Doomed lovers: mechanisms of isolation and incompatibility in plants

Adaptation to local conditions likely plays an important role in plant diversity and speciation. A fuller understanding of the role of adaptation in speciation requires connecting particular molecular events with selection occurring at individual, population, or community levels. Here I discuss five areas in which we understand the molecular basis of adaptation and isolation sufficiently to begin examining patterns. These examples highlight the importance of understanding both biotic and abiotic factors and the potential overlap between them, and demonstrate that understanding molecular mechanisms aids in interpreting pleiotropy and constraint. For example, mutations affecting anthocyanin production can affect both pollinator visitation and parasite attack, while edaphic adaptation can alter parasite susceptibility and reproductive timing. Adaptation is also implicated in postzygotic incompatibility: Potentially adaptive cytoplasmic divergence can lead to sterility or inviability; hybrid sterility genes may have pleiotropic effects in biotic or abiotic stress; and the plant immune system is implicated in hybrid failure.