When Vesta Meyer released the Cytoplasmic male sterile and fertility restorer strains with Gossypium harknessii cytoplasm in the early 1970's it appeared that commercial production of F₁ hybrid cotton would be practical. The discovery of the enhancer factor permitted the development of restorer strains which insured that F₁ hybrids would have satisfactory fertility under all growing environments except extremely high temperatures. Thus, during the past 15 years most of the research efforts on utilizing hybrid vigor in cotton has been on developing F₁ hybrids of CMS upland x upland restorer strains. A few years ago it became obvious that F₁ hybrids produced with the CMS system were inconsistent in their performance over years and locations. The average yield of these F₁ hybrids tended not to exceed adapted cultivars.

During the summer of 1983 we observed that in reciprocal crosses between restorer strains (G. harknessii cytoplasm) and cultivars (normal cytoplasm) that the F₁ hybrids with normal cytoplasm produced a significantly higher yield. During 1984 and 1985 three types of hybrids were tested for their yielding ability as follows: Cultivar as female parent x restorer Restorer as female parent x cultivar The CMS (A line) of the cultivar x restorer

The average yield results for 1984 and 1985 were consistent with the cultivars as the female parent producing the highest yield. The restorers as the female parent showed about an 8% reduction in yield. The CMS x restorer hybrids showed about a 5% reduction in yield. These results give strong indications that the G. harkriessii cytoplasm is harmful for yield.

The question arises as to why the hybrids made on CMS cultivars tended to produce a higher yield than when the restorer was used as the female parent. One possible explanation is that the male gamete carries a very small amount of cytoplasm and that a CMS strain with 15 backcrosses has some normal cytoplasm while a restorer strain produced after five or six backcrosses has a lesser amount of normal cytoplasm. Thus, with even more backcrosses the CMS strains might contain enough normal cytoplasm to overcome the apparent harmful effect of the harknessii cytoplasm and yet retain the male sterility. The apparent, but inconclusive, difference in yield for hybrids from R lines vs A lines as female parents should be more extensively tested under a range of environments.