Quantitative genetics of sugarcane : III. Potential for sucrose selection in Saccharum spontaneum.

1. A project has been initiated to explore the possibilities of selection for higher levels of sucrose storage in the wild species S. spontaneum, using a representative sample of parental clones under conditions of natural crossing. 2. The aim of the programme is the development of superior clones to represent the wild species in conventional breeding programmes, which involve repeated backcrossing to S. officinarum. 3. An analysis of the first generation following the intercrossing of 21 parental types without emasculation, has demonstrated extensive genetic variability within and between progeny groups for fibre, sucrose, reducing sugars and yield grade. 4. Fibre and sucrose percent fresh weight are characters of high repeatability (0.4-0.6), and show little evidence of non-additive genetic effects. Individual plant performance for these characters can therefore be taken as a satisfactory basis for selection in this material. 5. The average degree of self-fertilization occurring under conditions of natural crossing has been estimated to be 0.69±.13, based on analyses of the quantitative genetic data for fibre and sucrose percent fresh weight. 6. The within-family genetic variance observed is of the order of five times that expected if the parental clones were homozygous. Much of the genetic variability induced within-families is therefore due to self-fertilization of heterozygous parental material, though the exact proportion cannot be deduced from the present data. 7. The initial response to selection for increased sucrose percent fresh weight is predicted to be of the order of 50% of the population mean. Comparable responses in subsequent cycles of the programme appear to be possible, provided an adequate degree of outcrossing can be achieved by controlled pollination techniques. 8. The genetic correlations between sucrose and other characters of importance are favourable to the objectives of the breeder, or else of a low order. There is no general relationship between the somatic chromosome number of the parental clones and the mean sucrose level of their progeny.