Abstract
Numerous studies have examined the adaptive significance of seed color using natural variation within species or cultivars, yet our overall understanding remains limited. In this study, we used evolutionary contrasts between cultivated lines and their de-domesticated counterparts as a useful framework for investigating the adaptive significance of seed color. We hypothesized that the naturally expressed red-pericarp phenotype of de-domesticated rice may be associated with seed-based resistance to insect feeding. We compared Japanese weedy rice ( Oryza sativa ), a de-domesticated lineage characterized by a red proanthocyanidin pericarp, with a cultivated line ( Oryza sativa ) with a white pericarp. We compared the larval development of red flour beetles ( Tribolium castaneum (Herbst)) fed cultivated rice with that fed weedy rice under flour-based bioassay conditions. We predicted that if weedy rice conferred resistance to T. castaneum , the larval mortality rate and developmental time would increase, and adult body biomass at eclosion would decrease, respectively. Contrary to our hypothesis, whole grains (containing the endosperm and pericarp) of all four Japanese weedy rice strains exhibited lower apparent resistance to T. castaneum than the cultivated rice. Larvae fed weedy rice reached eclosion significantly earlier, and adult body biomass was higher than that of larvae fed cultivated rice, with no difference in mortality rates. To partially separate the contribution of the pericarp fraction from that of the endosperm, we reciprocally exchanged the pericarp between the weedy and cultivated rice. This experiment showed that mortality rate of beetles fed weedy rice with red pericarps was lowest (15%) among all treatments, which was lower than that without pericarp (85%). These results do not support the hypothesis that the red-pericarp phenotype of Japanese weedy rice confers resistance to T. castaneum under flour-based bioassay conditions. Collectively, these results suggest that red-pericarp weedy rice did not show resistance to T. castaneum under flour-based bioassay conditions, indicating that seed pigmentation re-emerging during de-domestication does not necessarily confer resistance to this stored-product pest and may instead be linked to other traits, such as seed dormancy.