Morphological puzzles posed by mosses - When morphology and molecules tell us different stories

Recent phylogenetic studies have shown that striking conflicts between the information we obtain from morphological characters and DNA sequence data might exist with different underlying reasons. Several hypotheses fail to explain these discrepancies between data sets. We present two case-in-point studies issuing this phenomenon in the pleurocarpous moss family Neckeraceae: i) Leptodon corsicus, a new and unique endemic moss species from Corsica and ii) the British and Macaronesian endemic Thamnobryum species. In the case of Leptodon corsicus, the species drastically differs morphologically from the other species of the genus, but molecular analyses show that it is deeply nested within Leptodon smithii. A plausible explanation would be to interpret the phylogenetic position of Leptodon corsicus as the result of a recent speciation process. Mutations involved in the process at one or a few coding loci, or differences in gene expression could have tremendous consequences for phenotypic appearance, and ancestral polymorphism in the non-coding sequences used for phylogenetic reconstruction could still be retained. Such an explanation might also apply to other plant species, which exhibit a striking morphology, and yet share identical non-coding sequences with the common species they derive from. The narrow endemic Thamnobryum species (T. angustifolium, T. cataractarum, T. fernandesii and T. rudolphianum) were investigated using nuclear (ITS1 & 2) and plastid (the rps4-trnT-trnL-trnF cluster) markers. The phylogenetic results indicate that the submerged multistratose leaved taxa (T. cataractarum, T. fernandesii and T. rudolphianum) have been independently derived from the surrounding T. alopecurum populations, a common terrestrial moss species, thus showing convergent evolution in response to the extreme aquatic habitat. The T. alopecurum complex provides an excellent study group for research on the genetic basis of morphological aberrations that would resolve if the morphological extremes are due to true speciation via genetic differentiation or simply up- and down-regulation of genes.

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