Comparative analysis of the genetic basis of Cry1F resistance in two strains of Spodoptera frugiperda originated from Puerto Rico and Florida.

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a major target pest of Bacillus thuringiensis (Bt) maize and cotton in America. Since the commercialization of Cry1F maize (event TC1507) in 2003, resistance to Cry1F maize in field populations of S. frugiperda has occurred in Puerto Rico, Brazil and the southeast region of the United States. In this paper, we conducted a comparative analysis of the inheritance of two Cry1F-resistant colonies of S. frugiperda originated from Puerto Rico (PR) and Florida (FL), respectively. The objective of the analysis was to determine if the genetic basis of the resistance was similar in the two different originated colonies. To accomplish the objective, besides PR, FL, and a known Cry1F-susceptible colony, 14 additional colonies were developed by reciprocal crosses among the three parents, F1 by F1 crosses, backcrosses, and intercolony-crosses between PR and FL. Larval mortalities of the 17 colonies were assayed on both Cry1F maize leaf tissue and Cry1F-treated diet at the concentrations of 3.16, 10.00, and 31.60µg/g. Resistance to Cry1F in both PR and FL was autosomal and recessive or incompletely recessive. Segregations in F2 and backcrossed generations associated with FL fitted the Mendelian monogenic model well, while with PR the segregations did not follow the single gene model in some bioassays. Further analyses with the intercolony complementation tests showed a similar level of resistance in the F1 progeny as their parents FL and PR. Together with the data, it was likely that a single (or a few tightly-linked) gene was involved in FL; PR shared the same locus of the major resistance gene as FL, but the resistance in PR might also be associated with additional minor factors. Information generated from this study should be useful in understanding the origin of Cry1F resistance in the U.S. mainland and developing effective strategies for Bt resistance management in S. frugiperda.