Risk Assessment of Gene Flow Associated with the Release of Virus-Resistant Transgenic Crop Plants

Abstract

An important environmental concern associated with the large scale cultivation of virus-resistant transgenic crop plants relates to the possible transfer of virus-derived transgenes into wild relatives through pollen flow. We addressed this issue by using two squash (Cucurbita pepo ssp. ovifera) species: The transgenic line CZW-3 (G pepo var. ovifera) containing the coat protein (CP) genes of cucumber mosaic cucumovirus (CMV), zucchini yellow mosaic potyvirus (ZYMV), and watermelon mosaic virus 2 potyvirus (WMV 2) and its wild relative C pepo var. texana. Viral transgene flow was evaluated in the field by studying the successive rate of introgression of the three CP genes from Fl hybrids (= G texana x CZW-3) and their backcrossed progeny (BC 1 = G texana x Fl hybrids) into G texana squash populations. Field plots were maintained under low or high disease pressure by deliberately introducing CMV, ZYMV, and WMV 2 in the latter plots. Introgression occurred under low but not high disease pressure. To assess fitness, test plants (G texana, CZW-3, Fl hybrids and BC 1 progeny) were evaluated for virus resistance, number of fruits produced, and number of viable seeds. Under high disease pressure, all G texana plants became severely infected and produced less than one fruit per plant and no viable seeds. Hybrids of G texana that contained the CP genes, on the other hand, exhibited virus resistance and produced a fair number of fruits with viable seeds. The implications of these data will be discussed.