THE GENETIC BASIS OF DIFFERENCES IN GROWTH AND BEHAVIOR OF SPECIALIST AND GENERALIST HERBIVORE SPECIES: SELECTION ON HYBRIDS OF HELIOTHIS VIRESCENS AND HELIOTHIS SUBFLEXA (LEPIDOPTERA).

Abstract

Two species of moths and their hybrids were studied to determine the genetic basis of host range. One species, Heliothis virescens (HV), has a broad host range and is an agricultural pest on cotton (Malvaceae), soybean (Fabaceae), and tobacco (Solanaceae). The other species, Heliothis subflexa (HS), has a narrow host range, feeding on plants in the genus Physalis (Solanaceae). Experiments were done to determine whether the abilities to feed on cotton, soybean, and tobacco were each under separate genetic control in HV (genetically uncorrelated) or whether feeding on all three hosts was genetically correlated. By repeatedly backcrossing hybrids to HS while selecting for high larval survival and weight on soybean, loci conferring the ability to feed on soybean were moved from HV into the genetic background of the specialist, HS. After six generations of selection on soybean we tested the selected line on soybean, cotton, Physalis, and tobacco to determine if ability to feed on soybean was correlated with feeding ability on these other hosts. We found no evidence to support the hypothesis that feeding on all three hosts was correlated and controlled by a single locus. Compared to HS, the survival and weight of the selected line were significantly higher on soybean, similar on tobacco and Physalis, and slightly, but not significantly, higher on cotton. We also conducted a behavioral choice test to determine if larval preference for soybean was correlated with preference for other hosts. The selected line larvae had higher preference for soybean and cotton than HS. Preference for tobacco was equal in the selected line and in HS. These results support the trends seen in survival and growth tests where performances on soybean and cotton appeared partially correlated and performances on soybean and tobacco were uncorrelated. In conclusion, the generalist, HV, did not have a single set of loci that governed feeding on soybean, cotton and tobacco. It is likely that some of the loci governing performance and preference for soybean also contributed to performance and preference for cotton and that the soybean loci examined did not interact negatively to lower performance and preference for Physalis, the specialist's host. This approach can be applied more widely to address evolutionary questions about host range and other ecological traits.