M.D. WILCOX
Forest Research Institute, New Zealand Forest Service,
Rotorua, New Zealand
SUMMARY
Tree breeding programmes in species widely planted internationally may benefit from new genetic material introduced as seed, scions, pollen, and tissue culture plantlets.
Importations can: introduce otherwise unobtainable provenances superior to those available locally; capitalise on someone else's skill and success in selecting and breeding superior genotypes; give faster growth as a result of evolution of superior exotic land races; and save time and work.
Importation of tree breeding material has no intrinsic merit itself as a method of tree improvement. With highly selected material, its value depends on the degree of phenotypic selection and accuracy of progeny or clonal testing, and on the relevance of selection criteria to recipient programmes.
INTRODUCTION
The theme of this paper is that exchanges of selected tree breeding material, as opposed to acquisition of “raw” seedlots for provenance trials, among countries growing the same forestry species can be beneficial, but are not necessarily so. The value of exchanges depends on the quality of the material, which in turn rests on the skill of the donor tree breeder; and, most importantly, on its provenance and breadth of genetic base represented by the founding seedlots of early exotic introductions. History plays an important part in the genetic make-up of cultivated stocks of many important species grown as exotics.
Species such as Pinus caribaea Morelet, Pinus radiata D. Don, Pinus patula Schiede & Deppe, Cupressus lusitanica Mill., and Eucalyptus grandis Hill ex Maid., have assumed great international importance, predominantly in exotic afforestation, and offer excellent scope for useful genetic exchanges because of the many countries actively cultivating and breeding them. Likewise, Pinus taeda L., Pinus elliottii Engelm., and Pseudotsuga menziesii (Mirb.) Franco, which are premier forestry trees within their native ranges in North America and the subjects there of large planting programmes and widespread tree breeding activity, present many opportunities for gene exchanges as they are also very important as exotics in several countries.
OBJECTIVES
No special genetic theories need be invoked to predict the value of international gene exchanges. Genetic gain in tree breeding programmes utilizing imported material is realised from the same principles of forest tree improvement as in independent breeding programmes:- provenance selection, intensive selection within the best populations, maintenance of a broad genetic base, progeny test selection for traits of low heritability, recognition of genotype x environment interaction effects, and avoidance of inbreeding in seed orchards. The only special precautions required in international exchanges of selected material are suitable quarantine measures and the realisation that selection criteria may differ markedly from one country to another, thereby possibly negating the attainment of genetic gains of economic significance.
Procurement and exchange of breeding material may fulfil four general purposes.
Immediate seed supply for planting programmes
Importation of seed orchard seed or superior clonal stock from other countries is potentially the fastest way of achieving genetic improvement in current planting programmes not able to be serviced by local seed supplies. The procuring country should preferably carry out prior performance tests of several alternative seed sources to identify the best overseas suppliers of improved seed.
Importations of seed orchard seed may be very useful in initiating afforestation projects. However, the genetic base of the resultant plantations could be far too narrow from which to develop a local breeding programme, should such be necessary. A more immediate disadvantage is that imported orchard seed may not perform as expected because of inappropriate selection criteria, and genotype x environment interactions.
New species introduction
It is common to find that major exotic tree species in many countries were originally introduced haphazardly, with only vague records of provenance or size of the genetic base.
International provenance trials are excellent for formally introducing a species for the first time, or for systematically re-introducing a species of proven commercial importance. Another good way of introducing a species is to use a wide range of selected open-pollinated seedlots from other countries, including selected samples from native stands. As long as the sample is large, there are obvious advantages in early obtaining material selected for universally desired characteristics such as better stem straightness and freedom from malformation.
Strengthening existing breeding programmes
Breeding programmes are usually founded on plus trees selected in local native stands or plantations and will therefore be comparatively narrowly based with respect to the genetic resources of the species as a whole. As the second and third generations of breeding are reached, worrisome coancestry builds up in the breeding population, giving fear that related clones in seed orchards could cause trouble from inbreeding depression. Furthermore, intensive selection for several characteristics may have reduced the scope for breeding for “new” characteristics in the future; in such a situation there will be considerable benefits in introducing new, improved material.
Another possibility is that better provenances than those used to initiate the breeding programme may be identified at a later stage; somehow, this better material has to be incorporated into, if not completely supersede, the current breeding population.
Acquisition of genetic material from overseas programmes, including those where the species is native, can obviously be very helpful in augmenting an otherwise isolated programme with an injection of unrelated or better material.
Rapid initiation of new programmes
Every year delayed in using genetically improved stock for afforestation represents lost opportunity for genetic and thus economic gain from improved strains once they are finally available. It is therefore important, if the species is really a certainty, to find or develop improved seed sources of it quickly. One way is to import the material “readymade” from another organization already well advanced in tree breeding and having far greater resources of trees for selection than presently available in the recipient country.
