Use of A \u3csub\u3e3\u3c/sub\u3e Cytoplasm to Reduce Risk of Gene Flow through Sorghum Pollen

A critical impediment to field testing and deployment of transgenic sorghum [Sorghum bicolor (L.) Moench] is the threat of gene flow to weedy relatives through pollen. A technique using sorghum with A3 cytoplasmic male sterility to control transgene flow through pollen while using nontransgenic pollinators is described and an experiment was designed to evaluate the risk of viable pollen flow using A3 hybrids under field conditions. Seed set under pollinating bags (an indicator of fertile pollen) was evaluated at the University of Nebraska Field Laboratory at Ithaca, NE, in 2001 and 2002 on selfed F2 progeny grown from open pollinated seed of 12 F1 hybrids produced in A1 and A3 cytoplasm. The F2 seed was produced in hybrid yield trials in 1997 and 1998 at Ithaca, NE. In each evaluation year, the experimental design was a split-split plot with seed production year the main plot factor, hybrid as the subplot factor, and cytoplasm as the sub-subplot factor. Cytoplasm effects were highly significant, with percent seed set on A1 F2 individuals averaging 74%, and on A3 F2 individuals averaging 0.04%. Upper confidence limits (P = 0.05) for percent seed set were 1.32% or less for the progeny from all A3 hybrids. Polymerase chain reaction (PCR) analysis confirmed that four male fertile individuals (from a population of 1007) were detected with A3 cytoplasm. These results support the hypothesis that gene flow through pollen can be severely restricted but not eliminated in sorghum by the use of A3 cytoplasmic male sterility