One-step generation of cytoplasmic male sterility by fusion of mitochondrial-inactivated tomato protoplasts with nuclear-inactivated Solanum protoplasts.
Mesophyll protoplasts of Lycopersicon esculentum were treated with iodoacetamide to inactivate mitochondria, and protoplasts of Solanum acaule and Solanum tuberosum were irradiated with gamma- or x-rays to inactivate nuclei. Mixtures of protoplasts thus modified were treated with Ca2+ and polyethylene glycol to obtain heterologous fusion products. Among the fusion products were some tomato plants that were indistinguishable from the original cultivars with respect to morphology, physiology, and chromosome number (2N = 24) but exhibited various degrees of male sterility (MS): complete lack or malformation of anthers, shrunken pollen, and normal-looking stainable pollen that could not germinate. The MS thus induced in five cultivars of different growth types, including one of subspecies L. esculentum cerasiforme, was inherited maternally over several generations and is, therefore, cytoplasmically determined MS (CMS). Analysis of mitochondrial DNA revealed that the mitochondrial genome of the CMS hybrids does not contain all elements of the mitochondrial DNA of either parent but includes sequences of a recombinational nature not present in either parent. The CMS hybrids, therefore, possess a true hybrid mitochondrial genome. The same procedure applied to fusion of tomato with Solanum lycopersicoides and Nicotiana tabacum cells did not produce CMS phenotypes. The advantages of this method over others for generating MS are as follows: (i) only one step is required; (ii) the nuclear genotype of the cultivar is unaffected; (iii) the prospect that cytoplasmic determination allows generation of 100% CMS progenies. The normal-appearing but nonfunctional pollen of certain CMS types might render them attractive to pollinating bumblebees that thus would facilitate production of hybrid seed.