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Genetic or Gene Conservation

W. S. Dvorak
North Carolina State University

Genetic conservation or gene conservation refers to the protection of diversity in forest tree species or populations that are being threatened by natural or man-made causes.  Generally two different approaches can be used, in situ or ex situ gene conservation.   In situ gene conservation is defined as the protection of a target tree species or population within its natural or original ecosystem or on a site formerly occupied by that ecosystem (FAO 1993). Ex situ gene conservation refers to a method that entails the removal of individual trees or propagating material (seed, pollen, tissue) from its site of natural occurrence (FAO 1993).  The goal of gene conservation is to “capture” enough genetic variation to ensure the continued existence of a species or population as a viable and dynamic breeding system.

Different forms of the same genes, called alleles, occur at an individual locus on a chromosome. Applied gene conservation efforts are really aimed at capturing alleles. Alleles can be of three types: common, rare, and private or unique. A common allele is one that is found at high frequencies in almost all populations of a tree species. A rare allele occurs at frequencies of 1% or less in a population. A private or unique allele, as the name implies, occurs only in one population. 

Alleles that are rare in one population can sometimes be found at higher frequencies in other populations. Sound gene conservation efforts are therefore based on knowing whether the traits to be protected in a species are controlled by a few or many alleles, and whether these alleles are at high or low frequency in a population. Generally it is thought that traits like adaptability and growth are controlled by many genes/alleles and traits like disease resistance might be controlled by only a few genes/alleles.  
 

In determining the most efficient method of sampling alleles in a tree species it is important to know its geographic range.  Generally, the larger the geographic range the greater the genetic diversity in the species (Hamrick et al. 1992) and the greater the number of populations that need to be sampled to maintain broad adaptability. The exception to this is when the species has gone through a genetic bottleneck.  A genetic bottleneck occurs when the species or population has descended from one to a few parent trees.

Examples of genetic bottlenecks in forest tree species are numerous (e.g. see Ledig et al. (1999)) but the classic example is Pinus resinosa. It has a large geographic range in northern United States and southern Canada, but exhibits little phenotypic variation in growth or stem form (Fowler and Morris 1977) and DNA marker assessments  show little differences among populations (Mosseler et al. 1992). Population geneticists suggest that its current geographic range is the result of expansion from a small refuge in the Appalachian Mountains after the retreat of the glaciers in North America.

 

Applied Gene Conservation Approaches

In situ Conservation

National  Parks, Biosphere Reserves and Protected Concession Lands all serve as suitable locations for in situ conservation areas for forest species.  The size of the area needed to adequately conserve a tree species is dependent on several factors that include its relative abundance in the designated protection area, its successional status, its mating system (animal ingested, animal, insect, wind-pollinated) and its reproductive biology (frequency of seed crops, seed germination rates etc.).  Improved surveys using Global Information Systems (GIS) have greatly aided in the mapping of the location of endangered tree species in remote areas of the Americas. Unfortunately, many in situ gene conservation programs have failed, especially in developing countries, because of inconsistent management, insufficient funds, and continued human encroachment and colonization.

Ex situ Conservation      

Ex situ conservation is complementary approach to in situ conservation.  In an applied program, the initial step includes a survey of the species’ natural range to determine patterns in environmental gradients. If possible, a range-wide assessment of genetic diversity is made using molecular markers to define those areas where sampling should be concentrated. Approaches become more scientifically sound and applied when molecular marker information taken from a number of natural populations is combined with provenance field results across many sites.

One of the most important decisions is to determine what frequency of alleles needs to be conserved ex situ in the species or population.   The lower the frequency of allele desired, the greater the number of trees that need to be sampled per population. Number of trees per population is also dependent on whether the stand is large and intact or small and fragmented (Dvorak et al. 1999). The number of selected trees per population and the estimated number of male pollinators that pollinate each tree determines the “effective population size” (Sewall 1931, 1938). 

In most applied ex situ conservation program that target wind-pollinated species, a 100 m distance is kept between each selected trees to reduce the risk of inbreeding.  Larger distances might be necessary for insect and animal pollinated species (e.g. see Boshier and Lamb 1997). If data are lacking on genetic diversity, a sample size of 20 to 25 trees per population from 6 to 10 populations across a species’ geographic range appears to sample most alleles at frequencies of greater than 5% in ex situ seed collections (Dvorak et al. 1999). This assumes that the species is one with a large geographic range and exhibits average levels of genetic diversity.

