Note. +, <100,000 ha were grown. The first commercial crops were planted in China during the early 1990s. The first commercial production in the United States was tomatoes during 1994. Several crops were first commercialized during 1995, including Bt corn. (a) Area by crop. Several minor crops are not listed. (b) Area by transgenic trait. Values do not always sum to the worldwide total because some crops have more than one transgenic trait. (c) Total area worldwide.

TABLE II Area of Bt Corn, Bt Cotton, and Bt Potato, 1995-1997 and 2000 for the United States Only; 1998-1999 for the World, Although Nearly All Was Planted in the United States

1995 1996 1997 1998 1999 2000 2001

Note. Corn and cotton data are in million hectares. Potato data are in hectares. na, data not available; +, <100,000 ha for maize and cotton and <1000 ha for potato.

Bt cotton area increased, perhaps tied to increased demand for herbicide-tolerant cotton.

Several factors probably account for the predominance of Bt genes in transgenic crops. First and foremost, transgenic technology is relatively new, and consequently the products are those that are technically feasible, are readily accomplished, and have a clear path to commercialization. The cry genes present in Bt have been technically easier to use than genes from plant or animal sources. The cry gene structure is simple, and Cry toxins require no posttranscriptional or posttranslational processing to be functional. Initially, cry genes did not express very well in plants, in part because bacterial DNA is A-T rich, whereas plant DNA is G-C rich. By directed mutagenesis, the "wobble" codon and other triplet redundancies were converted from A or T to G or C, and the resulting cry genes were expressed more consistently in plants. The commercialization path for Bt crops was believed to be relatively clear. Several Cry toxins have been used in commercial formulations of insecticides and these have been considered much safer for the environment and much less toxic to humans than nearly all other synthetic chemical insecticides. More recently, however, the differences between the Cry toxins in the insecticides and the Cry toxins in the transgenic plants have received increasing scientific and regulatory attention. Another reason for the predominance of cry genes is that as each particular Bt crop variety was approved for commercial use by U.S. regulatory agencies, the regulatory requirements became increasingly clear, making it increasingly easy to prepare for regulatory evaluation. Thus, the transformation technology, the regulatory environment, and the toxicological and insecticidal characteristics of Cry toxins have contributed to the present abundance of Bt crops.

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