Colura (Dumortier, 1831)

Heinrichs, Jochen, Dong, Shanshan, Yu, Ying, Schäfer-Verwimp, Alfons, Pócs, Tamás, Feldberg, Kathrin, Hentschel, Jörn, Schmidt, Alexander R. & Schneider, Harald, 2012, A 150 year-old mystery solved: Transfer of the rheophytic endemic liverwort Myriocolea irrorata to Colura, Phytotaxa 66 (1), pp. 55-64 : 59-60

Colura and Myriocolea

Colura is a large pantropical genus comprising about 70 species ( Jovet-Ast 1953, Gradstein et al. 2001, Grolle & Zhu 2002) of which eleven species were investigated in our study. Species of Colura are typically recognized by their leaf morphology: the presence of lobules forming an apical sac with an aperture mechanism consisting of a valve and a hinge. This sac varies greatly in size and shape, and may function for water retention and in some species also for zoophagy ( Barthlott et al. 2000). Colura species grow in wellilluminated sites from the lowlands to about 4,000 m above sea level, usually as epiphytes on twigs and trunks or on leaves, as epiphylls.

At first glance, the position of Myriocolea irrorata in the Colura sect. Colura clade is surprising. However, Thiers (1983) points out that the unique aspect of Myriocolea irrorata results from an exaggeration of conditions found elsewhere in the Lejeuneaceae whereas an exclusively Radula-type branching is otherwise unknown in Lejeuneaceae . However, Radula-type branches in combination with Lejeunea-type branches occur, e.g., in several species of Lejeunea ( Schuster 1994, Reiner-Drehwald 2000a, 2005).

Morphological support for the synonymy of Myriocolea and Colura comes from the number of underleaves. Usually, Lejeuneaceae have one underleaf per leaf pair, however, a few exceptions prove the rule ( Gradstein et al. 2003). The presence of one underleaf per leaf characterizes Colura , Diplasiolejeunea ( Spruce 1884: 301) Schiffner (1893: 121) , Macrocolura and Myriocolea , the latter being here identified as an element of Colura . Colura (Myriocolea) irrorata does not produce well-developed lobules; however, the ventral part of the hollow leaves may be interpreted as an incompletely inrolled, large lobule. Other members of Colura sect. Colura are characterized by a lobule consisting of a sac terminated by a long, conico-cylindric, elongate horn ( Grolle & Zhu 2002). Colura irrorata is not the only species of Colura without a well-developed lobule. In Colura sect. Heterophyllum Jovet-Ast (1983: 213) there is also a tendency to lose the lobule, which is very much reduced or completely missing in Colura corynephora (Nees, Lindenberg & Gottsche in Meyen 1843: 474) Trevisan de Saint-Léon (1877: 402), a phenomenon that parallels the situation in Colura irrorata . This fact also raises the classification problem of Calatholejeunea Goebel (1928: 8) , which generally resembles Colura by its pendular segmentation and Colura irrorata by its transversely inserted, hollow leaves with unsharply defined lobules. Calatholejeunea was morphologically compared to Colura by Mizutani (1984) but its molecular phylogeny has not yet been studied.

Colura irrorata is a rheophilous liverwort growing on twigs of shrubs on the periodically inundated riverbanks of the Río Topo. The rheophytic, nutrient-rich habitat may explain the untypical leaf development of Colura irrorata because a structure for water storage or zoophagy is not needed in such an environment. Rheophytes from different taxonomic groups exhibit a parallel development, because running waters and regular flooding shape them into flood resistant plants ( Van Steenis 1981). Accordingly, Colura irrorata shares its long, robust stems, pinnate branching, and the presence of numerous small gametoecial branches with other rheophytic Lejeuneaceae , namely Myriocoleopsis Schiffner (1944: 234) , Lejeunea subg. Neopotamolejeunea ( Reiner-Drehwald 2000b: 449) Gradstein & Reiner-Drehwald (2007: 484) ( Wilson et al. 2007a) and Cololejeunea stotleriana ( Gradstein et al. 2011: 13) . Non-rheophytic Colura species have a smaller size, grow usually attached to the substratum, have sac-like lobules, a lower number of gametoecia, and less robust stems. However, the phylogenetic distance between Colura irrorata and other members of C. sect. Colura is low ( Fig. 1 ), indicating that the rheophytic species C. irrorata originated in rather recent times. This scenario is also supported by the derived position of C. irrorata . It is likely that the Ecuadorian endemic C. irrorata evolved from a local population of a species close to C. calyptrifolia and C. tenuicornis , and that the morphological rearrangements of the gametophyte took place in a short period of time. A rapid reorganization of gametophytical traits has also been demonstrated for some epiphytic representatives of Plagiochila sect. Hylacoetes Carl (1931: 50) ( Heinrichs et al. 2003), providing some evidence that an occurrence in extreme habitats may occasionally lead to considerable changes in morphology. The molecular control procedures of such rapid rearrangements and their contribution to plant evolution are still incompletely understood ( Stern 2000, Carroll 2008, Theissen 2009, Frankel et al. 2011). However, rheophytic plants appear to be an excellent group on which to study the impact of selection on the establishment of rapid growth habit changes. Research on Japanese occurrences of the terrestrial fern Osmunda japonica Thunberg (1780: 209) and the rheophytic O. lancea Thunberg (1784: 330) showed that habitat conditions may lead to dramatic changes of the leaf morphology in sister taxa that otherwise share more or less the same genetic information ( Imaichi & Kato 1992; Yatabe et al. 2009).

Figures

FIGURE 1. Single most likely tree (ln= -9,559.44384) derived from a cp DNA rbcL-trnL-F – nrITS dataset. ML bootstrap values (bold) and MP bootstrap values (not bold) are indicated at branches. Two accessions of Myriocolea irrorata are nested in a robust subclade of Colura, and are placed sister to the Colura-generitype, C. calyptrifolia.