Systematics and Evolution of the Genera of the Marchantiales

This survey of the order Marchantiales contains descriptions of the order and of the genera including morphological, ontogenetic and cytological characters, reassessed with living plant material, ecological data, life history traits, geographical distributions and keys, as well as an outline of published evolutionary classifications and scenarios based on morphological, phytochemical and molecular sequence data. The relationships of the order with the other liverwort orders, with the mosses and with the land plants are summarized. The phylogenetic tree of the order is constructed with a cladistic parsimony analysis, based on 43 selected characters, with outgroup genera from the orders Sphaerocarpales, Monocleales and Metzgeriales. It results in a consensus tree in which the two main sister groups are the Ricciineae on one hand and the Corsiineae-Targioniineae-Marchantiineae on the other. Character analysis and screening of character evolution in character state graphs derived from the consensus tree show that the ancestors of the Marchantiales had relatively simple morphological traits. Progressively more complex sex-related gametophytic structures and sporophytes with larger foot, seta and capsules are seen in one of the sister groups, which includes the suborders Corsiniineae, Targioniineae and Marchantiineae, with the evolution of stalked gametangiophores, not found in any other order of liverworts. Reductions in these same characters are observed in the other sister group, the Ricciineae. Sex-related and sporophytic characters show higher stability, with fewer homplasies than the characters of the gametophyte. Seasonality of the former, and differences in the action of environmental conditions upon the haploid, exposed gametophyte as compared with the diploid, sheltered sporophyte, may be invoked as an explanation. Little support for the proposed phylogenetic reconstruction can be obtained from geographical distribution because many genera have worldwide ranges. No hypothesis on the place or time of origin of the order can be put forward. However, the most ancient stock of taxa seems to have inhabited temperate to hot-temperate, continental areas. The size of genera, species structure and differences in levels of genetic variability among species, are not reflected by tree topology. Instead, life history patterns appear to be linked to phylogeny. An independent statistical correspondence analysis, including 12 life history characteristics of 230 marchantialean species, discloses four groups of taxa with similar life history patterns. These groups show habitat specificity and are strongly determined by generic and familial membership. Life history traits appear to be shared as a result of common ancestry rather than of convergence due to habitat conditions. Diversification in life history patterns seem to have occurred at either the generic or the familial level, and seem to have been conserved in the course of evolution. The traditional assemblage of genera into families and of families into suborders is confirmed, except for the Targioniaceae and Peltolepis. Some taxa of subfamilial rank are paraphyletic. On the other hand, tree topology fails to confirm the basic hypotheses of both the antithetic and homologous theories, on which formerly published classifications and evolutionary scenarios, as well as the classification in the current use, were built. These theories postulated respectively progressive or reductive evolution of the sporophytic generation. The sporophytes remain small in all taxa of the Marchantiales. The phylogenetic reconstruction does not confirm most of the assumptions on evolution based on phytochemical data, or on molecular sequence data, sampled at present for only a small number of species. In the future, alanysis of other, especially ontogenetic, data and more complete sets of phytochemical and molecular sequence data will be necessary to test the robustness of the proposed phylogenetic hypothesis. A critical reevaluation of morphological characters might then be within reach.