Spermatogenesis in the Hepaticae : family Fossombroniaceae

Abstract

The spermatids of the bryophytes are the most structurally complex cell produced by the embryophytes. They consist primarily of the blepharoplast (multilayered structure, spline and basal bodies), nucleus, two mitochondria, two flagella and an energy reserve, the amyloplast. The family Fossombroniaceae, order Metzgeriales, includes three genera, two of which are found in the United States. Petalophyllum ralfsii (Wils.) Nees et Gott. is a monotypic, thallose species which has been reported from only four southern states: Texas, Louisiana, Mississippi and Arkansas. Fossombronia Raddi is a comparatively large genus with 14 leafy species of which eight are reported from Texas. Based on gross vegetative morphological characteristics, this family appears to occupy a pivotal position in the evolution of the Hepaticae between the leafy Jungermanniales and the purely thallose liverworts in the Marchantiales. The central question raised in this dissertation is whether or not the primative morphological position of this family is also substantiated by ultrastructural characteristics. The basis for taxonomic determinations in the genus Fossombronia is the ornamentation of the spores. This study is the first to apply scanning electron microscopy (SEM) to the resolution of species identification. Several specimens from prominent herbaria were found to be incorrectly labeled after the photo-micrographs of their spores were observed. Ultrastructural aspects of development of the antheridium are described using the statistically based methods of stereology. This allows the volume density of cell components, i.e., plastids, mitochondria, nucleus, cytoplasm, vacuole and flagella, to be quantitatively determined. This study is the first application of stereology to plant reproduction and specifically to spermatogenesis in the Bryophyta. This, along with the qualitative description of the cell components, gives a clearer picture of the remarkable cytological transformation which takes place during spermatogenesis with: 1) the de novo synthesis of the locomotive apparatus, 2) the organization and condensation of the nucleus and (3) the loss of extraneous cytoplasm. The ultrastructural characteristics of the sperms were found not to be useful for phylogenetic circumscription at the generic level but provided for good circumscription of the orders within the Hepaticae.