Generic concepts within hornworts: historical review, contemporary insights and future directions

Although the hornworts (anthocerotes) are a relatively small assemblage of approximately 150 species, generic boundaries and relationships within the group are controversial. The four prevailing classification schemes are based mainly on morphology and show little congruency. Here we set the foundation for contemporary phylogenetic and taxonomic studies by presenting an historical overview of generic concepts within the anthocerotes. An overview of recent morphological and molecular studies that concentrate on hornworts points to intuitive, novel relationships and a degree of diversity hitherto unknown in the group. Phylogenetically informative characters at the morphological level are identified, with emphasis on newly acquired ultrastructural data. A recent molecular analysis based on rbcL sequences is presented and the levels of suitability of several molecular markers to answer phylogenetic questions within the group are explored. On-going intensive studies that sample a wider range of species and utilise multiple genes and comprehensive morphological data are likely to revolutionise interpretations of the taxonomic relationships and character evolution within hornworts.