OCCURRENCE AND PHYLOGENETIC SIGNIFICANCE OF MONOPLASTIDIC MEIOSIS IN LIVERWORTS

Monoplastidic meiosis is reported for the first time in three seemingl y unrelated liverworts, namely Blasia pusilla (Metzgeriales), Monoclea gottschei (Monocleales), and Haplomitrium blumei (Haplomitriales). A second species of Haplomitrium, H. hookeri, is polyplastidic as previo usly reported. All three taxa represent isolated relicts of ancient li verwort lineages. Monoplastidy in these hepatics is evident in archesp orial tissue and is maintained through successive sporogenous cell gen erations. In archesporial mitosis, the single plastid divides-and the two resultant plastids are precisely positioned so that one is inherit ed by each daughter cell. In the nascent spore mother cell, the solita ry plastid undergoes two successive divisions and the resulting four p lastids become positioned in a tetrahedral arrangement. Concomitantly, the sporocyte assumes a quadrilobed shape, which is less exaggerated in Monoclea, and a single large plastid is situated in each lobe. Deta ils of plastid ultrastructure and morphology vary slightly among the t hree taxa. Evidence is presented that Blasia and Monoclea share a comm on ancestry and represent pivotal taxa in the evolution of the two mai n lines of liverworts. Haplomitrium is suggested to occupy a more basa l position in bryophyte phylogeny. Monoplastidy in meiosis of liverwor ts links the charophytes, the three bryophyte clades, and the lycopsid pteridophytes and supports a monophyletic interpretation of land plan t phylogeny.