Need for split: integrative taxonomy reveals unnoticed diversity in the subaquatic species of Pseudohygrohypnum (Pylaisiaceae, Bryophyta)

Abstract

We present an integrative molecular and morphological study of subaquatic representatives of the genus Pseudohygrohypnum (Pylaisiaceae, Bryophyta), supplemented by distribution modelling of the revealed phylogenetic lineages. Phylogenetic analyses of nuclear and plastid datasets combined with the assemble species by automatic partitioning (ASAP) algorithm revealed eight distinct species within the traditionally circumscribed P. eugyrium and P. subeugyrium. These species are therefore yet another example of seemingly widely distributed taxa that harbour molecularly well-differentiated lineages with narrower distribution ranges. Studied accessions that were previously assigned to P. eugyrium form three clearly allopatric lineages, associated with temperate regions of Europe, eastern North America and eastern Asia. Remarkably, accessions falling under the current morphological concept of P. subeugyrium were shown to be even more diverse, containing five phylogenetic lineages. Three of these lineages occur under harsh Asian continental climates from cool-temperate to Arctic regions, while the remaining two, referred to P. subeugyrium s.str. and P. purpurascens, have more oceanic North Atlantic and East Asian distributions. Niche identity and similarity tests suggested no similarity in the distributions of the phylogenetically related lineages but revealed the identity of two East Asian species and the similarity of two pairs of unrelated species. A morphological survey confirmed the distinctness of all eight phylogenetic lineages, requiring the description of five new species. Pseudohygrohypnum appalachianum and P. orientale are described for North American and East Asian plants of P. eugyrium s.l., while P. sibiricum, P. subarcticum and P. neglectum are described for the three continental, predominantly Asian lineages of P. subeugyrium s.l. Our results highlight the importance of nontropical Asia as a center of bryophyte diversity. Phylogenic dating suggests that the diversification of subaquatic Pseudohygrohypnum lineages appeared in late Miocene, while mesophilous species of the genus split before Miocene cooling, in climatic conditions close to those where the ancestor of Pseudohygrohypnum appeared. We speculate that radiation of the P. subeugyrium complex in temperate Asia might have been driven by progressive cooling, aridification, and increases in seasonality, temperature and humidity gradients. Our results parallel those of several integrative taxonomic studies of North Asian mosses, which have resulted in a number of newly revealed species. These include various endemics from continental areas of Asia suggesting that the so-called Rapoport's rule of low diversity and wide distribution range in subpolar regions might not be applicable to bryophytes. Rather, the strong climatic oscillations in these regions may have served as a driving force of speciation and niche divergence.

Keywords: Allopatry; Biogeography; Cryptic diversity; Disjunctive distribution; Hypnales; ITS; North Asia; Species distribution model; trnS-trnF.

Figure 1. Bayesian trees inferred from cp ATRKR (left side) and nr ITS (right side) datasets.

Statistical support indicating Bayesian posterior probabilities (PP) and maximum-likelihood bootstrap support (BS) inferred from matrices without (1) and with (2) indel coding is provided at the branches in the order PP1/PP2/BS1/BS2; a dash (-) indicates no support (PP < 0.7 and BP < 50). Dashed line indicates not or weakly supported clades.

Figure 2. Split trees of the crown clade of Pylaisiaceae, originated from the nr ITS (A) and cp ATR (B) datasets.

Colors indicate revealed lineages of subaquatic Pseudohygrohypnum. Bootstrap values are indicated at branches corresponding to genus- and/ or species level. The bootstrap values appeared from 1,000 iterations of the bootstrap analysis in Splitstree 4.

Figure 3. Crown part of bayesian tree inferred from expanded cp & mt dataset for Pylaisiaceae with nodes dated by Beast.

Temperature course is indicated by the black curve and also by colour-filling above according to Westerhold et al. (2020). Coloured bars in the temperature curve indicates estimated periods of basal radiation of the genus Pseudohygrohypnum (A), origin of mesophilous species of the genus (B), basal radiation of subaquatic lineages of the genus (C), further radiation of P. subeugyrium complex (D) in relation to temperature.

Figure 4. Habit of plants from the revealed lineages of subaquatic Pseudohygrohypnum.

(A–C) P. eugyrium lineages A–C correspondingly, (D–G) P. subeugyrium lineages A-D correspondingly, (H) recombinant plant P. subeugyrium D ×P. subeugyrium E, (I) P. subeugyrium lineage E.

Figure 5. Stem leaves in plants from the revealed lineages of subaquatic Pseudohygrohypnum.

(A–C) P. eugyrium lineages A–C correspondingly, (D–H) P. subeugyrium lineages A–E correspondingly.

Figure 6. Stem leaf tips in plants from the revealed lineages of subaquatic Pseudohygrohypnum.

(A–C) P. eugyrium lineages A–C correspondingly, (D–H) P. subeugyrium lineages A–E correspondingly.

Figure 7. Stem leaf base in plants from the revealed lineages of subaquatic Pseudohygrohypnum.

(A–C) P. eugyrium lineages A–C correspondingly, (D–G) P. subeugyrium lineages A–D correspondingly, (H) recombinant plant P. subeugyrium D ×P. subeugyrium E, (I) P. subeugyrium lineage E.

