Parallel Pleistocene amphitropical disjunctions of a parasitic plant and its host

PREMISE OF THE STUDY

Aphyllon is a clade of holoparasites that includes closely related North American and South American species parasitic on Grindelia. Both Aphyllon (Orobanchaceae) and Grindelia (Asteraceae) have amphitropical disjunctions between North America and South America; however, the timing of these patterns and the processes to explain them are unknown.

METHODS

Chronograms for the Orobanchaceae and Grindelia and their relatives were constructed using fossil and secondary calibration points, one of which was based on the inferred timing of horizontal gene transfer from a papilionoid legume into the common ancestor of Orobanche and Phelipanche. Elevated rates of molecular evolution in the Orobanchaceae have hindered efforts to determine reliable divergence time estimates in the absence of a fossil record. However, using a horizontal gene transfer event as a secondary calibration overcomes this limitation. These chronograms were used to reconstruct the biogeography of Aphyllon, Grindelia, and relatives using a DEC+J model implemented in RevBayes.

KEY RESULTS

Aphyllon had two amphitropical dispersals from North America to South America, while Grindelia had a single dispersal. The dispersal of the Aphyllon lineage that is parasitic on Grindelia (0.40 Ma) took place somewhat after Grindelia began to diversify in South America (0.93 Ma). Using a secondary calibration based on horizontal gene transfer, we infer more recent divergence dates of holoparasitic Orobancheae than previous studies.

CONCLUSIONS

Parallel host-parasite amphitropical disjunctions in Grindelia and Aphyllon illustrate one means by which ecological specialization may result in nonindependent patterns of diversity in distantly related lineages. Although Grindelia and Aphyllon both dispersed to South America recently, Grindelia appears to have diversified more extensively following colonization. More broadly, recent Pleistocene glaciations probably have also contributed to patterns of diversity and biogeography of temperate northern hemisphere Orobancheae. We also demonstrate the utility of using horizontal gene transfer events from well-dated clades to calibrate parasite phylogenies in the absence of a fossil record.

Bet-hedging against larval herbivory and seed bank mortality in the evolution of heterocarpy

PREMISE OF THE STUDY

Bet-hedging strategies maximize long-term geometric fitness at the cost of reduced arithmetic fitness by offsetting different mortality risks. Heterocarpic systems accomplish bet-hedging through the production of two or more fruit types that vary in dormancy and dispersal ability. It is unknown whether heterocarpy also offsets predispersal mortality risks. To address this question, we investigated whether heterocarpy in Grindelia ciliata (Asteraceae) also offsets mortality risks posed by a seed predator Schinia mortua (Noctuidae) to increase plant fitness.

METHODS

We conducted two manipulative experiments to quantify critical life history components of this plant-insect interaction. We measured predispersal achene mortality from herbivory, postdispersal achene mortality in the seed bank, and seedling emergence. These measurements were then used in deterministic models to evaluate evolutionary consequences of predispersal seed mortality in G. ciliata.

KEY RESULTS

Dormant achene types were less vulnerable to herbivory but more susceptible to mortality in the seed bank due to delayed seed emergence. Nondormant achene types experienced high predispersal mortality but low seed bank mortality due to rapid germination. Our herbivore-dependent model improved fit between observed and expected proportions of dormant and nondormant G. ciliata achenes and showed that heterocarpy could evolve in the absence of postgermination mortality.

CONCLUSIONS

Our study provides empirical support of how predispersal herbivory can be equally important to postdispersal seed mortality risks in the evolution and maintenance of a heterocarpic reproductive system and expands understanding of how bet-hedging theory can be used to understand this unique reproductive strategy.