Genetic Parameters and Improvement Strategies for the Pinus Elliottii Var
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Genetic parameters and improvement strategies for the Pinus elliottii var. elliottii x Pinus caribaea var. hondurensis hybrid in Queensland, Australia Dominic Paul Kain March 2003 A thesis submitted for the degree of Doctor of Philosophy at the Australian National University Chapter 1 - 1 Chapter 1 - Introduction This thesis addresses two complementary issues in tree improvement: the genetic improvement of artificial interspecific forest tree hybrid populations, and the genetic improvement of wood properties. The context of hybrid tree improvement A hybrid is an offspring of genetically dissimilar parents. Hybrids can arise from crosses between species (interspecific hybrids), or between types (eg varieties, cultivars, provenances, land races) within a species. Hybrids often display "hybrid vigour", or heterosis: an increase in vigour and yield above the mid-parent value, in one or more traits (Falconer and Mackay 1996). The favourable properties of some hybrids have been known for millenia: native North American Indian tribes, for example, exploited heterosis by ceremonially exchanging maize seeds during meetings between tribes - a practice known to maintain plant vigour and yield (Rife 1965). The utility of other traditional hybrids such as the mule and palomino horse is well known. The use of hybrids in industrialised agriculture, however, did not commence until the beginning of the 20th Century, with the work of Shull (1908) in maize. From 1930 to 1963, the average yield per acre of maize in the United States increased more than threefold, largely due to the substitution of hybrid seed for purebred seed selected using conventional methods (Hallauer 1999). Although numerous scattered tree hybridisation experiments occurred prior to this (Duffield 1981), it was largely the spectacular success of maize hybrids that inspired organised research in tree hybridisation directed at industrial deployment (Wright 1976). While many of the resulting tree hybrids demonstrated favourable characteristics, and these and various putative hybrids were sometimes deployed as unselected composite varieties (eg Venkatesh 1982; Ferreira and dos Santos 1997), it was primarily the difficulty of propagating the vigorous first generation (F1, filial 1) hybrid genotypes (eg Shelboume and Danks 1963; Potts and Dungey 2001) that thwarted organised hybrid breeding and deployment on an industrial scale. More recently, these Chapter 1 - 2 constraints have been overcome many taxa, through improved vegetative propagation (Brandao 1984; Walker et 1996) and pollination (Harbard et 1974; Nikles 1996) technologies. Largely as a result, artificial interspecific forest tree hybrids are currently enjoying a resurgence of interest in industrial plantation forestry programs globally (Griffin et al. 2000; V erryn 2000; Zobel et al. 1987), often yielding results reminiscent of those in maize: "The greatest advance in industrial plantation forestry of the past 20 years has undoubtedly been in the clonal deployment of hybrid genotypes" (Griffin et al. 2000). Increases in stem volume of 20-100% over preferred parental species are not uncommon in interspecific tree hybrids (eg Baltunis et al. 1998 in Larix, Rockwood and Nikles 2000 in Pinus), although enormous gains are often reported in some hardwoods (eg 600% for stem volume in Populus, Li and Wu 1997). Often more importantly than improvements in vigour, however, the gains from preferential inheritance of favourable parental characteristics in a broad range of traits have become an important motivation for adopting 'complementary' hybrid taxa (Nikles and Griffin 1992; de Assis 2000). Additionally, the often strong adaptive characteristics of tree hybrids (eg Potts and Dungey 2001, Verryn 2000) have allowed many forestry agencies to expand into marginal environments that would otherwise be unprofitable (Denison and Kietzka 1993). The number of organised tree hybrid breeding programs has burgeoned in recent years, with efforts in Eucalyptus in the Congo, South Africa and Brazil (Vigneron 1991; Verryn 2000; de Assis 2000), Pinus in Australia, North America and South Africa, and Larix and Populus in North America and Canada (Dungey and Nikles 2000). Large, recently established estates of hybrid Eucalyptus in Brazil, the Congo and South Africa form the bulk of an estimated c 0.5 million ha of hybrids currently in plantation internationally (Dungey and Nikles 2000). However, the focus of hybrid forestry, as in early maize hybridisation, has been on exploiting the gains from hybrid performance rather than seeking to understand its genetic basis or investigate the possibility of further improvement. Research efforts have mainly focussed on pre-first generation