METHODS
Seed
Seed is the most flexible way of introducing genetic material, and has the formidable advantage of allowing opportunity for progeny to be tested before a big commitment is made to them. Family tests can lead quickly and directly to seed production (through seedling seed orchards), provide a source of new selections for the next generation, and an evaluation of the donor's populations for future introductions.
What sort of select seed should be obtained? Open-pollinated (ex ortets), open-pollinated (ex clone bank or seed orchard ramets), controlled-pollinated polycross, or controlled-pollinated full-sib families are the main possibilities. Probably the most generally useful material would be seed of a set of unrelated full-sib families from random single-pair crossing amongst the best available parents in the best provenances. These could be excellent for development into seedling seed orchards, and for new selection programmes. The expectation of genetic gain would naturally be less with open-pollinated families from ortets, and the restricted pollen base of families derived from seed orchard ramets definitely limits this sort of material for long-term breeding.
Pollen
Pollen - the microspores of conifers and flowering plants - is haploid (n) genetic material comprising the male gametophyte, and ultimately producing the male gametes. Especially in conifers it can be an excellent means of effecting gene transfers among programmes as it can be collected in quantity, is easily packaged and posted, presents no problems with quarantine, and can be stored until required. Nevertheless, adequate storage facilities are essential, and there must be a proper mating design (e.g., polycross, nested, factorial) drawn up in advance, with a selection of good local trees on hand to serve as female parents in the design. Pollen of insect-pollinated species such as eucalypts is obviously more difficult to transfer.
Scions
Scions can be exchanged in the form of grafting material or as unrooted cuttings. They afford a direct way of introducing genotypes intact. The commonest transfers of scions have been with grafting material of plus trees, allowing recipients to establish their own clonal banks or seed orchards.
Quarantine problems and delays, transport, and assorted hassles in propagation greatly restrict the international exchange of scions and their successful utilization in breeding programmes.
Scion introductions for seed orchards should be accompanied by a full set of seedlots suitable for progeny testing the clones alongside locally selected candidates. There is a tendency for imported clones not to be properly tested either because no seed was available or because of a mistaken belief that imported material must be good and need not be tested.
Tissue culture plantlets
There have already been some successful transfers of tree genetic material in vitro from one research laboratory to another. Because cultures are strictly maintained under sterile conditions to prevent microbial contamination of the rooting media, healthy tissue cultures can safely be introduced with little risk, without the need for quarantine, thus saving time. Tissue cultures are also obviously more compact and efficient to handle than traditional scion material, allowing much more material to be moved in a single operation.
Needless to say, recipient organizations must have the proper facilities and expertise to make full use of tissue culture introductions.
There is nothing yet inherent in present tissue culture technique for trees to give it any particular advantage over the other methods of gene exchange in terms of genetic gain. It is purely a vehicle of exchange - not a method of breeding. The best scope for it could be as a means of transferring propagules of outstanding genotypes, and rapidly multiplying them.
EXAMPLES OF INTERNATIONAL GENE EXCHANGES IN TREE BREEDING PROGRAMMES
Poplar breeding in New Zealand
Cultivation and genetic improvement of poplars in several countries was initially based on local programmes of introduction, testing, and certification of promising clones from overseas. In New Zealand for example (van Kraayenoord, 1968), most poplar planting for erosion control and farm woodlots from 1958 to 1974 was with large unrooted cuttings of nine hybrid black poplar clones from Europe: Populus x euramericana cv. ‘Robusta PH’, P. cv. ‘Robusta Zeeland’, P. cv. ‘Eugenei’, P. cv. ‘Laevigiata’, P. cv. ‘I-30’, P. cv. ‘I-78’, P. cv. ‘I-214’, P. cv. ‘I-455’ and P. cv. ‘I-488’. Clonal tests in New Zealand had shown these to be the most suitable clones out of more than a hundred tested.
The spread of the poplar rusts Melampsora medusae Thum. (American poplar rust) and Melampsora larici-populina Kleb. (European poplar rust) from Australia to New Zealand in 1973 (N.Z. Forest Service, 1973; van Kraayenoord, Laundon and Spiers, 1974; Wilkinson and Spiers, 1976) effectively eliminated the “standard” nine hybrid black poplar clones from planting programmes - all were too susceptible to the rusts. Two replacement clones, P. × euramericana cv. ‘Flevo’ and P. cv. ‘I-154’, resistant to both rusts, were promptly selected from clonal archives and released for multiplication and planting in 1974 (Wilkinson and van Kraayenoord, 1979).
