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.

Keywords: brown planthopper; herbivory tolerance; heterosis; host plant resistance; leaffolders; plant physiology; rice herbivores; stemborers; whitebacked planthopper; yields.

Figure 1

Frequency of heterosis and heterobeltiosis for resistance/susceptibility to (A,B) BPH, (C,D) WBPH and (E,F) YSB among 15 hybrid rice lines from the IRRI breeding program. Heterosis and heterobeltiosis are indicated based on statistical differences in any response (i.e., development time, number of individuals, herbivore weights, etc.) between hybrids and (A,C,E) the A lines or (B,D,F) B lines. Light green portions of each pie chart indicate heterosis, dark green indicates heterobeltiosis resulting in improved resistance, and orange indicates heterobeltiosis resulting in increased susceptibility. Grey portions indicate no significant differences between plant types. Hybrid lines are listed; for full details, including comparisons of herbivore responses on lines related to each hybrid, see Tables S1 and S2 (A,B), Tables S4 and S5 (C,D), and Tables S6 and S7 (E,F).

Figure 2

Frequency of heterosis and heterobeltiosis for (A) plant tillering, (B) dry weight, (C) SPAD values, (D) the number of filled grain produced, (E) the size of grain (i.e., 1000-grain weight), and (F) the proportions of grain that were filled among eight hybrid rice lines from the IRRI breeding program. Heterosis and heterobeltiosis are indicated based on statistical differences between hybrids and their relevant B and R lines. Light green portions of each pie chart indicate heterosis; dark green indicates heterobeltiosis resulting in improved plant growth or yields in field plots. Grey portions indicate no significant differences between plant types. Hybrid lines are listed; for full details see Tables S9–S12.