BACKGROUND

The gametophytes of taxa in the S. involutus complex are strikingly different from those of their relatives. The leaves of moss gametophytes in the genus Syrrhopodon can be easily distinguished on the basis of a specialized zone of dead empty cells at their base. These tiny window-like regions are known as cancellinae. In members of the S. involutus complex, the cancellinae dominate the leaf area, so that green photosynthetic cells are restricted to the extreme tips and margins of the leaves. The morphology of these taxa is so striking that early taxonomic treatments placed S. involutus taxa (along with S. confertus, another taxon with expanded cancellinae) into a separate section, Leucophanella .

The S. involutus group is currently recognized as a either a single species, Syrrhopodon involutus, with 11-16 synonyms , or two taxa, S. involutus and S. rufescens, with 6 and 2 synonyms respectively (Eddy 1988). The taxonomic history of this group is indicative of its morphological peculiarity: A distinctive leaf morphology unites the group, yet details of leaf ornamentation, shape, and plant architecture vary substantially amongst different specimens. Furthermore, preliminary study of collection data indicate the complex displays a diversity of habitat preferences throughout its range. This high level of variation superimposed upon a fairly cohesive morphological phenomenon (hyaline cells composing more than half the leaf area) is interpreted by modern taxonomists as extreme morphological and ecological plasticity within a single taxon. However, some bryologists have questioned the legitimacy of this conclusion, and have emphasized the need for a more detailed assessment of S. involutus.

The taxonomic ambiguity of the S. involutus complex highlights a general problem in modern bryophyte systematics: the dearth of explicitly phylogenetic studies at lower taxonomic levels . Compounding this problem is a lack of systematic studies on bryophytes which apply a rigorous phylogenetic concept of species . More phylogenetic interspecific to population-level studies of mosses are needed, as they have the potential to expand our understanding of the diversification process in a biologically and historically unique group of organisms.

Members of the S. involutus complex are widely distributed throughout the Paleotropics, from Eastern Africa through Southeast Asia, Melanesia, Australia, Micronesia, and Polynesia. The adult leaves of all the members of the complex resemble the juvenile leaves of members of the genus Syrrhopodon. However, this similarity is accentuated towards the eastern end of its range, where the adult leaves are extremely simplified, displaying very little leaf cell ornamentation and minimal chlorophyllose (green) tissues. The morphological trend towards leaf simplification appears to be a gradual one, with vouchers examined from various South Pacific islands showing varying degrees of ornamentation and chlorophyllose cell production. This is an exciting observation, as this pattern has been described for other island plant taxa , but on a much more localized spatial scale (e.g. the Hawaiian archipelago). Its distribution on both mainland and islands coupled with the relative ease of culturing a single moss genotype (clone) and observing its ontogeny in many replicates makes the S. involutus complex an excellent system for the comparative study of morphological diversification on islands via heterochronic change in ontogeny.

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