Successful ex situ gene conservation effort are not determined by number of trees sampled for seed collections in natural stands, but by how many seedlings and trees  survive when grown in exotic conditions. Good species/site matches are critical to success. These can be based on field experience and/or computer models that are designed to search for areas of similar climate in both the donor and receptor countries. Important to all ex situ gene conservation efforts is knowledge about the potential of an introduced tree species becoming an invasive pest. Because of the limitations of in situ program described above, gene pools for some tree populations only reside in ex situ plantings, long distances from their place of origin.

Issues and Developments   

Between 1990 and 2005, the world lost 3% of its forested area (State of World’s Forests 2007). The future of the breeding base for many tree species will be dependent on how well genetic base populations are managed and conserved.  

There are a number of issues related to in situ and ex situ forest tree gene conservation that determine their effectiveness and success or failure, especially in developing countries. For in situ conservation approaches, genetic diversity results generated from molecular marker analysis seldom influence local governments to change their policy on whether to protect a tree species or specific populations. In situ conservation efforts need to be better linked to economic and social development. 

 
Ex situ

Recent trends in global climatic fluctuations combined with improved forest biotechnology approaches should strengthen efforts for sound gene conservation strategies.  Governments understand the ramifications of global warming for natural and planted forests. Higher temperatures with resulting increased insect and disease problems jeopardize natural and man-made forest plantations comprised mainly of only one tree species.  Risk management dictates the accumulation of a large, well-adapted base population for the commercial species, as well as the conservation of genetic base populations for alternative species and potential hybrid parents. Gene conservation will become more rather than less important in the future, and will be driven by the basic economic need to successfully grow trees on more marginal sites in areas where climatic fluctuations are pronounced. 

At the same time, there are great advances in functional genomics that explore the role of individual genes.  In the future, functional genomics might better explain the role of so- called “neutral genes” in genetic diversity assessments and will be able to link their importance to the expression of metric traits in provenance field trials (Burdon andWilcox 2007).  Furthermore, the new technologies might allow us to know the actual importance of rare alleles and their value for tree conservation programs.  The great need for genetic conservation of forest species and biodiversity does require that we examine all reasonable means to protect the gene pool of trees, and use this range of technological tools that are available.

 

References

Boshier, D. H. and A. T. Lamb. 1997.  Cordia alliodora-Genetics and Tree Improvement. Tropical Forestry Papers 36, Oxford Forestry Institute, Department of Plant Sciences, University of Oxford. 96 p.

Burden, R. D. and P. L. Wilcox.  2007.  Population management: potential impacts of advances in genomics. New Forests Vol. 34, No. 2:187-206

Dvorak, W. S. 2006. Ex situ conservation of forest trees: Effective strategy or overstated promise? (Abstract). In Proc. of the IUFRO Division 2 Joint Conference: Low Input Breeding and Conservation of Forest Genetic Resources. Edited by Fikret Isik. p. 41. Available from http://www.akdeniz.edu.tr/english/iufro/

Dvorak, W. S., J. Hamrick, and G. R. Hodge. 1999. Assessing the sampling efficiency of ex situ gene conservation in natural pine populations in Central America. Forest Genetics 6(1):21-28.

FAO 1993.  Conservation of genetic resources in tropical forest management. Principles and concepts.  FAO Forestry Paper 107. Food and Agricultural Organization. Rome, Italy.  105 p.

Fowler, D. P and R. W. Morris. 1977.  Genetic diversity in red pine: evidence for the low genic heterozygosity. Canadian Journal of Forest Research 7(2).  pp.343-347.

Hamrick J. L, M . J.W.  Godt and S. L Sherman-Broyles  1992.  Factors influencing levels of genetic diversity in woody plant species.  New Forests 6:95-124.

Ledig, F. T., M. T. Conkle, B. Bermejo-Velazquez, T. Eguiluz-Piedra, P. D. Hodgkiss, D. R. Johnson, and W. S. Dvorak.  1999.  Evidence for an extreme bottleneck in a rare Mexican pinyon: Genetic diversity, disequilibrium, and the mating system in Pinus maximartinezii. Evolution: 53(1):91-99.

Mosseler, A, K. N. Egger and G. A. Hughes. 1992. Low levels of genetic diversity in red pine confirmed by random amplified polymorphic DNA markers. Canadian Journal of  Forest Research. 22(9):1332-1337 

State of the World’s Forests 2007.  Food and Agricultural Organization of the United Nations. Rome, Italy. 144 pp. 

Wright, S. 1931.  Evolution of Mendelian populations. Genetics. 16:97-159.

Wright, S. 1938.Size of population and breeding structure in relation to evolution. Science. 87:430-431

Posted 30 September 2007