Premise of the study: Species delimitation in parasitic organisms is challenging as traits used in the identification of species are often plastic and vary depending on the host. Here, we use species from a recent radiation of generalist hemiparasitic Euphrasia to investigate trait variation and trait plasticity. We test whether Euphrasia species show reliable trait differences, investigate whether these differences correspond to life history trade-offs between growth and reproduction, and quantify plasticity in response to host species.
Methods: We perform common garden experiments to evaluate trait differences between eleven Euphrasia taxa grown on a common host, document phenotypic plasticity when a single Euphrasia species is grown on eight different hosts, and relate our observations to trait differences recorded in the wild.
Key results: Euphrasia exhibit variation in life history strategies; some individuals transition rapidly to flower at the expense of early season growth, while others invest in vegetative growth and delay flowering. Life history differences are present between some species, though many related taxa lack clear-cut trait differences. Species differences are further blurred by phenotypic plasticity—many traits are plastic and change with host type or between environments.
Conclusions: Phenotypic plasticity in response to host and environment confounds species delimitation in Euphrasia. When grown in a common garden environment it is possible to identify some morphologically distinct taxa, though others represent morphologically similar shallow segregates. Trait differences present between some species and populations demonstrates the rapid evolution of distinct life history strategies in response to local ecological conditions.

Data collection is detailed in the associated manuscript. Post collection data processing can be viewed at:

https://github.com/Euphrasiologist/phenotypic_plasticity_euphrasia

"Manyhosts.csv" contains morphological measurements from one Euphrasia arctica population from North Berwick, Scotland, grown with eight hosts.

"Manyspecies.csv" contains morphological measurements of five Euphrasia species and six natural Euphrasia hybrids grown on a single host, Trifolium repens.

"Earlylate.csv" contains repeated growth measurements at different times of year, used in correlations of height at end of season.

"Wildcommon.csv" contains Euphrasia species grown in the common garden experiment and wild collected plants for trait comparisons.