Reproductive isolation of parasitic plant host races

Is there selection for increasing pre-zygotic barriers to reproduction between host races?

Parasites have been extremely successful throughout the history of life on Earth, comprising the majority of species by some estimates. New parasite species can often form following infection of a novel host species if barriers to successful reproduction between parasites on the two species exist. A question of longstanding debate in evolutionary biology is how these barriers can arise in the absence of geographic obstacles.

This work tests established theory about the relative roles of pre-zygotic and post-zygotic isolation mechanisms in sympatry and allopatry using two host races of a native aerial hemiparasite of plants, desert mistletoe (Phoradendron californicum), across their overlapping ranges. Populations of the host races infecting catclaw acacia (Senegalia greggii ) and velvet mesquite (Prosopis velutina) in sympatry (locally interspersed) and allopatry (> 3 km from nearest infected host of other species) will be studied at multiple sites across Arizona. I am conducting selection gradient analyses for flowering time across these populations to test if selection for divergent flowering times (a pre-zygotic isolation mechanism) between the host races is stronger in sympatry than in allopatry.