Pre-breeding utilizing the improved intraspecific clones of Saccharum spp

Abstract

Sugarcane breeding has been a success all over the world by intercrossing the original interspecific hybrids but a very few Saccharum species clones have been exploited in the breeding programme. The exploitation of the unutilized clones of cultivated and wild species of Saccharum in breeding program was suggested as early in 1960’s (Berding & Roach, 1987) who indicated that the repeated use of few parents in hybridization have resulted in narrow genetic diversity in modern sugarcane cultivars. Currently sugarcane cultivars under cultivation have a narrow genetic base tracing back less than 20 S.officinarum, two S.spontaneum and a couple of S. barberi and S.sinense clones which offers a major threat to the sugarcane productivity and demands concerted efforts to utilize new germplasm. Pre-breeding aims at creating new base population by gene introgression that enhances genetic variability in the germplasm for direct use in breeding programmes. It can also assist in identifying heterotic patterns for hybrid programs. Backcrossing is an efficient way to transfer characteristics controlled by one or two genes, although it can be used also for higher number of genes, including quantitative characters. Eberhart (1971) suggested that the first backcross to adapted germplasm is the best base population to start selection. In our study, elite improved clones of S.officinarum and S.robustum produced through intra population improvement programme at ICAR-Sugarcane Breeding Institute (Nair et al..,SBI Annual Report 1991-1996) and commercials were utilized in introgression through backcross breeding. The back cross generations and nobilized hybrids (F1 BC1, BC2) were explored for their heterotic pattern and breeding potential. Selection during nobilisation :The prebred clones developed (utilizing improved S. officinarum, improved S. robustum and commercial Co canes), from three nobilized generations and different mating groups exhibited extensive variability for both qualitative and quantitative traits.They expressed wide variability for various traits like cane population, cane height, cane diameter, canopy colour, internode shape, internode color, spines and flowering. Significant variation was recorded for cane growth and yield characters.The means for the F1, BC1,BC2 generations showed progressive increase of yield traits and sucrose % with successive stages of nobilisation. Heritability estimates varied over the different nobilising generations. Heritability estimates from this study, indicate that selection of parents for sucrose % during the nobilisation process will be effective. The results indicate that gain from selection for yield traits like number of millable canes and single cane weight at the BC1 and BC2 stages was moderate to high. In general, sucrose % have shown moderate to moderately high heritability and clump yield of low to moderate heritability. Genetic improvement in the backcross generations: In the improved S.robustum x improved S.officinarum group, the first generation nobilized hybrids showed an improvement of 15.12% for sucrose % at 360 days over the improved S.officinarum parents. BC1 hybrids showed an improvement of 16.71% and 34.09 % for sucrose % at 360 days in comparison with improved S.officinarum and S. robustum parents respectively. BC2 hybrids showed an improvement of 22.36% and 40.58 % for sucrose % at 360 days in comparison with improved S.officinarum and S. robustum parents respectively. Number of millable canes, cane height and single cane weight exhibited a substantial improvement compared to their improved S.robustum S. officinarum parents. BC1 and BC2 exhibited improvement for both yield and quality parameters suggesting further exploitation. The hybrids from the improved S.officinarum x improved S. robustum group also exhibited improvement for both yield and quality traits. There was an improvement of 23.57 % for sucrose in F1, 30.37 % in BC1 and 33.35 % in BC2 in comparison with improved S.officinarum parents. Similarily an improvement of 41.97 %, 49.78 % and 53.21 % was observed in F1, BC1 and BC2 respectively in comparison with improved S. robustum parents. The BC3 hybrids from these two mating groups also exhibited improvement for yield and quality traits suggesting further studies on cytogenetical behaviour and breeding potential in further backcross generations. Breeding potential of pre-bred clones From the study, the crosses PIR 001157 x PIO 00845, PIO 001057 x PIR 00 1062, PIR 03-107 x PIO 96-475, PIO 94-345 x PIR 96-258, PIR03-107 x PIR 96-475 ,PIO 001100 x PIR 001174 gave more selections that combined yield and quality traits. Crosses with PIR 96-285, PIR 001188, 99 -269 as one of the parents were identified as potential with high quality recombinants. Significant variation was recorded for growth and yield characters in the prebreeding population. Among the prebred materials, 26 clones recorded single cane weight > 1.25 kg, 45 clones with brix % above 20 % and 41 clones with juice sucrose above 18.5 % and most of them were derived from the crosses involving improved S.officinarum x Improved S.robustum.The clones viz., 13-36,13-38,13-39,13-44,13-78,13-80,13-125,13-150,13-151,13-198,13-272,13-266 and 13-250 with good field stand, yield and quality parameters comparable with Co 86032 were identified as genetic stocks for further utilization. The clones 13-69,13-103 and 13-251 showed substantial improvement for sucrose % at 360 days. The clone 13-147 recorded the maximum single cane weight of 1.45 kg.The pre-bred clones were tested for tolerance to water deficit conditions, red rot and for natural incidence of smut. The clones involving PIR 001057, PIR 001058, PIR 96-285, PIR 00 1022 as parents in the back crosses were moderately resistant and moderately susceptible to red rot. The promising clones did not show natural incidence of smut. Under water defcit conditions, 25 clones exhibited good growth and response, coupled high yield (95-98.0t/ha) and sucrose (19.85 -20.94%) indicating their ability of adaptation under changing climate. IOR 360 showed significant improvement for cane yield (21.05%) and sucrose (10.14 %) followed by IOR 396, 258, 1100, 1136 and 1128. These clones also combined resistance to red rot. Earlier studies have indicated that clones with S. robustum cytoplasm performed better at high temperature and water deficit conditions and hence the pre-bred clones with genetic base of S. robustum might prove as a potential donor for tolerance to high temperature. Forty prebreeding clones were selected based on genetic diversity and were screened for molecular diversity using sugarcane specific markers. Variation in allelic size was detected within the range of 122-1136 bp and maximum of up 33 alleles. Among the SSR primers used, NKS 34142 & 185 were specific to PIR 00 1057, marker NKS 8181, 312 & 336 were specific to PIR 00 1057, Maximum molecular diversity was detected in the crosses involving PIR 00-1174, PIR 00-1163, PIR 001057 as female parents indicating the use of these clones as diverse parents in hybridization.

Though introgression of wild germplasm is time consuming and requires considerable efforts and resources, it is very important to improve the commercial breeding populations. Pre-breeding strategy in our study through back crossing has helped in identifying clones combining yield, quality and resistance to red rot and smut. Utilization of these clones by the breeders will help in the development of new breeding populations to work upon and on identifying clones to improve resistance to red rot, smut and tolerance to drought and salinity stresses