hybrid issues, such as useful hybrid combinations and propagation methods. Due to the high risk of uneconomic outcomes, hybridisation has usually been performed in small research experiments Chapter 1 - 3 ancillary to pure species breeding programs: as a result, hybrid breeding populations in most taxa have been small and poorly structured (Payne and Miller 2000). The lack of structured hybrid experiments incorporating pure species controls has contributed to the lack of empirical and theoretical genetic information available to support decisions in many breeding programs now seeking to improve interspecific tree hybrids. The recent inception of hybrid forestry on a large scale, in many countries, has created a strong need for genetic research in aid of evaluating and developing recurrent improvement strategies suitable for tree hybrids. Issues in improvement of interspecific hybrid trees The recurring central issue in interspecific hybrid tree improvement is the high cost of genetic gain per unit capital per unit time invested, relative to pure species breeding. This is largely due to the involvement of multiple populations in hybrid improvement, yet only a single population in pure species improvement. Hence, while a large genetic gain may be made very quickly upon creating and deploying hybrids, the genetic gain per year achieved from recurrent selection thereafter is likely to be less than that achievable using pure species breeding under the same resource constraints. In addressing this central issue, a primary consideration is to ensure that the basis of hybrid superiority is well understood in terms of the hybrid's performance relative to its parental species, in the traits of greatest economic importance. A second, increasingly critical consideration is to revise, improve, and develop cost-effective interspecific hybrid tree improvement strategies based on appropriate genetic parameter estimates, other empirical evidence and practical considerations. To these ends, four key research priorities may be identified as: 1. Assess and understand the basis of hybrid superiority relative to pure species, in terms of trait values; 2. Assess the importance of hybrid testing relative to pure species testing, and thereby choose between existing breeding strategy options, for Fr hybrid improvement; 3. Investigate methods for reducing hybrid breeding cycle interval and expense through early and indirect selection for traits of economic importance, and; Chapter 1 - 4 4. Investigate the modes of gene action contributing to hybrid performance, and use this information to assess the potential for advanced generation hybrid breeding strategies. 1. The basis of hybrid superiority The assessment of hybrid superiority is a primary consideration in hybrid development. The expense of hybrid improvement necessitates careful assessment of benefits from the choice of a hybrid taxon over pure species alternatives. This choice must ultimately be justified based on the economic value of, or profit from, products obtained from the taxa. Although profit can rarely be measured directly, comparison of taxa based on measurable characters known to affect profit can provide a useful indication of superiority. Growth, stem form,. disease and frost resistance are examples of traits commonly assessed for this purpose. Wood density (Harding et al. 2000; Greaves et al. 2000; Borralho et al. 1993) and wood variability (Malan 1997; Wright et al. 1996; Zobel and Sprague 1998) have been shown to strongly affect the value of many wood products, yet have rarely contributed to decisions between taxa in forestry (Zobel and Jett 1995). The inclusion of such traits in taxon comparisons may be critical in some instances. Where taxa differ only slightly in economic value, the choice of pure species improvement may yield large cost efficiencies in breeding, and be preferable to hybrid forestry (Shelbourne 2000; Potts and Dungey 2001). Where the hybrid is clearly economically superior, an understanding of wood variation patterns in the hybrid and parental taxa, and their possible environmental and genetic causes, is likely to be critical for sustained genetic improvement of product value. 2. The choice among breeding strategies for F 1 hybrid improvement In hybrids, the task of further genetic improvement beyond the Ft becomes more complex. While in pure species, improvement can be achieved simply by forward selection in the traits of interest (Cotterill and Dean 1990), this practice has traditionally been avoided in hybrids due to "hybrid breakdown": the deterioration of hybrid performance in forward selected Ft progeny, evidenced from early breeding experiments in model organisms (eg Shull 1908). Consequently, the conventional approach in crops and trees has been recurrent improvement of vigorous F1 hybrid populations.