Ordinal relationships.

The relationships and classification of the Hedwigiaceae were difficult to asses because morphological characters informative at the Order level are absent or were missinterpreted.

Features of the peristome are missing in the family. Moreover, the gametophyte branching system was wrongly considered as being pleurocarpous.

It is now clear that the plant habit in the Hedwigiaceae is acrocarpous. The archegonia are terminal on the main shoot, and the branching pattern is sympodial.

The main axis of each plant consists of a succession of subterminal innovations, rather than a single shoot of indeterminate growth. The plants are plagiotropic and are pleurocarpous only in appearance (De Luna, 1990).

The branching system in Hedwigia consists of a series of sympodial modules

The Hedwigiaceae was sometimes placed in the Grimmiales, Orthotrichales, or Leucodontales.

Currently, the Hedwigiaceae is best placed in the Order Hedwigiales, likely sister to Order Bartramiales, among the acrocarpous mosses.

Three families are grouped in the Order Hedwigiales. The Rhacocarpaceae is sister to the Hedwigiaceae, and the Helicophyllaceae is sister to both families.

BRYOPHYTA Schimp.

SUBDIVISION BRYOPHYTINA Engler

CLASS BRYOPSIDA Rothm.

   SUBCLASS BRYIDAE Engl.

      SUPERORDER BRYANAE (Engl.) Goffinet & W. R. Buck

 ORDER SPLACHNALES (M. Fleisch.) Ochyra

 ORDER HEDWIGIALES Ochyra

Hedwigiaceae Schimp.

Rhacocarpaceae Kindb.

Helicophyllaceae Broth.

 ORDER BARTRAMIALES D. Quandt, N.E. Bell & Stech

 ORDER BRYALES Limpr.

 ORDER ORTHOTRICHALES Dixon

 ORDER RHIZOGONIALES (M. Fleisch.) Goffinet & W. R. Buck

 ORDER AULACOMNIALES N.E. Bell, A.E. Newton & D. Quandt

 ORDER ORTHODONTIALES N.E. Bell, A.E. Newton & D. Quandt

     SUPERORDER HYPNANAE W. R. Buck, Goffinet & A. J. Shaw

From: Cox CJ, et al. 2010. Moss diversity: A molecular phylogenetic analysis of genera. Phytotaxa 9: 175-195.

The phylogenetic placement of the Hedwigiales among the orders of acrocarpous mosses seems to be sister to the Bartramiales, at the base of the Bryidae (Cox et al 2010).

A moss phylogeny by Chang & Graham (2014) also revealed a relationship of the Hedwigiales at the base of the Bryidae.

Fig 1. Moss phylogeny inferred in an unpartitioned likelihood analysis of plastid data with base frequencies optimized (using Garli). Bootstrap values for parsimony and likelihood analysis (the latter an unpartitioned Garli analysis with optimized base frequencies), respectively, are indicated beside branches; those with < 50% support are indicated with a dash (–), and those with > 90% bootstrap support in both analyses are indicated with thick lines. Names of major taxa are indicated, including orders in subclasses Bryidae, Dicranidae, and Funariidae. Clades along the phylogeny referred to in the text are indicated with labels; see Table 2b for bootstrap support in other analyses. This is a subset of a larger green plant tree, shown in Fig. S2. The scale bar indicates estimated substitutions per site. (from Chang and Graham, 2014)

Relationships within the Hedwigiaceae.

The Hedwigiaceae consists of four lineages ranked at generic level:  Hedwigia, Pseudobraunia, Hedwigidium, and Braunia. The current circumscription of the Hedwigiaceae with four genera is based on morphological (De Luna 1995) and molecular phylogenetic analyses (Cox et al 2010).

Bryowijkia and Rhacocarpus were sometimes included in the Hedwigiaceae. A phylogenetic analysis of morphological data suggested to exclude the two genera from the family (De Luna, 1995).

According to Cox et al (2010) Bryowijkia is nested in the Dicranidae, sister to a clade formed by Aongstroemia, Garkea, Cladophascum and Eccremidium. The same analysis by Cox et al (2010) places Rhacocarpus as sister to the Hedwigiaceae.

Within the Hedwigiaceae, Hedwigia and Pseudobraunia are sister taxa (De Luna 1995; Cox et al 2010), although Pseudobraunia californica is sometimes ranked as species in Hedwigia (Buchbender et al 2014; Ignatova et al 2016).

Similarly, Braunia and Hedwigidium are sister genera (De Luna 1995, 2021; Cox et al 2010), although Hedwigidium imberbe is sometimes ranked as species in Braunia (Buchbender et al 2014; Ignatova et al 2016).

The molecular phylogenies by Buchbender et al (2014) investigated the relationships of several species in Hedwigia for the first time.

They corroborated Pseudobraunia is related to Hedwigia, but found different relationships for Rhacocarpus: their likelihood analyses placed Rhacocarpus sister to Braunia.

A recent phylogeny of species of Hedwigia by Ignatova et al (2016) suggests phylogenetic relationships congruent with those found in De Luna (1995) and Cox et al (2010).

One clade includes Braunia and Hedwigidium (as Braunia imberbis); this is sister to a clade which includes Pseudobraunia (as Hedwigia californica) and Hedwigia (Ignatova et al 2016).

Rhacocarpus was considered outgroup to the clade including the four genera of Hedwigiaceae.

These phylogenetic relationships grouping four taxa in the Hedwigiaceae are congruent with the distribution of  synapomorphic morphological characters (globular protonema and papillose pseudoparaphylia) recorded for the four genera (De Luna 1995).

