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Polytrichales: towards a modern phylogenetic monograph and the development of a model of sporophyte evolution

Final Activity Report Summary - POLYPHYMON (Polytrichales: Towards a modern phylogenetic monograph and the development of a model of Sporophyte evolution)

Mosses are the second most diverse group of land plants after the flowering plants or Angiosperms, and yet are relatively little studied. The Polytrichopsida are one of the major groups of mosses, with fewer species than the largest group, the Bryopsida, but considerable variety of form. Some species are very large for mosses with well developed vascular systems and are abundant in certain habitats. The Polytrichopsida represent an evolutionary counterpart to the Bryopsida, with many unique features as well as others that are also found outside of the group but may have evolved independently.

In this project we used molecular sequences from all three genomes (nuclear, chloroplast and the mitochondrial) as well as some novel features of the sporophyte (particularly the peristome, the structure that regulates spore release) to construct a model of the evolutionary history of the group (a phylogeny). We also made an extensive study of the peristome to determine how it has evolved within the Polytrichopsida and to discover whether it is an independent development or homologous to superficially similar structures found in other mosses.

Our results show that the earliest lineages in the group all lack peristomes and that the Polytrichopsidan peristome has arisen subsequently, i.e. it is not derived from the precursors of the structurally rather different peristomes found in the Bryopsida. We also discovered many surprising new relationships within the group, including ones that demonstrate convergent evolution of the morphology corresponding to the genus Oligotrichum in northern and southern hemisphere temperate areas and the artificiality of the genus Polytrichastrum. Our molecular results were incongruent (i.e. they disagreed) on some key aspects of the phylogeny. One possible explanation for this is that the nuclear genome has a slightly different evolutionary history from the chloroplast and mitochondrial genomes due to occasional past hybridisation between lineages that have now diversified into different groups but were once more similar; we were able to explain the incongruence between our datasets in terms of four hypothesised events of this type.

Extensive new information about peristome structure additionally provides an entirely new source of data for understanding the development of this apparatus within the group, as well as some aspects of its function. Our results are being formulated in terms of a comprehensive classification. This is the foundation of an ongoing database project that will be made available online as a primary resource for information about the group.