Evolutionary genetics of plant adaptation

Abstract

Plants provide unique opportunities to study the mechanistic basis and evolutionary processes of adaptation to diverse environmental conditions. Complementary laboratory and field experiments are important for testing hypotheses reflecting long-term ecological and evolutionary history. For example, these approaches can infer whether local adaptation results from genetic tradeoffs (antagonistic pleiotropy), where native alleles are best adapted to local conditions, or if local adaptation is caused by conditional neutrality at many loci, where alleles show fitness differences in one environment, but not in a contrasting environment. Ecological genetics in natural populations of perennial or outcrossing plants can also differ substantially from model systems. In this review of the evolutionary genetics of plant adaptation, we emphasize the importance of field studies for understanding the evolutionary dynamics of model and nonmodel systems, highlight a key life history trait (flowering time) and discuss emerging conservation issues.

Figure 1

In spatially heterogeneous landscapes, divergent selection in contrasting environments can result in the evolution of local adaptation [31]. Owing to their sedentary nature, many plant species are adapted to local conditions [31], which can be graphically represented as elevated fitness in their home environment and depressed fitness in an alternative environment (A). In all panels of this figure, the genotype or allele is labeled with its environment of origin (1 or 2). The genetic basis of local adaptation has only rarely been investigated in plants. Local adaptation at the organismal level could be due to antagonistic pleiotropy at the single locus or QTL level (B), where native alleles have a fitness advantage relative to foreign alleles. Alternatively, an allele could be beneficial in its native environment, but have no fitness costs in the contrasting environment (C and D). This pattern of conditional neutrality can contribute to local adaptation at the whole-genome level when several independent loci interact to influence fitness, and alleles derived from contrasting environments are favored at different loci. For example, at one locus, an allele derived from environment 1 is conditionally advantageous (C), whereas an allele from environment 2 is conditionally advantageous at a second locus (D). Investigating local adaptation requires reciprocal transplant or common garden experiments in the native habitats.

Box 2, Figure I

(A) A set of 6 hypothetical populations subdivided into northern and southern groups, which have experienced divergent natural selection and genetic drift. (B) Divergent selection favors different phenotypic optima in the North and South. (C) Uncorrected genome wide association study reveals several genomic regions significantly associated with phenotype (the dotted red line indicates significance threshold). Regions with high association scores (metric of the relationship between genotype and phenotype) result from natural selection causing local adaptation (red), as well as historical population structure (green) and random false positives (blue). (D) Statistical controls for population structure eliminate false positives (unselected loci) and cause false negatives (adaptive loci). False negatives could potentially be detected via family-based QTL mapping of North by South controlled cross.