Evolution and consequences of paternal inheritance of plastid DNA in conifers

In most seed plants, plastid DNA is inherited maternally. In conifers, however, plastid DNA is transferred paternally with pollen. The evolution of this strategy is unknown. With maternal transfer, plastid DNA is selected based on its effect in the sporophyte (plant) only, irrespective of its effect on the male gametophyte (pollen). This becomes problematic with antagonistic selection and when the pollen phase is long and stressful. With paternal transfer, only successful fathers pass on their plastid DNA to their offspring, which selects for sequences supporting pollen viability. If siring success is heritable, paternal inheritance of plastid DNA can evolve. We refer to this idea as the Male Gametophyte Selection Hypothesis. With paternal inheritance, plastid DNA alternates between sporophyte and male gametophyte, allowing selection in both phases. We review the consequences of mode of transfer for evolution of plastid DNA. The non-photosynthesis gene ycf1 is longer and under positive selection in conifers, making it an interesting candidate gene for further analysis.