Through international co-operation fostered by the International Poplar Commission of FAO, and good personal contacts between poplar breeders in New Zealand and overseas, New Zealand was quickly able to import several new rust-resistant clones from Korea, Japan, Argentina, Australia, Italy, Turkey, Yugoslavia, Holland, and the United States. Past experience with imported selected poplar clones is that clone x environment interactions are often large internationally, necessitating stringent testing and re-selection under local conditions. There are nevertheless some clones such as ‘I-214’ that have grown very well in all countries with a suitable climate.
Poplar breeders in New Zealand have imported or exchanged tree breeding material in the form of woody cuttings, seed, and tissue cultured plantlets. New Zealand regulations require that imported cuttings be quarantined for 2 years before multiplication and clonal testing can begin. Cultured plantlets in test tubes or plates have proved to be an easy and effective means of exchanging material. No quarantine period is required after arrival of stock in vitro because healthy, sterile cultures are already guaranteed free of microbial infection (except viruses). Rapid multiplication of material in the recipient country is therefore possible.
Pinus radiata
Pinus radiata D. Don., a native of California in the United States, is the major tree planted for forestry in Chile, New Zealand, and Australia, and is also commercially important in Spain, South Africa, and Kenya. It shows promise in Italy, Tunisia, Morocco, France, Portugal, and Ireland, but is not planted much in its native country.
International exchanges of seed, pollen, and scions have featured in several tree improvement programmes established in the species (Fielding, 1966; Pederick and Griffin, 1978; Thulin and Wilcox, 1968).
Grafting scions from 23 of New Zealand's original group of 40 plus trees selected in the 1950's from local stands were sent to various organizations in Australia. New Zealand clones are still important in several Australian orchards today. The immediate value to Australia of these importations in the 1950's was in obtaining material of very intensively selected plus trees to augment individual local selection programmes. Apart from saving time and work, the real benefits of sending the New Zealand clones to Australia need to be judged in terms of breeding value (as determined from progeny tests) and orchard performance (graft compatability and seed production) in Australia of the New Zealand clones in comparison with Australian clones. The 23 New Zealand clones constitute only 4.5% of the 506 first-generation clones now in the Australian Plus Tree Register (Pederick and Griffin, 1978), and one widely-tested clone, NZ 850–55, has consistently ranked with the best local clones in Australian progeny tests (Eldridge, 1974). The value to Australia of these early scion transfers of untested clones appears promising, but has not really been properly assessed; the question of whether phenotypic selection in New Zealand stands was effective also for Australian sites is unanswered. Nor is there any evidence that distinctive land races that might have evolved in two or three generations of cultivation in New Zealand are better or worse in Australia than local land races.
Potentially the most useful and mutually beneficial international exchange of selected genetic material in P. radiata is the co-operative family experiment organized by Shelbourne (1973). In this project, 319 seedlots from plus trees (open-pollinated families from ortets and/or orchard ramets), controlled full- and half-sib pollinated families, and some open-pollinated families from native stands, were assembled from New Zealand, Australia, Kenya, South Africa, France, and California. In addition to the donor countries, Chile, Tunisia, and Morocco planted out test plantations of the families. The general belief is that the international gene pool is “a good thing”, but how much direct use can be made of the material in local breeding programmes will depend on performance of the families and the needs of individual programmes. In New Zealand, the best imported families are seen as a potential source of new plus trees. For countries not yet heavily involved in their own breeding programmes, the gene pool could be a very good foundation for local selection programmes leading to seedling and/or clonal seed orchards.
Eucalypts
Countless transfers of eucalypt seed have been made from Australia to nearly all parts of the world. These importations have resulted in arboretum or roadside plantings, species trials, provenance trials, as well as full-scale industrial plantations and the base for local breeding programmes. Many introductions have been haphazard and of unknown quality in terms of provenance, number of seed trees represented in seedlots, and degree of selection (other than for a heavy seed crop) exercised by seed collectors. For most eucalypts the native Australian forests nevertheless remain by far the most important source of genetic material (Turnbull, Nikles, and Brown, 1980).
As a future base for breeding in recipient countries, it is suggested that the most valuable types of eucalypt seed importations from Australia are:
commercial seedlots from numerous trees in a specified region (e.g., samples from large seedlots collected from logging areas for air-seeding or planting programmes);
special collections from numerous individual trees of a range of provenances (e.g., CSIRO provenance study collections, or open-pollinated family seedlots from Australian breeding programmes).
