Occurrence and phylogenetic significance of monoplastidic meiosis in liverworts

Abstract

Monoplastidic meiosis is reported for the first time in three seemingly unrelated liverworts, namely Blasia pusilla (Metzgeriales), Monoclea gottschei (Monocleales), and Haplomitrium blumei (Haplomitriales). A second species of Haplomitrium, H. hookeri, is polyplastidic as previously reported. All three taxa represent isolated relicts of ancient liverwort lineages. Monoplastidy in these hepatics is evident in archesporial tissue and is maintained through successive sporogenous cell generations. In archesporial mitosis, the single plastid divides and the two resultant plastids are precisely positioned so that one is inherited by each daughter cell. In the nascent spore mother cell, the solitary plastid undergoes two successive divisions and the resulting four plastids 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. Details of plastid ultrastructure and morphology vary slightly among the three taxa. Evidence is presented that Blasia and Monoclea share a common ancestry and represent pivotal taxa in the evolution of the two main lines of liverworts. Haplomitrium is suggested to occupy a more basal position in bryophyte phylogeny. Monoplastidy in meiosis of liverworts links the charophytes, the three bryophyte clades, and the lycopsid pteridophytes and supports a monophyletic interpretation of land plant phylogeny. Key words: chloroplast, liverwort, meiosis, monoplastidy, phylogeny, sporogenesis.