Studies of protonemal morphogenesis in mosses. XI. Bryum and allied genera: a plethora of propagules

Abstract

Asexual propagules found in the genus Bryum include rhizoidal and chloronemal tubers, axillary bulbils, chloronemal brood cells, and foliar and protonemal gemmae. While tubers and, to a lesser extent, axillary bulbils have been described in detail and used widely in the taxonomy of the genus, reports on the occurrence and morphology of protonemal gemmae are at best patchy, thus explaining their exclusion from any taxonomic and ecological evaluations of Bryum. Reported here is the first systematic survey of the occurrence of protonemal gemmae in 36 Bryum species and several related genera based on both wild and in vitro-grown specimens. Protonemal gemmae are described for the first time in 19 species; indeed the majority of Bryum species analysed produced protonemal gemmae in culture and subsequently these were also found in nature in many of the species. Compared with tubers (19 species) and bulbils (five species) protonemal gemmae (32 species) are the most ubiquitous asexual propagule in the genus. Overall gemma morphology within Bryum is fairly uniform: the gemmae are filamentous chloronemal derivatives and separate from the parent filament by the formation of a tmema (abscission) cell. Notable exceptions are the critically endangered B. schleicheri where clusters of apical choronemal branches detach by schizolysis and the highly branched gemmiferous protonemata of B. radiculosum which form from axillary rhizoids. While in the latter two species these unique gemma morphologies can be used for identification, overall protonemal gemmae are of much more limited use in Bryum taxonomy than rhizoidal tubers. Scanning electron microscopy, however, reveals that the protonemal gemmae in some species are highly ornamented, thus suggesting their possible future use in taxonomy alongside rhizoid papillosity. Although most of the genera allied to Bryum have vegetative propagules, very few produce protonemal gemmae. In those that do, only the gemmae of Mielichhoferia bryoides and Rhodobryum roseum have tmema cells and are remarkably similar to those of Bryum capillare and associated species. Gemma detachment in Epipterygium, Plagiomnium, Rhizomnium and Mnium is via schizolysis. Consideration of these gemma liberation mechanisms in relation to recent phylogenies of the Bryaceae suggests that schizolysis may be a synapomorphy for the Mniaceae. In vitro, gemma production is intrinsically linked to high nutrient availability in the substratum and there are indications that the same may also be true in nature. Ecologically, protonemal gemmae, presumably less long lived and less tolerant to desiccation than tubers, function in increasing the potential for initial establishment and localized spread of a given taxon, especially in situations where sporophytes are rarely produced.