Submitted to: American Journal of Potato Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2004
Publication Date: 2/10/2005
Citation: Bamberg, J.B., Palta, J.P., Vega, S.E. 2005. Solanum commersonii cytoplasm does not improve freezing tolerance in substitution backcross hybrids with frost sensitive potato species. American Journal of Potato Research. 82:251-254. Interpretive Summary: Potato is the most important US vegetable, and one of the best hopes for feeding an increasingly hungry world. But efficient potato production is limited worldwide by frost sensitivity of all commercial varieties. Some wild relatives of potato can withstand very hard frosts, so could be useful in breeding. How can the genetics of frost hardiness be best transferred to future varieties? One consideration is that if any cold hardiness genetics exists in the cytoplasm of frost hardy species, it will only be transmitted if the cross is done when the frost hardy parent is the mother, because cytoplasmic genetics are contributed only by the mother plant. So, to test the potential contribution of cytoplasm, hybrids were made between very frost hardy and very sensitive species using the hardy species as mothers. Several more generations were created by always crossing to the sensitive parent as the father, resulting in hybrids with almost exclusively frost sensitive genetics in the nucleus, but with the cytoplasm of the very frost hardy species. Since these hybrids were always found to be just as frost sensitive as their sensitive parent, we conclude that cytoplasm does not contribute to frost hardiness, so it is one less thing breeders have to worry about in their efforts to create frost hardy varieties.
Technical Abstract: Solanum commersonii Dun. (cmm) is the most frost hardy wild potato species known, being able to tolerate an acute freezing episode to about -5C and further acclimate to tolerate to about -10C after being exposed to chilling temperatures for several days. Breeding with this species to incorporate its frost hardiness traits can be accomplished by standard sexual hybridization or protoplast fusion. Both of these methods can result in hybrids that vary in contribution of the cmm plastome. To test the effect of cmm cytoplasm, cytoplasmic substitution backcross hybrids were made with three very frost sensitive species, S. brachistotrichum, S. cardiophyllum and S. pinnatisectum, by using S. commersonii as the female to make an F1, then performing repeated backcross (BC) using the sensitive species as males. Relative freezing tolerance (RFT) of all genotypes was assessed by measurement of ion leakage of excised terminal leaflets subjected to a controlled ice nucleation and simulated freeze-thaw stress. Even against the background of these very sensitive species' genomes, the cmm cytoplasm of substitution hybrids promoted no detectable improvement in frost hardiness or ability to acclimate. We conclude that either 1) cmm cytoplasm does not contribute to frost hardiness, or 2) if cmm cytoplasmic frost hardiness genes do exist, they must be epistatic to (depend on the presence of) nuclear hardiness genes for expression.