Streptophyte Terrestrialization in Light of Plastid Evolution.

Key steps in evolution are often singularities. The emergence of land plants is one such case and it is not immediately apparent why. A recent analysis found that the zygnematophycean algae represent the closest relative to embryophytes. Intriguingly, many exaptations thought essential to conquer land are common among various streptophytes, but zygnematophycean algae share with land plants the transfer of a few plastid genes to the nucleus. Considering the contribution of the chloroplast to terrestrialization highlights potentially novel exaptations that currently remain unexplored. We discuss how the streptophyte chloroplast evolved into what we refer to as the embryoplast, and argue this was as important for terrestrialization by freshwater algae as the host cell-associated exaptations that are usually focused upon.