Spore wall development has been studied in all three of the traditional bryophyte groups (reviewed in Brown and Lemmon 1988, 1990). In the majority of liverworts, immediately after meiosis, a polysaccharide wall (the spore special wall) is laid down outside the plasma membrane (Brown and Lemmon 1985). In many liverworts, this spore special wall seems to function as a primexine in which the pattern of exospore ornamentation is established (Brown and Lemmon 1993).
However, in some liverworts exospore ornamentation appears to be determined by exospore precursors produced by the diploid sporocyte prior to meiosis and formation of the haploid spores (Brown et al. 1986). The exospore develops centripetally (Brown and Lemmon 1993) based on WLCL formed outside the spore cytoplasm. At completion, the entire exospore comprises sporopollenin deposited on WLCL.
At maturity, the lamellate structure thus formed is clearly discernible and is highly characteristic of the liverwort exospore. Liverworts lack a tapetum and there is therefore no input from this source. The innermost layer of fibrillar intine is the final wall layer to be formed (Brown and Lemmon 1993).
Studies of spore wall development in hornworts are limited. As with liverworts, a spore special wall is formed after meiosis and functions as a primexine in which the exospore is set down. It was initially thought that the exospore formed in the absence of WLCL, but Taylor and Renzaglia have recently demonstrated their presence (W.A. Taylor, University of Wisconsin-Eau Claire, USA, pers. comm., 2011).
Recent analyses of Phaeomegaceros fimbriatus have shown that the mature spore wall has a thin perine-like outer layer, but this represents the remnants of the spore mother cell wall rather than extra-exosporal material derived from a tapetum (Villarreal and Renzaglia 2006).
Three types of spore wall have been recognized in mosses: Bryopsida type, Andreaeidae type and Sphagnidae type (Brown and Lemmon 1990). All three of these types appear to form in the absence of a spore special wall. Bryopsida-type spore walls are homogeneous except for an inconspicuous foundation layer (Fig. 2). This foundation layer forms first via sporopollenin accumulation on WLCL.
Subsequently, the homogeneous exospore layer is laid down outside the foundation layer in a centrifugal manner. This layer is probably mainly extrasporal in origin. Sometimes additional homogeneous material is also deposited inside the foundation layer. This layer is almost certainly derived from the spore. Following the accumulation of homogeneous material, the spores are coated by an additional extra-exosporal layer, referred to as the perine or perispore, which is derived from the tapetum. Finally, the intine forms.Fig. 2
Proposed model of spore wall development in physcomitrella. The exine foundation layer is laid down first by way of sporopollenin accumulation on WLCL. The rest of the exine layer is deposited outside the foundation layer centrifugally. Note the appearance of callose in the inner exine, which is confined to the expanded aperture region at the proximal pole.
Spore wall development in the Andreaeidae type is unique among mosses in that they have a spongy exospore that appears to form in the absence of WLCL (Brown and Lemmon 1984). By studying Andreaea rothii, Brown and Lemmon (1984) demonstrated that the exospore is instead initiated as discrete homogeneous globules within the coarsely fibrillar network of the spore mother cell.
These globules accumulate and form an irregular layer with numerous interstitial spaces. The sequence of spore wall layer development is essentially the same as that of other mosses and the mature wall consists of an inner intine, a spongy exospore and an outer perine (Brown and Lemmon 1984).
Sphagnidae-type moss spore walls are more complex than those of the other mosses and consist of five layers (Brown et al. 1982). Unlike other mosses, the exospore of Sphagnidae type comprises two layers: an inner lamellate layer (A-layer) and a thick homogeneous outer layer (B-layer). In addition to the exospore, there is an intine, a unique translucent layer and the outermost perine.
The A-layer is the first to form and does so by sporopollenin accumulation on WLCL, and develops evenly around the young spore immediately after meiosis. The homogeneous B-layer is deposited outside the A-layer. Overlying the exospore is a translucent layer that consists of unconsolidated exospore lamellae in a medium of unknown composition. The tapetally derived perine is deposited on top of this unique layer. The study of spore wall development in Sphagnum lescurii by Brown et al. (1982) suggests that the ontogeny of the wall layers is not strictly centripetal.