Heterocladium

Taxonomy of the genus Heterocladium View in CoL

The genus Heterocladium Bruch, Schimper & W. Gümbel was described for two species, H. dimorphum and H. heteropterum , but the type species of the genus in unclear (it is not indicated in Tropicos, accessed on 19 May 2019). Different authors have reported one or other of the two species as the type: H. dimorphum was mentioned as the type of the genus by Watanabe (1972) and Ignatov & Ignatova (2004), whereas Grout’s (1928) typification, supposedly the earliest one, was with H. heteropterum (cf. also Enroth et al., 2019). For a long time the genus was placed in the Thuidiaceae ( Fleischer, 1923; Brotherus, 1925), but later Buck & Crum (1990) suggested transferring it to the Pterigynandraceae (along with Myurella , Habrodon , Iwatsukiella and Leptohymenium ), where it is accepted in some recent publications ( Magill, 2014).

The early analysis of Gardiner et al. (2005) showed that Heterocladium was unrelated to the Thuidiaceae , where it was traditionally placed, but at the same time without any apparent affinity. Thus Ignatov & Ignatova (2004) segregated it in a family Heterocladiaceae . Limited sampling did not allow any further splitting, so despite low support all the species were assumed to form one lineage. Most subsequent analyses have separated the Heterocladium species in two different clades, and Heterocladium s. str. (the species grouped around H. heteropterum ) has usually been placed within the Neckeraceae or Lembophyllaceae or between them in a grade (cf. Enroth et al., 2019), while the species related to H. dimorphum were usually in a more distant position.

The family has been accepted in some floras, e.g. by Hedenäs et al. (2014), but recently it was pointed out that the earlier name Heterocladiaceae Decaisne for a family of red algae has been overlooked ( Enroth et al., 2019). This family is based on the genus Heterocladia Nageli , which is still in current use and therefore the name cannot be a subject for conservation.

The present analyses of different loci resulted in somewhat different topologies regarding the position of these two groups of Heterocladium . The ITS and atpB analyses resolve Heterocladium s.str. in a clade with the Lembophyllaceae ; rpl16 shows the same, but in this case the H. dimorphum clade is found sister to the Heterocladium s.str. clade. The analysis of trnS-F found the Heterocladium s.str. clade in the grade from Lembophyllaceae to Neckeraceae , between the core clade of the Neckeraceae and the “ Pinnatella -clade”. It seems that the position of Heterocladium in the Lembophyllaceae would be the best solution in the present state of knowledge.

The similarity in morphology between Heterocladium s. str. and core Lembophyllceae ( Tangney, 1997; Quandt et al., 2009) is not very high; however, some other marginal groups which have been referred to the family based on molecular phylogenetic inference have likewise not much in common with the core of the family, e.g. Nogopterium and Mawenzhangia ( Enroth et al., 2018) . Multipapillose laminal cells, especially conspicuous in H. macounii , would be odd in the new circumscription of the family; it may be inconvenient, but the loss of the “smooth laminal cell” character from the diagnosis of the Lembophyllaceae has so much in parallel with the case of Pseudanomodon that it is already not surprising. Note that in contrast to the Anomodontaceae , the papillae in Heterocladium occur only on the dorsal leaf surface, as in the Thuidiaceae and Leskeaceae .

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The position of the Heterocladium dimorphum group is more indefinite. It forms clades with Anomodontella in the ITS tree (with low support, Fig. 1 View Fig ) and with Heterocladium in the rpl16 analysis ( Fig. 3 View Fig ), but more commonly keeps a separate position in polytomy or grade near Ortostichellaceae, Echinodiaceae , Lembophyllaceae and Neckeraceae ( Figs. 2-6 View Fig View Fig View Fig View Fig View Fig ), being in most cases maximally supported. Considering such genetic differentiation, we suggest segregating the H. dimorphum -group of species in a separate genus and family.

Heterocladiellaceae Ignatov & Fedosov , fam. nova

Monogeneric family, type: Heterocladiella Ignatov & Fedosov

Diagnosis. Stems rather regularly pinnately branched, densely foliate; central strand present; epidermal cells smooth; paraphyllia absent, proximal branch leaves often compound. Stem leaves squarrose, from broadly ovate, appressed base tapered to triangular acute acumen, broadly decurrent; margins serrate to serrulate; costa short, double; laminal cells quadrate to rhomboidal, on dorsal surface with a massive papilla closer to distal end of cell, on ventral surface smooth. Branch leaves distinctly differentiated, erect to spreading, ovate, broadly acute. Dioicous. Capsule horizontal, short-cylindrical, curved. Operculum conic-apiculate. Peristome perfect, cilia long. Spores small.