Field Evaluation of Yield and Lepidopteran Pest Resistance in Three Transgenic Cotton Lines

The most threatening factor in cotton production worldwide is Lepidoptera pests. The pest management using chemicals imposes huge costs and environmental side effects. Introduction of genetic engineering as a modern technology has opened a new insight for investigators to control the pests through transgenic pest resistant varieties. Cotton was the first transgenic crop which received cry genes, and introduced to farmers after numerous field trials. The field trials on evaluation of transgenic lines are a complex process in three levels comprising mini, mid and large scale trials, but the first step in this process is evaluation of transgene in the field condition along with considering other important crop properties. There are several Lepidoptera pests causing damage to cotton in nearly all cotton producing areas worldwide. However, bollworm (Helicoverpa armigera Hübner) and spiny cotton bollworms (Earias vittella Fabricius and Earias insulana Boisduval) are the most the most important pests in Iran. Also, there are many biotypes among different geographical populations. This fact implies that the transgenic lines should be tested in different locations within a country. A number of research works has been conducted to produce transgenic cotton via both modern biotechnology and traditional backcrosses in Iran. As a result, some different transgenic lines harbouring cry genes were produced. Primary evaluation of these transgenic lines was resulted to a few lines exhibited the most resistance characters against the pests. More precise evaluation of the lines for pest resistance traits and yield was done in this study.

Three transgenic lines harbouring cryIAc gene comprising A, B and CRI lines along with a non-transgenic variety as negative control (Varamin) were used in the experiment. The experiment was conducted in a Lepidoptera pest affected farm at Minab, Hormozgan from June to July of 2015. A complete randomized block design with eight replications was adopted as the statistical model. Each plot consisted of eight lines with 8 meters length in which the distance between lines were 80 cm. All agricultural operations including irrigation, fertilizer and weeding were carried out according to local recommendations. During growing season, some properties related to the pest resistance including number of bolls per plant, percentage of contaminated bolls, percentage of contaminated locule, and percentage of boll injuries were recorded, and damage index was then calculated. The varieties yield was recorded as well. All traits were recorded on two separate dates i.e. June and July, 2015. The data were analyzed using SAS software according to the adopted statistical model.

Analysis of variance showed that all recorded traits in the experiment were significant for cultivars effect, except for boll injuries percentage. Primarily, the current results demonstrate that all cultivars were exposed to the pest equally. Other traits including number of bolls per plant, percentage of contaminated bolls, percentage of contaminated locule, damage index and yield differed significantly between the cultivars. The mean comparison for number of boll per plant showed that transgenic A and B (transgenic) lines produced and kept the most boll number, and CRI transgenic line and Varamin cultivar ranked as the second. This trait is very important, because it not only acts as a yield component, but also involves in resisting against pests. The mean comparison for contaminated boll and contaminated locule traits showed that non-transgenic Varamin cultivar had the largest contamination amounts. In contrast, transgenic A and B lines exhibited the least contamination for both traits.
The most important trait expressing resistance is damage index. The lowest damage index was calculated for B line (0.69 and 0.46% in June and July, respectively) and A line (0.75 and 0.56% in June and July, respectively), and CRI line ranked as the second (2 and 5%). Meanwhile, the damage index of Varamin cultivar in this experiment was 3.8 and 11.5% in June and July, respectively. The damage index for A and B transgenic lines in the first and second recording show a downward pattern in contrast to CRI line and Varamin cultivar. Hence, Lepidoptera pests does not succeed in feeding on the A and B lines. Moreover, A and B lines exhibited the best yield (5041 and 4466 kg/ha, respectively), and the CRI line ranked as the second (3382 kg/ha). The Varamin cultivar yield was 3071 kg/ha which was the least record.

In general, two out of three transgenic lines comprising A and B lines had the best yield, the minimum contaminations and injuries by Lepidoptera pests. Varamin cultivar, as non-transgenic control line showed the least tolerance and yield and the CRI cultivar were ranked between the superior transgenic and non-transgenic lines. Both A and B transgenic lines have proper agronomic traits and the best resistance characters against Lepidotera pests. It can be concluded that these lines, as the first Iranian transgenic varieties, can be used for registration and commercialization.