The genus Rhytidhysteron Speg.
Anales Soc. Ci. Argent. 12: 188 (1881)
By Eric W.A. Boehm
The genus Rhytidhysteron (Clade E). A-D. Rhytidhysteron opuntiae (GKM 1190 [BPI 879805], Kenya).
E-J. Rhytidhysteron rufulum (GKM 361A [BPI 879806], Kenya).
1. Rhytidhysteron rufulum(EB 0382 [BPI 879808], Ghana).
2. Rhytidhysteron rufulum(EB 0381 [BPI 879807], Ghana).
3. M. Rhytidhysteron hysterinum(EB 0351 [BPI 879804] France, photo by Alain Gardiennet).
Scale bar (habitat) = 1 mm; Scale bar (spores and asci) = 10 μm.
Fig. 10 from Boehm et al. 2009b.
The genus Rhytidhysteron is characterised by ascomata that are at first closed and navicular (e.g., Fig. 10K), somewhat resembling those found in the Hysteriaceae, but then later opening by a longitudinal sulcus to become irregularly apothecioid at maturity, often with incurved margins (e.g., Fig. 10M) – a feature never observed in the Hysteriaceae. The peridium in Rhytidhysteron is somewhat gelatinous when wet, as compared to the hard, carbonaceous peridium found in the Hysteriaceae. Although ascomata may possess striations, in Rhytidhysteron these are perpendicular to the long axis (Fig. 10K), rather than parallel, as in the Hysteriaceae (e.g., Figs. 1A, 2B, and 6A). The ascospores in Rhytidhysteron tend to be heavily pigmented and thick-walled, as opposed to lightly pigmented and thin-walled in the Hysteriaceae. These features, among others, have been used to place Rhytidhysteron within the Patellariaceae (e.g., Kutorga & Hawksworth 1997). Samuels & Müller (1980) revised the genus, providing a number of synonyms, and accepted only two species, namely the type, R. rufulum (Spreng.) Speg. (Fig. 10E-K), with 3-septate phragmospores, and R. hysterinum (Dufour) Samuels & E. Müll. (Fig. 10M), with 1-septate spores, both darkly pigmented and thick-walled. Anamorphs have been characterized as Diplodia– and Aposphaeria-like (Samuels & Müller 1980). Subsequently, another two species have been accepted in the genus, namely R. dissimile (P. Karst.) Magnes (Magnes 1997), with 5-septate phragmospores, and R. opuntiae (J.G. Br.) M.E. Barr (1990b), from the American South West, with short pigmented dictyospores (Fig. 10A-D), reminiscent of those found in Hb. mori.
Dictyospores of both R. opuntiae and Hb. mori are similar in shape, obovoid, with obtuse ends, and are also similar in size and septation. In both, the longitudinal septum is usually associated with the mid-cells, but on occasion it can be found obliquely in the end cells. However, unlike Hb. mori, the spores of R. opuntiae are thick-walled, verruculose and darkly pigmented. The most surprising morphological feature of R. opuntiae is that the spores are not borne within patellarioid ascomata, as in other members of the genus. Rather, the ascomata are hysterithecioid, that is, carbonaceous, navicular, with an invaginated longitudinal sulcus (Fig. 10A-B). In hindsight, it is remarkable that Barr (1990) recognised R. opuntiae as a member of Rhytidhysteron, transferring it from Hysterographium opuntiae J.G. Br., despite the presence of hysterithecioid ascomata. In this study we were fortunate to acquire an isolate of R. opuntiae from Kenya (GKM 1190 / BPI 879805). Rhytidhysteron opuntiae falls distant from R. rufulum and R. hysterinum, lying outside of Clade E altogether (Fig. 1). Although both morphological and molecular data suggest that R. opuntiae should be removed from the genus Rhytidhysteron, this is based only on a single specimen, and clearly needs to be substantiated with other isolates.
The six isolates of R. rufulum included one from Kenya (GKM 361A / BPI 879806; Fig. 10E-J), four from Ghana (EB 0381 / BPI 879807, Fig. 10L; EB 0382 / BPI 879808, Fig. 10K; EB 0383 / 879809; EB 0384 / BPI 879810), and one from Europe (CBS 306.38). Also included was one isolate of R. hysterinum from France (EB 0351 / BPI 879804). Three of the Ghanian isolates clustered together in Clade E (Fig. 1), but one (EB 0381 / BPI 879807) associated in another subclade, along with the Kenyan (GKM 361A) and European (CBS 306.38) accessions of R. rufulum. The morphology of the ascomata (Fig. 10L) of R. rufulum EB 0381 (BPI 879807) differs from other more typical specimens of R. rufulum (e.g., Fig. 10 K), although the 3-septate spores in both are identical. Finally, molecular data indicate that R. hysterinum, with 1-septate spores, falls outside of the R. rufulum subclades, while still within Clade E (Fig. 1).
Boehm et al. (2009) were the first to provide sequence data indicating that Rhytidhysteron does not lie within the Patellariaceae. Although initially based on only a single isolate of R. rufulum (CBS 306.38), the genus was tentatively noted to be associated with the Hysteriaceae. In the current study, a total of eight isolates, representing three species, clearly indicates that that the genus Rhytidhysteron belongs to the family Hysteriaceae, and not to the Patellariaceae, the latter defined in this study to include Hysteropatella clavispora (CBS 247.34), Hp. elliptica (CBS 935.97), and Patellaria atrata (CBS 958.97).
Earlier, Barr (1987) had noted the differences between Rhytidhysteron and other members of the Patellariaceae, stating: “Rhytidhysteron rufulum illustrates the problem: paraphysoids and a well-developed pseudoepithecium are conspicuous, but the structure of the peridium, thickened base of ascoma, cylindric asci, are all features attributed to members of the Hysteriaceae. When the heterogeneous family Patellariaceae is revised, Rhytidhysteron should be segregated in its own family”. Samuels & Müller (1980) also noted that “The genus does not have any close relatives in the heterogeneous Patellariaceae“. However, other authors (Bezerra & Kimbrough 1982) presented arguments against the inclusion of Rhytidhysteron within the Hysteriaceae, based on patterns of centrum development. Nevertheless, molecular data presented here, necessitate a radical reappraisal of the Hysteriaceae to include patellarioid forms.
Key to the species of Rhytidhysteron
- Ascospores mainly 1-septate; Europe → Rhystidhysteron hysterinum
- Ascospores with more than one septum → 2
- Ascospores mainly 3-septate → 3
- Ascospores with five or more septa; Europe → Rhystidhysteron dissimile
- Ascospores with three transverse, but also one or more longitudinal septa; Southwestern United States, East Africa → Rhystidhysteron opuntiae
- Ascospores transversely 3-septate, with no longitudinal septa; cosmopolitan → Rhystidhysteron rufulum