Exine formation in Chamaecyparis lawsoniana (Cupressaceae) and a discussion on pteridophyte exospore and gymnosperm exine ontogeny

Abstract

Exine ontogeny in Chamaecyparis lawsoniana is similar to that in other gymnosperms investigated as far as modes of wall layer formation are concerned. However, it differs markedly in timing of layer development, inasmuch as endexine elaboration is initiated at the cell surface prior to appareance of tectum and infratectum components at the level of the microspore surface coat. It is observed that the tectum and infratectum are formed through deposition of sporopollenin on receptor sites provided by the microspore surface coat, whereas the foot layer appears to result from accumulation of similar sporopollenin against the endexine surface. It is proposed that the microspore surface coat is part of the microspore glycocalyx and mediates tectum and infratectum formation, while foot layer deposition must be mediated by a distinct, innermost part of this glycocalyx. Comparison of gymnosperm exine and pteridophyte exospore ontogeny shows that, (1) the whole solid, structureless outer part of the exospores is formed through the same process as the foot layer of gymnosperm exines; (2) the ontogenetic and structural differences between the two types of walls are mainly due to the presence of the microspore surface coat in gymnosperms, which produces formation of exine outer strata having very varied organizations, usually including large gaps. The entire pteridophyte exospore appears thus to be ontogenetically homologous to the endexine and foot layer of the gymnosperm exine. It is assumed that the “para-exospore” of Isoetaceae and Selaginellaceae microspores and the comparable extra-exosporal wall of certain fossil spores are formed through intervention of a microspore surface coat-like matrix, and are thus homologous to tectal and infratectal structures of gymnosperm exines.