Hybrid Sterility over Tens of Meters Between Ecotypes Adapted to Serpentine and Non-Serpentine Soils

Abstract

The development of hybrid sterility is an important step in the process of speciation, however the role of adaptive evolution in triggering these postzygotic barriers is poorly understood. We show that, in the California endemic plant Collinsia sparsiflora, ecotypic adaptation to two distinct soil types is associated with the expression of intrinsic F1 hybrid sterility between ecotypes, over spatial scales of less than 1 km. First, we show that hybrids between soil-adapted ecotypes are less fertile than hybrids within soil ecotypes. Second, we show that between-ecotype postzygotic incompatibility is insensitive to soil growth environment, and can therefore operate under conditions relevant to both ecotypes in the wild. Third, we confirm there is little genetic differentiation between ecotypes using molecular markers, indicating that these postzygotic barriers are recently evolved. Finally, we explore specific soil attributes that might be the source of selective differentiation that confers hybrid sterility. Our results indicate that hybrid barriers are developing in response to strong adaptive differentiation between adjacent and very recently diverged lineages, despite likely ongoing gene exchange.