Classification and macroevolution

Recent phylogenetic inferences from sequence data challenge morphology-based concepts of liverwort taxonomy and the recognition of three main lineages defined by the architecture of the gametophyte (Heinrichs et al. 2005, Forrest et al. 2006, He-Nygren et al. 2006). Indeed, the simple thalloid liverworts are not monophyletic; the circumscription of the Marchantiales (the complex thalloid liverworts) has been revised; and it is clear that, although most leafy liverworts compose a well-supported clade, leaves evolved independently several times in the course of liverwort evolution. The consequence of this novel hypothesis is that many characters used traditionally to define the three lineages are homoplastic and thus poor indicators of phylogenetic relationships. Furthermore, some lineages now lack morphological characteristics. Exploring the biochemical, morphological and onto-genetic diversity of liverworts de novo in search of new synapomorphies that would support the shared ancestry of newly aligned taxa has therefore become the latest challenge for liverwort systematists.

The relationships among the major lineages of liverworts are summarized in Fig. 3.19. A detailed account of the phylogeny is presented by Crandall-Stotler et al. (2009). Haplomitrium and Treubia compose the sister group to the remainder of liverworts, within which the primary cladogenetic events separate the complex thalloid lineages from the simple thalloid and leafy

Classification and macroevolution

Haplomitrium

Haplomitriidae

Treubia

Treubiidae

Blasia

Blasiidae

Marchantia

Marchantiidae or

complex thalloids

Pellia

Pelliidae or

simple thalloids I

Metzgeria

Metzgeriidae or

simple thalloids II

Pleurozia

Porella

Porellales or

Leafy I

Jungermannia

Jungermanniales or

Leafy II J

Fig. 3.19. Simplified liverwort phylogeny (redrawn from Forrest et al. 2006).

Haplomitriopsida

Marchantiopsida

Jungermanniopsida

Leafy II J

Fig. 3.19. Simplified liverwort phylogeny (redrawn from Forrest et al. 2006).

liverworts. Simple thalloid liverworts are now interpreted as a paraphyletic grade composed of the Pelliidae (also known as simple thalloids I) and the Metzgeriidae (simple thalloids II; Fig. 3.1d). The Pelliidae are thalloid and occasionally foliose (e.g. Noteroclada) and include taxa with a central strand containing dead water-conducting cells within the thallus mid-region (see Section 1.4.2). The Metzgeriidae lack any internal differentiation and comprise simple thalloid species forming a sister clade to the leafy Pleurozia (Fig. 3.20). This heterogeneous lineage composes the sister group to the remainder of the leafy liverworts, or Jungermanniidae, split in two lineages, Leafy I (or Porellales) and Leafy II (or Jungermanniales). The Porellales include Porella, Radula and the most diverse family of liverworts, the Lejeuneaceae. The Porellales are characterized by leaves with a small ventral lobule (Fig. 3.1i) and, except for Ptilidium, the endosporic germination of spores (Heinrichs et al. 2005). Incubous leaf insertions are shared by all taxa but are not diagnostic of the clade, since this character is also expressed in its sister group. The Jungermanniales comprise a diverse assemblage of taxa that lack any conspicuous shared morphological innovation. The leaves are entire or lobed (with the dorsal lobe smaller), the insertion is transverse and either incubous or succubous and branches arise ventrally or laterally.

A primary source of homoplasy is reverse evolution, whereby synapomorphic character-states are lost during the diversification of a clade. Character loss has occurred repeatedly within the complex thalloid taxa with varying degrees of severity (Boisselier-Dubayle et al. 2002). The genus Monoclea, for example, was

Fig. 3.20. Pleurozia gigantea looks like a leafy liverwort, but is nested within a clade of simple thalloid liverworts (photo L. Zhang). See plate section for colour version.

traditionally considered intermediate between the complex and simple thalloid liverworts: the gametophyte bears scattered oil cells and the antheridia are clustered on receptacles, as in complex thalloid liverworts, but the thallus lacks air chambers and pores and the seta is very long as in simple thalloid liverworts (Johnson 1904, Schuster 1984b). Analyses of sequence data as well as secondary metabolite chemistry (Gradstein et al. 1992) reveal that Monoclea is nested within a group of complex thalloid taxa. The lack of internal differentiation in the vegetative body is thus best interpreted as a reduction. Blasia, another taxon previously considered to have affinities to the simple thalloid taxa (Schuster 1984b), is now known to share a unique ancestor with the complex thalloid lineages. Whether this relationship indicates a derivation of the complex anatomy from a simple thallus, or an origin of Blasia through reduction, is not clear.

Parallel evolution of innovations is another source of homoplasy. The novel phylogeny inferred from DNA sequences suggests, for example, that the evolutionary history of liverworts is characterized by several transitions between thalloid and leafy organization. Whether leaves or a continuous thallus constitute the innovation that has arisen more than once is ambiguous, due to the uncertainty of the primary cladogenetic event at the base of the tree leading to thalloid (complex thalloid) and leafy taxa (Haplomitrium and Treubia; Fig. 3.1a, b). Recent phylogenetic reconstructions also reveal the shared ancestry of taxa with highly dissimilar morphologies. Most notable is the alliance of Pleurozia with Metzgeriidae II. Pleurozia has a leafy gametophyte with lobulate leaves (Fig. 3.20) that lacks any commonality with the simple ribbon-like thallus of Metzgeria and its relatives, except for the apical cell that is lenticular in shape (Fig. 3.2c), and likely inherited from a common ancestor. In all other leafy liverworts, the apical cell is tetrahedral (Fig. 3.2a, b). Although DNA data may be essential for accurately reconstructing the phylogeny of liverworts, continuous exploration of morphological and onto-genetic characters is imperative for interpreting the evolutionary trends and identifying the innovations defining the various lineages.

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