionally, we examined whether linked identified haplotypes associated with BPH genes are likely to occur across different genes in resistant rice varieties. Our new pipeline sheds light on the genomic architecture of BPH resistance, indicating functional SNP and indel haplotype arrangements, among Thai rice accessions and provides a novel bioinformatics tool that should aid rice (and other crop) breeding across the globe.