Imported open-pollinated families from breeding programmes or provenance collections in Australia can be very good supplements or alternatives to any local families selected, provided the provenances are suitable. In both Eucalyptus fastigata Deane & Maid. (Wilcox, Rook and Holden, 1980) and Eucalyptus regnans F. Muell. (Wilcox, Faulds, Vincent and Poole, 1980), many of the Australian families imported have proved to be far superior to local families in frost resistance in New Zealand, mainly because the best provenances are not fully represented in local plantations.
There can also be benefits in a eucalypt breeding programme in obtaining exotic seedlots from other countries growing and/or breeding the same species. These benefits are unpredictable, but could include capitalising on effective natural and artificial selection in even-aged plantations or provenance tests, fortuitous choice of an excellent founding provenance, or avoidance of troublesome hybridisation in local stands. Examples are the introduction of very frost-hardy families of E. fastigata from Natal in South Africa to New Zealand (Wilcox, Rook and Holden, 1980), and the good results obtained in Brazil with seed of Eucalyptus grandis Hill ex Maid. imported from South Africa and Rhodesia (Zimbabwe) (Campinhos and Ikemori, 1978).
Pinus caribaea
One of the justifications claimed for the much-discussed international co-operative breeding programme with Pinus caribaea Morelet var. hondurensis Barrett & Golfari was that advantageous exchanges of genetic material could be arranged among co-operators (Nikles, 1978, 1979; Nikles and Burley, 1978). The good performance in Fiji of certain families from Queensland has been taken as evidence that international gene exchanges in this species have good potential. Subsequently, plans have been advanced to institute international progeny testing and seed orchard programmes.
Foremost in any plans for breeding P. caribaea should be results obtained in the international provenance trials sponsored by the Commonwealth Forestry Institute, Oxford (Greaves, 1978). These seedlots represent a precious source of information and a genetic base of immense potential for future selection in all countries growing this species. Thus, the value of future exchanges of selected tree breeding material rests heavily on the provenance being optimal for the recipient programme.
Areas such as Queensland and Fiji (Bell, 1979) with substantial plantations old enough for selection have been able to achieve considerable improvement in stem straightness in P. caribaea var. hondurensis from phenotypic selection in first-generation stands derived from seed importations from Mountain Pine Ridge, Belize. However, there is now evidence in Queensland that certain coastal lowland provenances from Honduras, Belize, and Nicaragua are better there than the Mountain Pine Ridge strain in several important features, namely wind resistance, stem straightness, and height growth.
Consequently, incorporation of these good characteristics into the current breeding programme is under investigation. One possibility is the importation of pollen from plus trees selected in exotic stands (e.g., in Brazil) derived from coastal lowland provenances, followed by artificial hybridisation with the local base of Mountain Pine Ridge strain clones.
CONCLUSIONS
Importations of tree breeding material from other countries are likely to be beneficial and worth the effort involved if they give :-
better provenances than those presently available in local plantations or readily obtainable from native stands;
better quality individual genotypes (i.e., clones), F1-hybrids, gametes (pollen), or families than those which could be selected locally; “better” in terms of desired or relevant selection criteria, selection intensities, and proven performance in tests;
greater numbers of “new” or “different” genotypes unrelated to existing local stocks, thereby minimising inbreeding problems in seed orchards, and broadening the base of genetic variability for future selection;
better growth from seedlots collected in large exotic plantations than from seedlots collected in native stands or stands of limited genetic base as a result of natural selection and adaptation to “new” environments, from silvicultural selection, and from higher rates of out-crossing;
a large saving in time and/or work.
Seed, scions, and pollen each have their place in gene exchange activities :-
seed transfer, in many species, is the easiest, the most versatile, and generally the most useful method of gene exchange;
pollen is potentially the fastest, most direct, method of incorporating new material into well-advanced breeding programmes having the necessary resources of recipient female parent clones;
scion transfers can involve delays or losses in quarantine and are justified mainly for the procurement of the very best, thoroughly pre-tested, or uniquely superior genotypes;
tissue culture offers a compact method of transferring clonal material, without the problems of quarantine.
Good planning is required to make the best possible use of international gene exchanges :-
when requesting material from another organization, give adequate details of how it is proposed to use the material and why it is needed;
benefits of gene exchanges will be enhanced if there is also preparatory groundwork in the form of sustained personal contacts, letters, exchanges of reports and publications - all helpful in fostering mutual understanding of the programmes involved.
LITERATURE CITED
Bell, T.I.W., 1979 Pinus caribaea seed from Fiji. Fiji Pine Research Paper No. 4. Fiji Pine Commission, Lautoka, and Fiji Forestry Department, Suva, 4p.
Campinhos, E. Jnr. and Ikemori, Y.K. 1978 Tree improvement program of Eucalyptus spp. Preliminary results. Proceedings of Third World Consultation on Forest Tree Breeding, Canberra, 1977, pp. 717–738.
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