A comparative study of two Eucalyptus hybrid breeding strategies and the genetic gains of these strategies

Abstract

The hybrid combination between Eucalyptus grandis and Eucalyptus urophylla plays a significant role in the production of pulp in the South African Forestry Industry. Superior clones of this hybrid combination have been bred using a conventional hybrid breeding strategy and planted successfully in the subtropical coastal region of South Africa, namely Zululand. Although the conventional hybrid breeding strategy of these species has delivered superior clones for commercial production, it is a time consuming strategy to follow. One of the constraints with the conventional hybrid breeding strategy is the time required to first test the hybrid material as seedlings before clonal testing commence. In order to address this, an accelerated version of the conventional hybrid breeding strategy is being investigated with the main focus on reducing the testing time of E. grandis × E. urophylla hybrid material as seedlings. With this in mind, this study was set out with two main aims, firstly to review the conventional hybrid breeding strategy, and secondly to do a comparative study between the conventional hybrid breeding strategy and the accelerated hybrid breeding strategy. In order to review the conventional hybrid breeding strategy, as well as to identify the impact of accelerating this strategy has, information on the genetic control of the traits of interest is needed for the E. grandis and E. urophylla pure species populations, as well as hybrid seedling and clonal populations. Additional information such as the correlation between parents’ performance in pure species and hybrid combination, as well as the ortet-ramet correlation of the hybrids is also essential for developing an effective hybrid breeding strategy. The main objectives of this study were, therefore, to first estimate the genetic parameters and indentify selections of E. grandis and E. urophylla pure species populations in Zululand; secondly to estimate genetic parameters of E. grandis × E. urophylla hybrid seedling and clonal populations; thirdly to investigate the correlation between E. grandis and E. urophylla parental General Combining Ability (GCA) and their General Hybridising Ability (GHA); fourthly to determine the correlation between E. grandis × E. urophylla hybrid seedling ortets and their ramets and lastly to do a comparative study between the conventional hybrid breeding strategy and the accelerated version of this strategy. Results of the E. grandis and E. urophylla pure species populations indicated that genotype by environment interaction (G×E) effects were present, but would be practically negligible for growth in Zululand and a single breeding population will therefore be appropriate. In general, volume was under low to moderate genetic control, with narrow-sense heritabilities ranging between 0.14 and 0.48 for E. urophylla, and between 0.16 and 0.23 for E. grandis. Best linear unbiased predictor (BLUP) estimates indicated that elite selections could produce genetic gains of approximately 60% and 30% over the E. urophylla and E. grandis population means respectively. With regards to the E. grandis × E. urophylla hybrid populations, results indicated that non-additive genetic variation explained the majority of the total genetic variation and ranged between 68% (seedling population) and 88% (clonal population from accelerated strategy). For the E. grandis × E. urophylla seedling population, most of the additive genetic variance was contributed by the E. urophylla parents (h2male = 0.23) and a correlation of 0.58 (p<0.007) was detected between the GCA and GHA values of the E. urophylla parents. However, for the E. grandis × E. urophylla clonal populations, narrow-sense heritabilties were low and ranged between zero (female) and 0.08 (male). Majority of the non-additive genetic variation was explained by the proportion of dominance variance ( =0.16), and less by the clone within family effect ( =0.12). Accelerating the conventional hybrid breeding strategy by shortening the testing time of E. grandis × E. urophylla seedlings resulted in an increase in percentage realised volume gains per year (from 1.9% to 3.7%) when compared to the E. grandis × E. urophylla commercial clone.