ABSTRACT

Variation in overall expression levels of Cry1Ac (i.e. Bt) among Bollgard® cotton (event 531) have been correlated to survival levels in various Lepidoptera indicating that all cultivars of transgenic Bt cotton do not provide the same level of larval control. Although genetic transformation events often modify the agronomics of the transgenic variety compared to the corresponding parental variety (e.g. plant maturity and mean height), few studies have been published that examine differential expression of Bt toxin among different plant parts and varieties. Factors that have been proposed to influence the level of expressed Bt among varieties are still not fully understood, but site-of-gene insertion and cultivar or parental background has been implicated. Furthermore, by not providing a high-dose strategy to control the intrinsically tolerant Lepidoptera (i.e. armyworms, loopers, and bollworms), managing resistance to these insects may be further complicated by differential expression of Bt among plant parts and varieties that could create a temporal source for resistance to develop. The purpose of this research was to determine if differences in expression among Bollgard® varieties are under genetic control. These studies are much needed to determine if transgenic crops can be selected based on their plant-insect resistance traits (i.e. highest expression varieties) in addition to their agronomic traits. As with any foliar insecticide and herbicide, the efficacy of each variety must be determined to ensure the best recommendation to growers.

To determine if expression differences among Bollgard® varieties are under genetic control, crosses were made from plants bred from distantly related backgrounds. A Stoneville Pedigree Seed Co. variety (ST4691B) and a Delta and Pineland Co. variety (PM 1218B/RR) were chosen to cross to two other varieties (NuCOTN 33B and DP 458BR: Delta and Pineland Co.). Both NuCOTN 33B and DP 458BR express significantly more Bt in all plant structures compared to other common mid-south varieties, while both ST4691B and PM 1218BR have been shown to express significantly less Bt than NuCOTN 33B or DP 458BR (Figure 1). NuCOTN 33B and DP 458BR's recurrent conventional variety was DP 5415. Reciprocal crosses were made between DP 458BR X PM 1218BR plants and NuCOTN 33B X ST4691B plants. Seeds from the F1 crosses were then planted in the greenhouse and seeds from the F2 generation were harvested. Parents, F1 and F2 crosses were planted on May 1, 2002 in field plots located in Stoneville, MS. Plots were arranged in a randomized complete block design with 6 genetic types and 4 blocks. The F2 crosses had 3 plots in each block (i.e. an extra level of replication), while the parents and F1 crosses only had one level of replication per block. All plots were maintained according to local management practices. The amount of Bt was quantified on a per plant basis by using a commercially available quantification kit (Envirologix, Inc., Portland, ME).

In these studies the expression differences in Bt among varieties of Bollgard® are under simple genetic control. Significant differences among expression means (ppm) were found among the crosses and parents (Tables 1 and 2.). In addition, the variances for the F2 crosses were much higher than the F1 crosses and the parents (Tables 3 and 4). Further analysis indicated that the variance attributed to genetic factors was much higher than the variance attributed to environmental factors (Tables 5 and 6). Tests for dominance and epistatsis were not significant for the NuCOTN 33B and ST4691B cross (Table 5); however, the initial test for dominance was significant for the DP 458BR and PM 1218BR cross (Table 6). Estimation on the number of genes conferring expression differences was calculated using a modification to the Castle and Wright formula suggested by Cockerham. An estimation of a small number of genes was concluded for both crosses (NuCOTN 33B X ST4691B = 0.98; DP 458BR X PM 1218BR = 1.08). There was no difference in reciprocal F1 and F2 populations for Bt expression.

This initial study provides key information regarding genetic effects of expression of Bt among different varieties. Future work will involve planting progeny rows of additional F2 crosses to determine if plants can be selected for the highest expressing individuals in a given population. Hopefully, these data will provide valuable insight on selecting the best plants in a transgenic breeding program.