Attempts to identify candidate genes of adaptive importance, and to relate genetic variation in these genes to phenotypic expressions in multi-locality field trials, have typically been hampered by a complex polygenic architecture
[71][35], and a limited understanding of the physiological trade-offs
[48][23]. For instance, theoretical expectations dictate that local selection at a single locus will promote local adaptation in the absence of gene flow (i.e., selection–migration balance
[65][28]). However, more complex polygenic quantitative adaptation can even be established and maintained in the presence of high gene flow
[72][36]. While the discipline of molecular quantitative tree genetics
[73][37] merges with the field of ‘big data’ analytics
[74][38], an expanded view of complex traits is arising, moving from a polygenic framework to a view in which all genes are liable to affect adaptation to biotic stresses (the omnigenic model described above,
Figure 2b
[16][34]), so that most heritability can be explained by the effects of rare variants, their second order epistatic interactions, and with epigenetic factors, even accounting in this way for transgenerational epigenetic inheritance
[75][39]. However, looking back, the metabolic basis of tree evolution still has the potential to improve plantations’ yields because natural selection has tested more options than humans ever will. Mining the molecular footprint of selection and adaptation from
in situ sampling for tree pre-breeding and climate adaptation will benefit from bridging the gap between phenotyping and genotyping across provenances, and the more deterministic quantitative and population genetic models.
A useful type of polygenic model, yet to be calibrated within an omnigenic framework
[16][34], is genomic prediction (GP)
[76][40]. Predictive breeding via GP allows assessing genetic estimated values (GEVs) for biotic stress resistance
[77][41]. GP uses historical resistance data to calibrate marker-based infinitesimal additive predictive models
[78][42], which provide a more comprehensive representation of a quantitative polygenic trait than traditional genetic mapping
[79][43], a tendency that several biotic resistances have started exhibiting
[80][44]. Therefore, GP offers a key path to assist the introgression breeding of biotic stress resistance from key donors (via genomic-assisted recurrent backcrosses—GABC, as successfully applied in the breeding program for blight resistant in American chestnut trees
[81][45]). GP’s predictive ability can be significantly enhanced after performing
a priori weighted resistance mapping through more conventional methods such as quantitative-trait loci (QTL) mapping or genome-wide association studies (GWAS)
[82][46]. These mapping strategies enable choosing target SNP arrays for high throughput genotyping of multi-parental populations
[83][47] via SNP-Chips (
Figure 21c).
GP may also go beyond pre-breeding efforts, and feedback on restoration optimization
[84][48] and provenance characterization
[85][49] (e.g., by predicting biotic resistance and yield) even across thousands of half-sib families that could hardly be tested at once in field and lab trials for pests and herbivore resistance
[86][50]. Expectations within these half-sib families are likely similar to the ones previously discussed, which are as follows: (i) a nascent trend towards a more polygenetic architecture of the resistance, and (ii) the occurrence of pleiotropic genes in response to multiple biotic stresses despite the apparent absence of phenotypic correlations in the components of resistance
[77][41].
2.3. Genetic Edition Coupled with Gene Drives May Enable Tree Defense Responses
Genetic drift refers to random allelic fluctuations within genepools
[87][51]. It is typically a consequence of limited population size and rampant selection, and thus becomes stronger in secluded hosts and pathogens’ populations. Rare alleles are likely to disappear completely from populations, while previously polymorphic loci might become fixed. Remarkably, in some cases, pathogens may overcome natural genetic drift by utilizing genetic elements from their host as a way to develop resistance to plant defenses. For instance, whitefly, through a horizontal gene transfer event, acquired the plant-derived phenolic glycoside malonyltransferase gene (BtPMaT1), which allows whiteflies to neutralize phenolic glycosides
[88][52].
On the other hand, modern CRISPR/Cas9 gene editing technology is capable of modifying the immune response function in eukaryotic cells via a highly specific RNA-guided complex
[89][53]. This technology has broad applications in all biological fields, including tree pathology
[90][54]. Bottlenecks are the availability of fine-mapped candidate genes for resistance with major effects, in vitro protocols for tissue culture, and legal regulation
[91][55]. Still, the prospect for gene editing remains open. Interestingly, coupling gene editing with selfish elements in Mendelian segregation distortion due to meiotic drive
[92][56] may efficiently introgress resistance at the population level
[93][57] in a snowball manner
[94][58]. Although promising, combining gene editing with gene drives remains speculative because factual trajectories may prove undesired.
2.4. Harnessing Data Access
Joint research efforts to study more systematically the genomics of forest pathology across the enviromics continuum must be envisioned
[95][59]. At the genomic and breeding level, similar initiatives already exist, such as the European EVOLTREE consortium (
http://www.evoltree.eu/, accessed on 16 September 2021), and North Carolina State University’s Central America and Mexico Coniferous Resources Cooperative (CAMCORE,
https://camcore.cnr.ncsu.edu/, accessed on 16 September 2021) a not-for-profit international tree breeding organization partnered with private companies in the forestry sector around the world. Both alliances may serve as inspiration to build a stronger networking around breeding for biotic resistance in forest trees. Ultimately, promoting more of these partnership efforts will enhance multi-locality trials and data sharing among countries
[96][60], while improving the understanding of the dynamics of co-evolutionary antagonistic interactions in forest ecosystems through genomic, ecological, and evolutionary studies.
3. Conclusions
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Forest pathology must start integrating more thoroughly disciplines that allow understanding the biology and natural evolution of trees under biotic stress, seeking the conservation of the mechanisms by which species have defended themselves from biotic antagonistic agents.
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Polygenetic biotic resistance must be acknowledged as an equally plausible pre-adaptation as Mendelian inheritance.
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Another prerogative must focus on deepening our ecological understating at the pathogen–species–environment interface, while better integrating this classical PDT paradigm with the modern disciplines of forest genomics, molecular biology, phylo-geography, and predictive breeding (i.e., genomic prediction).
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Promoting open access and information agreements among national and international parties (i.e., research centers, tree breeding cooperatives, and industries form the forestry sector) is equally relevant to build more cohesive input datasets to ultimately leverage these ‘big data’ integrative approaches for forest pathology breeding.