Methods of selection and estimation of genetic variances in population of sugar cane

Abstract

Few studies have been made on the applicabilitynof classical quantitative genetic theory to sugar cane, and few studiesnhave examined the efficiency of selection systems used in sugar cane.nFor this thesis, five experiments were conducted to investigatenvarious aspects involved in the estimation of genetic variance componentsnin sugar cane, Including the applicability of the classicalntheory. These experimentsalso provided information on the efficiencynof selection systems in original seedlings. A sixth experimentnexamined selection in 30-sett plots, with particular reference tonthe benefits conferred by weighing the plots.nMany of the assumptions underlying classicalngenetic theory are unlikely to be valid in sugar cane. The implicationsnof non-validity were tested in Experiment II which allowedncomparison of independent estimates of genetic variances derivednunder different genetic assumptions. The character Brix was measurednwith good statistical precision, and the estimates of genetic variancesnsuggested that the effect of non-validity of the assumptions wasnlimited. For other characters, measurement was less precise, andngreater differences occurred between estimates of genetic variances.nHowever, the differences were not statistically significant.nAdditive genetic variance was more important thanndominance genetic variance for m s t characters. However, substantialndominance genetic variance was shown for tons cane per acre and tonsnBrix per acre in two experiments. Epistatlc genetic variance wasnapparently small.In general, differences between reciprocal crossesnwere small. Maternal effects were unimportant except in Experiment IVnin which they could be attributed to the effect of accidental self-pollination.Cross X environment interactions were generallynsmall in all experiments in which they were estimated. However, dominancengenetic variance interacted with years for several charactersnin Experiment 1. The male parent x year interaction was significantnfor all characters ln Experiment II, but this was attributed to yearsnbeing completely confounded with crop class (plant or ratoon); thatnis, the interaction was due to differential behaviour of male parentsnin the plant and ratoon crops.Estimates of variance components from plant andnratoon crops were similar for Brlx, but differed for the otherncharacters. The differences were attributed to the Interaction betweennmale parents and crop class. Estimates of variance components fromnseed-planted and sett-planted material were found to be comparablenfor most characters. These data suggested that estimates from seed-plantednmaterial could be applied to sett-planted populations, whereasnestimates from plant crops may not be applicable to ratoon crops.nWithln-family competition had a major effect onnthe estimation of both environmental and genetic variance components,nparticularly in the ratoon crop of Experiment II. Many negativenestimates of variance components were obtained, and most of thesencould he attributed to the effect of competition. In particular,nmany estimates of dominance genetic variance were biased negatively.nHerltability and degree of genetic determinationnon an Individual basis in original seedling populations were lown( l0.3) for all characters except Brix which had a value of approximatelyn0.5. For 30-sett plots, the degree of genetic determinationnfor yield relative to a standard was 0.5, Indicating that selection for yield in these plots should be more effective than in originalnseedlings.Few characters exhibited a high degree of geneticnvariability as measured by the genetic coefficient of variability.nFor most characters, the coefficient was less than 20 per cent and,nfor Brix, it was only six to eight per cent. Thus, although Brixnhas a high degree of genetic determination, progress from selectionnwill be slight due to the restricted genetic variability in thenpopulation.Genetic, phenotypic, and environmental correlationsnbetween important characters were estimated, and their implications innselection were discussed. The important genetic correlation betweennBrix andyyield of cane was variable, but was concluded to be small andnnegative, in general.nA simulated selection experiment on a population ofnoriginal seedlings was conducted using the results of Experiments Inand II. Mass selection based on a phenotypi: seore (called a grade)nwas the most effective method tested.In Experiment VI, weighing of 30-sett plots wasncompared with visual estimation of yield. Weighing of plots resultednin a gain from selection for grade of approximately five per cent.nMany elite clones that would have been discarded on the basis of visualnestimation of yield in 30-sett plots were retained by selection basednon weights.n