Heritability and ontogeny of phytochemical traits in Piper sancti-felicis

Casey S Philbin, Lee A. Dyer, Christopher S. Jeffrey and Lora A. Richards, Hitchcock Center for Chemical Ecology, University of Nevada, Reno, Reno, NV, Heather L. Slinn, Integrative Biology, University of Guelph, Guelph, ON, Canada, Trevor M. Faske, EECB, University of Nevada, Reno, Reno, NV

Background/Question/Methods
Phytochemical variation serves as a hub for multitrophic interactions in plant-herbivore communities. However, the genetic and environmental underpinnings of this variation, how they interact, and how they affect community assembly, are poorly understood. While modern genetic techniques allow the estimation of relatedness at relatively fine scale, the expression of phenotypic traits can be confounded by biotic and abiotic heterogeneity in field experiments, which can also confound studies of ontogenetic phytochemical variation. To standardize resource availability and herbivory, we have examined the heritability of, and ontogenetic changes to, the chemical phenotype (chemotype) of Piper sancti-felicis in a common garden experiment. Cuttings, seeds and foliar tissue for chemical analyses were collected concurrently from 100 individuals spatially distributed across La Selva Biological Station, Costa Rica. Clonal and sexual offspring grown in a common garden were harvested as seedlings or after first inflorescence. Compositional and structural facets of variation in foliar phytochemistry were measured by liquid chromatography-time of flight mass spectrometry (LC-TOF) and 1H nuclear magnetic resonance (NMR) spectroscopy, respectively.
Results/Conclusions
Heritability of phytochemical variation was calculated as Pearson correlation between parent and offspring of composite variables derived from network modules summarizing NMR data, and principal components analysis (PCA) of LC-TOF features and NMR modules. Compositional chemical variation (LC-TOF features) of P. sancti-felicis parent plants was highly heritable for clonal offspring (R = 0.95) and sexual offspring (R = 0.84), as was structural chemical variation for adult sexual offspring (1H NMR modules, R = 0.44 - 0.96). LC-MS data revealed three distinct compositional chemotypes for all parents and offspring which proved highly heritable, with all clonal, and 92% of sexual offspring inheriting parental chemotype, which were also reflected in structural chemical variation (PC1, PC2 of NMR modules). These results suggest an inherited biosynthetic machinery producing phytochemicals with related structural motifs. Mantel tests of phytochemical and spatial distance matrices showed a weak but statistically significant (R = 0.12, p < 0.001) compositional-spatial relationship as measured by LC-TOF. However, a structural-spatial relationship was not apparent via NMR, which cannot distinguish variation in structurally similar, but distinct, phytochemicals. Despite diverging phytochemical composition between seedlings and their respective progenitors and adult siblings, a subset of their phytochemistry was still heritable (R = 0.65), and representative of their adult siblings (R = 0.61). These results demonstrate the high heritability of phytochemical variation in P. sancti-felicis and serve as a benchmark for investigating phytochemical variation in subsequent field experiments.