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This study was conducted to determine the inheritance of altered traits in plants regenerated from Texas male‐sterile cytoplasm (T) maize (Zea mays L.) tissue cultures. Plants were regenerated from AlSSTcms (rf1rf1;Rf2Rf2) tissue cultures grown on control media or on media containing different levels of streptomycin. Of 169 regenerated plants, eight expressed unexpected changes to male fertility and/or resistance to Helminthosporium maydis pathotoxin even though the cultures had not been selected for pathotoxin resistance. The male fertility and disease resistance traits had a cytoplasmic mode of inheritance. Two regenerated plants were male fertile and toxin susceptible, but their first generation progeny were either male fertile and resistant or male sterile and susceptible. Some regenerated plants apparently were chimeric for two cytoplasm conditions, one causing male fertility and toxin resistance and the other causing male sterility and toxin susceptibility. Mitochondrial DNA (mtDNA) was extracted from six variant lines and analyzed by restriction enzyme digestion and agarose gel electrophoresis. Five of the six variants were distinguishable from control A188Tcms (rf1rf1;Rf2Rf2) by virtue of an alteration(s) in a mtDNA sequence of about 6,600 base pairs (6.6 Kb). A 6.6 Kb fragment characteristic of A188Tcms (rf1rf1;Rf2Rf2) mtDNA was not detected in the five variant lines when mtDNA was digested with the restriction enzyme Xhol; however, no difference could be detected between the sixth variant line and A188Tcms (rf1rf1;Rf2Rf2). A possible relationship between the mtDNA changes and the cytoplasmically controlled changes to male fertility and toxin resistance is proposed.

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Cassava is an important subsistence crop grown only in the tropics, and represents a major source of calories for many people in developing countries. Improvements in the areas of resistance to insects and viral diseases, enhanced nutritional qualities, reduced cyanogenic content and modified starch characteristics are urgently needed. Traditional breeding is hampered by the nature of the crop, which has a high degree of heterozygosity, irregular flowering, and poor seed set. Biotechnology has the potential to enhance crop improvement efforts, and genetic engineering techniques for cassava have thus been developed over the past decade. Selectable and scorable markers are critical to efficient transformation technology, and must be evaluated for biosafety, as well as efficiency and cost-effectiveness. In order to facilitate research planning and regulatory submission, the literature on biosafety aspects of the selectable and scorable markers currently used in cassava biotechnology is surveyed. The source, mode of action and current use of each marker gene is described. The potential for toxicity, allergenicity, pleiotropic effects, horizontal gene transfer, and the impact of these on food or feed safety and environmental safety is evaluated. Based on extensive information, the selectable marker genes nptII, hpt, bar/pat, and manA, and the scorable marker gene uidA, all have little risk in terms of biosafety. These appear to represent the safest options for use in cassava biotechnology available at this time.