Figure 8. Distribution of the revealed lineages of subaquatic Pseudohygrohypnum.

upper map –P. eugyrium A (blue squares), B (violet squares) and C (green squares); lower map –P. subeugyrium A (lemon circles), B (orange circles), C (red circles), D (dark blue circles), E (pale blue circles) and recombinant specimens P. subeugyrium B × C (orange stars) and D × E (pale blue stars). Colour shades indicate distribution of the lineage with concolorous symbols obtained from SDMs.

Figure 9. Distribution of the revealed lineages of subaquatic Pseudohygrohypnum along the nine selected CHELSA bioclimatic variables based on observed occurrences.

Colour legend for the lineages under consideration follows the one in Fig. 8.

Figure 10. 3-D PCA scatterplots of the observed occurrences (small symbols) and those predicted by model under 80% threshold (clouds) based on their distribution along the CHELSA bioclimatic variables used for modelling for P. eugyrium A–C (A) and P. subeugyrium A-E (B).

Figure 11. Pseudohygrohypnum appalachianum (from holotype).

(A, B, J) Habit. (C) Stem transverse section. (D) Capsule. (E, G) Mid-leaf cells. (F) Upper leaf cells. (H, I, K) Leaves. (L) Basal leaf cells. Scale bars: 5 mm for B; 2 mm for A, D; 1 mm for H, I, K; 100 μ m for C, E-G, L.

Figure 12. SEM images of peristomes and spores of selected subaquatic species of the genus Pseudohygrohypnum (all images from Holotypes except for I).

Peristome (A) Pseudohygrohypnum appalachianum. (B) P. orientale. (C) P. sibiricum. (D) P. subarcticum. (E) P. neglectum. Distal portion of tooth from the outside (F) Pseudohygrohypnum appalachianum. (G) P. orientale. (H) P. sibiricum. (I) P. purpurascens (from Ignatov & Ignatova 13-1388, HyF52). (J) P. neglectum. Fragments of peristome with outer endostome surface (K) P. orientale. (L) P. sibiricum. (M) P. subarcticum. Spore (N) P. sibiricum. (O) P. neglectum.

Figure 13. Pseudohygrohypnum eugyrium (from Austria, 16.10.2005. G. Schlüsslmayr, HyF15).

(A, B) Habit. (C) Stem transverse section. (D) Upper leaf cells. (E, G) Mid-leaf cells. (F–I) Leaves. (K) Basal leaf cells. Scale bars: 5 mm for B; 2 mm for A; 1 mm for F–I; 100 μ m for C–E, G–K.

Figure 14. Pseudohygrohypnum orientale (from holotype).

(A) Capsule. (B–D) Habit. (E–G, J) Leaves. (H) Mid-leaf cells. (I) Upper leaf cells. (K) Stem transverse section. (L) Basal leaf cells. Scale bars: 5 mm for B, C; 2 mm for A, D; 1 mm for E–G, J; 100 μ m for H, I, K, L.

Figure 15. Pseudohygrohypnum subeugyrium (from UK, Scotland, Rothero, 2019001, dupla in MW, HyF35).

(A, B) Habit. (C) Stem transverse section. (D) Upper leaf cells. (E, I) Mid-leaf cells. (F–H) Leaves. (J) Basal leaf cells. Scale bars: 5 mm for B; 2 mm for A; 1 mm for F–H; 100 μ m for C–E, I, J.

Figure 16. Pseudohygrohypnum purpurascens (from Primorsky Territory, Pidan Peak, Ignatov & Ignatova 06-2197 MW, HyF61).

(A, B, J) Habit. (C) Stem transverse section. (D) Upper leaf cells. (E–H) Leaves. (I) Mid-leaf cells. (K) Basal leaf cells. Scale bars: 5 mm for A, J; 2 mm for B; 1 mm for E–H; 100 μ m for C, D, I, K.

Figure 17. Pseudohygrohypnum sibiricum (from isotype MW9077400).

(A) Stem transverse section. (B) Upper leaf cells. (C, H) Mid-leaf cells. (D, F, G, I) Leaves. (E) Habit. (J, K) Basal leaf cells. Scale bars: 2 mm for E; 1 mm for D, F, G, I; 100 μ m for A–C, H, J, K.

Figure 18. Pseudohygrohypnum subarcticum (from holotype).

(A, C, M) Habit. (B) Capsule. (D) Stem transverse section. (E, I–L) Leaves. (F) Mid-leaf cells. (H) Upper leaf cells. (N) Basal leaf cells. Scale bars: 5 mm for A, M; 2 mm for B, C; 1 mm for E, I–L; 100 μ m for D, F–H, N.

Figure 19. Pseudohygrohypnum neglectum (from holotype).

(A) Stem transverse section. (B) Upper leaf cells. (C, D) Habit. (E, G–I) Leaves. (F) Mid-leaf cells. (J) Basal leaf cells. Scale bars: 5 mm for D; 2 mm for C; 1 mm for E, G–I; 100 μ m for A, B, F, J.

Figure 20. Pseudohygrohypnum subarcticum ×P. neglectum (from HyF5 Yakutia, Kyurbelyakh, Ignatov & Ignatova 11-2242 MW, HyF5).

(A, B) Habit. (C) Upper leaf cells. (D) Mid-leaf cells. (E) Stem transverse section. (F–H) Leaves. (I) Basal leaf cells. Scale bars: 5 mm for B; 2 mm for A; 1 mm for F–H; 100 μ m for C–E, I.