No Evidence of Reproductive Character Displacement between Two Sister Fungal Species Causing Anther Smut Disease in Silene

Abstract

Reproductive isolating mechanisms that are stronger for sympatric populations than for allopatric populations of a given species pair are indicative of reproductive character displacement, that is, selection for increased barriers to avoid the costly production of hybrid offspring. Evidence of reproductive character displacement in nature remains equivocal and requires further experimental studies. The genus Microbotryum includes species of anther‐smut fungi, which castrate pathogens specialized on different plants in the Caryophyllaceae and which serve as excellent models for studying mechanisms of speciation. Microbotryum lychnidis‐dioicae and Microbotryum silenes‐dioicae are sister species that show no assortative mating and relatively high hybrid viability and, yet, display a lack of gene flow in natural populations. We wanted to test whether these apparently contradictory results could be explained by reproductive character displacement. We first confirmed the absence of detectable gene flow between the two species in two sympatric populations. Then, using experimental crosses and inoculations of host plants (Silene latifolia and Silene dioica), we compared intrinsic reproductive barriers between M. lychnidis‐dioicae and M. silenes‐dioicae for sympatric versus allopatric populations. We found no evidence for strong reproductive character displacement at any of the following stages: selfing propensity, assortative mating, or hybrid infectivity. Altogether, our results suggest that ecological differences and a tendency for high selfing rates constitute barriers that are strong enough to effectively prevent interspecific gene flow.