Genotype-environment interaction and the maintenance of genetic variation: an empirical study of Lobelia inflata (Campanulaceae)

Genetic variation in natural populations often occurs at levels suggesting the action of processes such as frequency-dependent selection, heterozygote advantage, and variable selection. However, the maintenance of genetic variation in fitness-related traits remains incompletely explained.  Obligately self-fertilizing populations of Lobelia inflata(Campanulaceae L.) are characterized by high levels of genetic variation, strongly implying balancing selection.  L. inflataoffers an exceptional opportunity for an empirical test of genotype-environment interaction (GxE) as a variance-maintaining mechanism under fluctuating selection: L. inflatais monocarpic and reproduces only by seed, facilitating assessment of lifetime fitness; genome-wide homozygosity precludes some mechanisms of balancing selection, and microsatellites are, in effect, genotypic lineage markers. Here, we find support for the temporal GxE hypothesis using a manipulated space-for-time approach across four environments: a field environment, an outdoor experimental plot and two differing growth chamber environments.  High genetic variance was confirmed: 83 field-collected individuals consisted of 45 distinct microsatellite lineages with on average 4.5 alleles per locus. Rank-order fitness, measured as lifetime fruit production in sixteen replicated multilocus genotypes, changed significantly across environments.  Phenotypic differences among microsatellite lineages were detected.  Although results support the GxE hypothesis in principle, evaluation of the effect size of this mechanism and fitness effects of life-history traits will require long-term study of fluctuating selection on labelled genotypes in the field.