Genetic Diversity Parameters Associated with Viability Selection, Reproductive Efficiency, and Growth in Forest Tree Species

Abstract

Biological diversity, “biodiversity” for short (e.g.,Wilson 1988), encompasses all levels of the variability of life, i.e., the diversity within species, among species, among ecosystems, and among biomes. Our contribution addresses the genetic variation within species (intraspecific biodiversity, commonly designated as “genetic diversity”) which is quantified as variation within populations (within and among individuals), among populations, and within metapopulations. The study of functions of biodiversity may follow a hierarchical structure, i.e., coding genes, individuals, ecotypes, species, and various other biological communities. Functionally relevant are also the dynamics of populations and species in time and space, as well as specific characteristics such as abundance, evenness, and reproducibility. At the species level, genetic variation of populations – particularly their potential to create genetic variation (“genetic variability”) – may play a major role in their ability to adapt to heterogeneous environmental conditions and unpredictable host–parasite interactions. Genetic variability is expected to determine the totality of adaptive abilities of populations (e.g., Gregorius 1991; Ziehe et al. 1999). In addition, genetic variation is expected to be correlated with fitness in various plant and animal species (Mitton and Grant 1984; Allendorf and Leary 1986). Biodiversity can also be considered as being redundant (e.g., Lawton and Brown 1993; Yachi and Loreau 1999). The question of whether or not genetic variation (variability) can be expected to be redundant refers to various components of the genetic system of a species including reproduction, gene flow, and the response to stress, but this question has not yet been studied in detail. Concerning functions of biodiversity, forest ecosystems (i.e., natural forests and forest plantations) have a high indicative value because of their longevity compared to other plant species and the wide range of occupied globe. The forest cover substantially affects atmospheric carbon exchange (Waring and Schlesinger 1985) and supplies various economic options such as utilization of timber and many other natural products for various industrial purposes and for energy production. In contrast to agricultural systems, the majority of forests still contain biodiversity in a non-domesticated status. Due to great environmental heterogeneity in space and in time, tree populations are exposed to a variety of biotic and abiotic stresses.Control of stress is not possible in the case of exposure to heat, frost, UV-B radiation, drought, and air pollution, and is strictly limited in the case of biotic stress following pathogen infection or insect attack. Prophylactic disease control such as in agriculture is not possible in forest systems. Adaptational and survival abilities are challenged predominantly by the discrepancy between long generation cycles of immobile trees in contrast to very short generation cycles of their mostly mobile parasites. In addition, most forest tree species suffered from old genetic bottlenecks following post-glacial re-immigration and from severe exploitation, fragmentation, and devastation, particularly since the medieval period. Further challenges for adaptation and survival arise from air pollution and climate change (for survey see Karl et al. 1997; Geburek 2000) which particularly affect longlived forest ecosystems and will result in uncertain future response of forest tree populations, particularly with respect to host–parasite interactions and corresponding changes in susceptibility. In natural and long-lived tree populations, “stress” usually cannot be defined as a single component, but as a complex and dynamic system of highly variable abiotic and biotic factors that affects individuals, populations, and ecosystems alike in terms of dieback of individuals and corresponding reductions of density and size of populations.At the population level, individuals with different genotypes respond differently under stress conditions (e.g., Scholz et al. 1989; Müller-Starck 1993; Ziehe et al. 1999; Geburek 2000). Viability selection following stress causes genotype-dependent elimination of individuals and, consequently, induces changes in the frequency distribution of the corresponding population. Resulting modified frequency distributions are the necessary condition for populations to adapt to the given environmental conditions and to reproduce. The genetic response to stress is manifold such as by mutations, gene regulation, viability, and fertility selection, as well as by losses of genetic variation. Consequently, genetic markers can be employed as a tool for indication of stress (“bioindication”) in various forest ecosystems (Müller-Starck and Schubert 2000). In the marker development and corresponding assessments of genetic diversity in forest tree populations, problems arise from the fact that genome analysis in trees such as Picea abies (Norway spruce) is still rudimentary as compared to species such as Arabidopsis thaliana. One major reason for this gap in knowledge in P. abies is its outstandingly large genome size and pro- The A. thaliana genome is fully sequenced while the analysis of only a minor part of P. abies with low density linkage maps is in progress (e.g., Paglia et al. 1998). Furthermore, handling of individuals is easy in the small A. thaliana as compared to the extraordinarily large individuals in P. abies, with heights up to 35 and 40 m. Also, the reproduction in A. thaliana, with its regular annual flowering, is easy to observe in comparison with P. abies,with its non-regular reproduction starting at a late ontogenetic stage at the age of two to three decades.Natural selection is intensive and indicative of short- and long-term responses to complex environmental stress due to the natural longevity of P. abies populations. Generally, the research community is extremely small in the case of P. abies (not more than 20 molecular genetic groups worldwide), while nearly 1,000 groups study the model species A. thaliana – a situation reciprocal to the economic and also the ecological significance of these two species. In Table 5.1, the genomic peculiarities of Picea abies are highlighted in contrast to a collection of reference species including humans. The specificity of this tree species can be seen by comparing the number of gene loci in relation to the size of the genome. The resulting quotient is, for instance, 935 in Escherichia coli, 212 in case of A. thaliana, and 1 in Picea abies (Table 5.1). The objective of our contribution is to survey experimental studies in longlived tree populations that illustrate differential aspects in the functional significance of genetic diversity. Our main focus is to demonstrate the indicative potential of diversity for vital functions such as reproductive efficiency, growth, and response to environmental stress.