Orchid phylogenetics and evolution: history, current status and prospects.

Abstract

Background: Orchidaceae are one of the two largest families of angiosperms; they exhibit a host of changes -- morphological, ecological and molecular -- that make them excellent candidates for evolutionary study. Such studies are most effectively performed in a phylogenetic context, which provides direction to character change. Understanding of orchid relationships began in the pre-evolutionary classification systems of the 1800's that were based solely on morphology, and now is largely based on genomic analysis. The resulting patterns have been used to update family classification and to test many evolutionary hypotheses in the family.

Scope: Recent analyses with dense sampling and large numbers of nuclear loci have yielded well-supported trees that have confirmed many longstanding hypotheses and overturned others. They are being used to understand evolutionary change and diversification in the family. These include dating the origination of the family, analysis of change in ecological habit (from terrestrial to epiphytic and back again in some cases), revealing significant plastid genome change in leafless holomycotrophs, studying biogeographic patterns in various parts of the world, and interpreting patterns of fungal associations with orchids.

Conclusion: Understanding of orchid relationships has progressed significantly in recent decades, especially since DNA sequence data have been available. These data have contributed to an increasingly refined classification of orchids and the pattern has facilitated many studies on character evolution and diversification in the family. Whole genome studies of the family are just beginning and promise to reveal fine-level details underlying structure and function in these plants, and, when set in a phylogenetic context, provide a much richer understanding of how the family has been so successful in diversification.