Cytoplasmic genomes are of interest both for the processes controlled by organellar genomes, and for the interaction of nuclear and organellar genomes. The interaction of organellar genomes with nuclear genomes is best studied in cytoplasmic male sterility (cms) in several systems. It is also studied in products of protoplast fusion. The transfer of cytoplasms is also of interest for the study of nuclear/organellar genomes, and to try to create cms. Andersen (1963) reported that transfer of L. esculentum cytoplasm into L. pennellii created an apparent cms system. This rasied the question of whether the transfer of L. pennellii cytoplasm into L. esculentum would also create a cms system.
A crossing barrier, termed unilateral incongruity (UI), exists between L. esculentum and most of the wild Lycopersicon species, including L. pennellii. This barrier prevents crosses in which L. esculentum is not the female parent both in making the F1 and in the BC generation. Since the female parent provides the cytoplasm of offspring, UI usually prevents transfer of L. pennellii cytoplasm into esculentum. We reported a rare success in making the cross L. pennellii x (L. esculentum x L. pennellii) (Mutschler and Cobb, 1985), and the beginning generations of tranferring L. pennellii cytoplasm into esculentum. Using the L. esculentum line New Yorker (NY), the F1 NY X L. pennellii (LA716), and embryo culture, the near-isocytoplasmic line New Yorker^Lp^ (NY^Lp^) has now been developed according to the pedigree:
(((LA716 x F1) x F1) X F1) X NY^(10 times)^
NY^Lp^ has pennellii (LA716) cytoplasm, and has sufficient backcrosses to contain greater than 99% of the esculentum genome. Southern analysis, using total DNA from each of the plants involved in this pedigree, proves that the original pennellii x F1 plant in the pedigree above, and all subsequent generations, contain pennellii cytoplasm, since they contain the chloroplast and mitochondrial DNA of pennellii. NY^Lp^ appears identical to NY for all mophological traits, in development and maturity, and is visually indistinguishable from NY.
Percent stainable pollen of NY^Lp^ (94%) is the same as that of normal tomatoes. NY^Lp^ is male and female fertile, is readily pollinated by esculentum, and is fully self fertile. The male fertility of NY^Lp^ demonstrates that simple transfer of pennellii cytoplasm to esculentum is not sufficient to create cms in tomato. It remains possible that recombinations between different mitochondrial genomes in the process of protoplast fusion could alter critical genes and lead to the creation of cytosteriles, but such recombination is highly unlikely in the sexual transfer of an alien cytoplasm in tomato.
Literature Cited
Anderson W.R. 1963 TGC 13: 7-8.
Mutschler, M.A. and E. Cobb 1985 TGC 35: 14.