Latitudinal clines in the phenology of floral display associated with adaptive evolution during a biological invasion

Abstract

PremiseThe timing and pattern of flowering -- flowering phenology -- strongly influences reproductive success in plants. Days to first flower is easy to quantify and widely used to characterize phenology, but reproductive fitness depends on the full schedule of flower production over time.MethodsWe examined floral display evolution associated with rapid adaptive evolution and range expansion among thirteen populations ofLythrum salicariapopulations, sampled along a 10-degree latitudinal gradient in eastern North America. Growing these collections in a common garden field experiment at a mid-latitude site, we used Principal Coordinates Analysis (PCoA) and quantitative metrics analogous to central moments of probability distributions (i.e., mean, variance, skew, and kurtosis) to quantify flowering schedules..Key ResultsConsistent with adaptation to shorter growing seasons at higher latitudes, we found that populations from higher latitudes had earlier start and mean flowering day for both population average (i.e., among individuals) and population aggregate (i.e., pooled flowers). Clines in population average and aggregate flowering skew increased with latitude while kurtosis decreased, consistent with a bet-hedging strategy arising in biotic environments with more herbivores and greater competition for pollinators. Although clines were generally similar at the average vs aggregate level, we found that individual flowering schedules from more southern populations tended to flower later than their population aggregate.ConclusionsHeritable genetic changes in flowering schedule are consistent with biotic and abiotic responses to natural selection, potentially contributing to rapid evolution and range expansion of this invasive species.