Ontogeny and Phylogeny in Tortula (Musci: Pottiaceae)

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Abstract
The development of gametophores of 38 populations belonging to 11 species of Tortula Hedwig [T. andicola, T. cainii, T. chisosa, T. fragilis, T. latifolia, T. mucronifolia, T. muralis, T. norvegica, T. obtusissima, T. princeps, and T. ruralis] was studied. The observed sequence of juvenile leaves was compared to the diversity of mature leaf-forms found in various species within this monophyletic group (Tortula sensu stricto) in the context of their cladistic relationships. Several features common to all species examined allow some generalizations. Juvenile leaves have an acute apex, no hairpoint, a costa ending well below the apex, and plane margins. The upper leaf-cells of the very first juvenile leaves are usually smooth, while later leaves have progressively more mammillose and papillose leaf-cells. Depending on the species, mature leaves have an acute, obtuse, or emarginate apex, hairpoint, excurrent costa, and/or recurved margins. Some striking parallels can be seen between at least some characteristics of juvenile leaves and the mature leaves of certain species, as well as between juvenile leaves and the differentiated brood leaves (specialized asexual propagules) produced by some species. Character polarity assessments based on a previous cladistic analysis of these species indicate that some of these parallels are instances of paedomorphosis, while others are instances of recapitulation. Some of the instances of paedomorphosis are best interpreted as cases of neoteny, in particular, the neotenic retention at asexual "maturity" of the ancestral juvenile leaf-form by species such as Tortula fragilis. Juvenile leaves have a much greater ability than mature leaves to proliferate. However, the mature leaf morphology of species such as T. obtusissima confers certain advantages for water relationships, particularly in providing capillary systems functioning in the external conduction of water. An adaptive trade-off is likely to exist between the prolific asexual reproductive capabilities of juvenile leaves and the efficient water relationships of the mature leaves of some species.

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