Rick, C. M.
The mutant cm was first reported by Andrus (TGC3:32) in PI 196,297, collected at El Recreo, Nicaragua. Several years ago I noticed a similar phenotype in my LA292, a cerasiforme from the Galapagos Islands. Now the same condition has appeared in collections that I made recently in coastal Ecuador. All of the cultivated forms that I have tested from markets at Guayaquil and other coastal towns and from various fields in the same area show the cm phenotype when grown under the proper conditions at Davis. The same characteristic has appeared in accessions of L. piminellifolim from Pichilingue, but not in others from Daule, Puna, and Cerecita. Test crosses between these races and PI 196,297 indicate that they all carry cm or a very similar allele. The presence of cm in LA292 and in all tested cultivated tomatoes from the Ecuadorian coast and its absence in all other accessions from the Galapagos provide additional evidence that LA292 is not likely native to the islands, but was probably introduced recently by settlers. The expression of cm in the Ecuadorian tomatoes, if anything, is more severe than in the original accession from Nicaragua. The simptoms so cripple some lines that they perform poorly throughout the season. As in the original cm, the symptoms here are most extreme in May shortly after the plants are set in the field; thereafter they gradually diminish until midsummer, but gradually reappear again in the fall, never regaining the intensity observed in May. Much needs to be done before we know the environmental conditions that are critical for expression of cm.
The intriguing aspect of cm in tropical tomatoes is its widespread distribution. While travelling in Ecuador I could see no manifestation of cm in any growing plants; in fact, I was totally unaware of its presence in any of the lines. It appears very likely that the phenotypic expression that we associate with cm is masked by the temperature and light conditions prevailing through the year in the native habitat. If this is granted, the problem then arises as to why this gene has become fixed in so many wild and cultivated forms and in regions so distant as Ecuador and Nicaragua. It seems very likely that it offers some selective advantage under tropical conditions, but I could not even guess what advantage cm could offer when its phenotypic expression is apparently completely masked by the environment.