Heterosis for Resistance to Insect Herbivores in a 3-Line Hybrid Rice System

Abstract

Three-line hybrid rice is produced by crossing male sterile (A line) rice with a fertility-restorer (R line). Fertile lines (B lines) are also required to maintain A line seed for breeding programs. We used a range of hybrids and their parental lines to assess the frequency and nature of heterosis for resistance to the whitebacked planthopper (Sogatella furcifera), brown planthopper (Nilaparvata lugens) and yellow stemborer (Scirpophaga incertulas). Heterosis is defined as trait improvement above the average of the parental lines as a result of outbreeding. Based on the results from a greenhouse study that challenged hybrids and their parental lines with each herbivore species, we found that susceptibility to planthoppers was associated with one of the eight A lines tested, but resistance was improved by crossing with a relatively resistant restorer. Higher frequencies of heterosis for susceptibility in comparisons between hybrids and their B lines suggest that susceptibility was not related to the cytoplasmic genomes of the associated sterile A lines. Furthermore, because none of the parental lines possessed currently effective resistance genes, improved resistance against planthoppers was probably due to quantitative resistance. In a related field trial, hybrids had generally higher yields than their fertile parents and often produced larger grain; however, they were often more susceptible to stemborers, leaffolders (Cnaphalocrocis medinalis) and other caterpillars (Rivula atimeta). This was largely a consequence of hybrid heterosis for plant biomass and was strongly affected by crop duration. We make a series of recommendations to improve hybrid breeding to reduce the risks of herbivore damage.

Heterosis for Interactions between Insect Herbivores and 3-Line Hybrid Rice under Low and High Soil Nitrogen Conditions

“…Therefore, in total, we used 30 separate rice lines (genotypes) in the experiments. These same varieties have been used in previous studies, including as part of a wider study on the frequency of heterosis for resistance within the IRRI hybrid breeding program [ 11 ] and in field trials to assess plant development and yields and the field occurrence of planthoppers and stemborers [ 11 , 18 ]. The main results from these studies with respect to each of the eight hybrids are summarized in Table S1 .…”

“…However, hybrids—particularly early-generation hybrids—have often been associated with high damage from insect herbivores and diseases [ 1 , 9 , 10 ]. The careful selection of breeding lines can reduce herbivore damage to hybrids [ 11 ]; however, susceptible hybrids are still widely available [ 12 , 13 , 14 ].…”

“…These practices increase the attractiveness of hybrids and the growth of pest populations by enhancing host plant quality (i.e., fertilizers and some resurgence pesticides) and reducing the diversity and abundance of natural enemies (i.e., resurgence pesticides) [ 1 , 15 , 16 ]. (2) Some hybrid varieties have relatively low anti-herbivore resistance (the ability to defend against herbivore oviposition (antixenosis) and growth and development (antibiosis)) because of relatively fast hybrid growth rates (e.g., planthoppers and leafhoppers), efficient assimilation of nitrogen by the plants (e.g., planthoppers, leafhoppers and stemborers), thick stems (e.g., stemborers) and high leaf and shoot biomass (e.g., leaffolders, stemborers and other caterpillars) [ 1 , 11 , 17 , 18 ]. There is also evidence that some hybrids may be ‘hyper-susceptible’ to planthopper damage, possibly associated with the female cytoplasm (i.e., from the male-sterile parent) [ 10 , 19 , 20 ].…”

“…Heterosis can be enhanced by increasing the genetic distance between parental lines [ 22 , 23 ]; this will also limit the movement of certain pests, such as planthoppers, between fields with different hybrid rice varieties [ 1 , 24 ]. Breeders can further avoid herbivore-susceptible parental lines and promote specific combinations of parental lines that achieve heterobeltiosis for herbivore resistance [ 11 , 25 , 26 ]. However, there are limitations on the choice of parents for hybrid breeding programs, as not all lines can restore fertility [ 5 , 6 ].…”

“…Furthermore, tolerance to certain pests and diseases can be enhanced under high fertilizer conditions [ 18 ]. Despite its importance, only a few studies have examined heterosis for resistance to rice herbivores in hybrid rice [ 11 , 25 , 26 ]. This is partly because parental lines are generally restricted as intellectual property for commercial use.…”