SYNAPOMORPHIES FOR THE HEDWIGIACEAE

Globular protonema in the Hedwigiaceae

A combined morphological and molecular phylogenetic analysis of the Hedwigiaceae is still missing.

Morphological synapomorphies for the Hedwigiaceae include

• a globular protonema (De Luna, 1990) and
• papillose pseudoparaphyllia (De Luna 1995).

To date, these two characters have not been observed in Rhacocarpus, Bryowijkia, or any other moss.

The protonema in Bryowijkia is endosporic, and the protonema in Rhacocarpus is filamentous, as is the majority of mosses (De Luna 1992).

Fig. 6-8, papillose pseudoparaphylia ine the Hedwigiaceae (from De Luna 1995)

References

Buchbender V, H. Hespanhol, M. Krug, C. Sérgio, A. Séneca, K. Maul, L. Hedenás & D. Quandt. 2014. Phylogenetic reconstructions of the Hedwigiaceae reveal cryptic speciation and hybridisation in Hedwigia. Bryophyte Diversity and Evolution 36(1): 1-21.

Chang Y. & S. W. Graham. 2014. Patterns of clade support across the major lineages of moss phylogeny. Cladistics 30: 590-606.

Cox C.J., B. Goffinet, A. J. Shaw, N.J. Wickett & S. B. Boles. 2010. Moss diversity: A molecular phylogenetic analysis of genera. Phytotaxa 9: 175–195.

De Luna E. 1990. Protonemal development in the Hedwigiaceae (Musci), and its systematic significance. Systematic Botany 15: 192-204.

De Luna E. 1990. Developmental evidence of acrocarpy in Hedwigia ciliata (Musci: Hedwigiaceae). Tropical Bryology 2: 53 60.

De Luna E. 1992. Multicellular spores and false anisospory in Bryowijkia ambigua (Musci: Trachypodaceae). LINDBERGIA 16: 73-79.

De Luna E. 1995. The circumscription and phylogenetic relationships of the Hedwigiaceae (Musci). Systematic Botany 20: 347-373.

Ignatova EA, OI Kuznetsova, VE Fedosov & MS Ignatov. 2016. On the genus Hedwigia (Hedwigiaceae, Bryophyta) in Russia. Arctoa 25: 241-277.

De Luna, E. 2021. Seta length variation and the refutation of Hedwigidium = Braunia (Hedwigiaceae, Bryopsida). Acta Botanica Mexicana. 128: e1810. DOI: https://doi.org/10.21829/abm128.2021.1810.

Goffinet, B., W.R Buck and A.J. Shaw (2008), Morphology and classification of the Bryophyta, in Goffinet & Shaw (eds.) Bryophyte Biology 2nd edition, p. 55–138, Cambridge University Press.

References:

FROM THE ONLINE VERSION OF CENTRAL AMERICAN MOSSES

Hedwigiaceae.

Discussion

The Hedwigiaceae consist of four, very similar genera: two with exserted capsules (Braunia Bruch & Schimp. in B.S.G., Pseudobraunia (Lesq. & James) Broth.), and two with immersed ones (Hedwigia P. Beauv., Hedwigidium Bruch & Schimp. in B.S.G.). The gametophytes of Hedwigia and Pseudobraunia are so similar they are sometimes considered synonymous (De Luna 1992) despite their different setae lengths. Hedwigia and Hedwigidium have also been synonymized (Nyholm 1960;
Scott & Stone 1976; Smith 1978; Casas 1991; Hedenäs 1998; Cortini Pedrotii 2001; Smith 2004).
Braunia, with 23 species (Crosby et al. 2000), is by far the largest genus. That genus, however, exhibits such great gametophytic variation that if setae length is disregarded one might consider merging all four genera.
The Hedwigiaceae genera share a remarkable number of features. The family is acrocarpous (De Luna 1992), but occurs in tufts or mats due to a combination of sympodial and adventitious branching.
All members of the family have unbranched or rarely branched rhizoids that occur in clusters at the abaxial side of the leaf insertions. The stems of the Hedwigiaceae lack central strands, paraphyllia, and pseudoparaphyllia. All members of the family have ecostate, more or less oval to oblong leaves. Leaf areolation is a strong unifying feature of the family. The leaf cells are thick-walled throughout, often sinuose, and densely papillose. They are short above, elongate-rectangular in the basal median regions, and oblate, quadrate, or short-rectangular in the alar regions. All members of the family have eperistomate capsules, and globular protonemata development (De Luna 1990).
The Hedwigiaceae are usually placed near the Leucodontaceae (Brotherus 1925; Crum & Anderson 1981; Vitt 1984; Crum 1994a) or Orthotrichaceae (Nyholm 1960; Smith 1978; Walther 1983; De Luna 1995; Hedenäs 1998; Buck & Goffinet 2000). However, the presence of sympodially branched, acrocarpous stems in the genus Schoenobryum Dozy & Molk. may indicate a relationship of the family to the Cryphaeaceae. The presence of acrocarpous stems in the Orthotrichaceae is also a primary reason for placing the Hedwigiaceae near that family. Evidence for a relationship between the Hedwigiaceae and Leucodontaceae is present in a number of features: rhizoidal form and rhizoidal initial
position, ecostate leaves, and a distinctive leaf cell areolation in which the cells are thick-walled throughout, short above, elongate-rectangular in the basal median regions, and oblate, quadrate, or
short-rectangular in the alar regions. Both families also have some species with endosporically germinated protonemata.