Phylogeny and taxonomy of Phyllactinia species (powdery mildew: Erysiphaceae) occurring on the ash trees (Fraxinus spp.)

Abstract

The genus Fraxinus (Oleaceae), known as ash trees, currently comprises 43 recognized species that are distributed in temperate and subtropical regions of the Northern Hemisphere. Two Phyllactinia species, P. fraxini and P. fraxinicola, have been known on Fraxinus spp. so far. In this study, powdery mildews belonging to Phyllactinia were collected on Fraxinus spp. from different areas of the world to make molecular and morphological analyses. These specimens are divided into four distinct molecular phylogenetic groups, which are distinguishable by their morphology and/or host preference. Two new species, viz. P. japonica occurring on F. sieboldina and F. lanuginosa f. serrata, and P. fraxini-longicuspidis on F. longicuspis, are proposed in this study. An epitype is designated for P. fraxini. This study indicates very high host specificity among the four Phyllactinia species on Fraxinus, suggesting that genetic isolation by host specificity played a more important role than geographic segregation in the speciation events of these Phyllactinia species. Evolutionary timing calculated by molecular clock analysis suggests that these powdery mildews diverged in accordance with host phylogeny after divergence of host plants.

Keywords: Phyllactinia fraxini­longicuspidis; Phyllactinia japonica; biogeography; epitype; new species.

Fig. 1– Phylogenetic analysis of the 5'-end of 28S rDNA (including domains D1 and D2) for 46 sequences from Phyllactinia fraxini s. lat. This is one of the equally parsimonious trees with 35 steps, which were found using a heuristic search. Horizontal branch lengths are proportional to the number of substitutions that were inferred to have occurred along a particular branch of the tree. Sequences retrieved from type specimens were shown as boldface (HT: holotype, ET: epitype). BS (≥70%) values by the maximum parsimony (MP) and maximum likelihood (ML) methods were shown on the respective branches. Rooting point was inferred by a preliminary analysis using P. roboris and P. guttata as outgroup taxa. Divergence time of the major clades calculated by molecular clock of ITS region were shown on/under the respective nodes (mya: million years ago).

Fig. 2– Chasmothecia produced on host leaves. A: Phyllactinia fraxini on Fraxinus excelsior (TSU-MUMH1726). B: P. fraxinicola on F. japonica (TSU-MUMH7104). Bars: A, B 200 µm.

Fig. 3– Phyllactinia fraxini on Fraxinus excelsior (KR-M-0048247). A–L: Conidia M–Q: Conidiophores. R–U: Appressoria. Bars: A–L 25 µm; M–U 10 µm.

Fig. 4– Phyllactinia fraxini on Fraxinus mandshurica (TSU-MUMH1239). A–C: Chasmothecia. D–M: Asci. N–S: Penicillate cells. Bars: A–C 200 µm; D–M 40 µm; N–S 15 µm.

Fig. 5– Phyllactinia fraxinicola on Fraxinus japonica (TSU-MUMH4150). A–C: Chasmothecia. D–H, L–P: Asci. I–K, Q–S: Penicillate cells. Bars: A–C 200 µm; D–H, L–P 40 µm; I–K, Q–S 25 µm.

Fig. 6– Phyllactinia japonica on Fraxinus lanuginosa f. serrata (TSU-MUMH7224). A, B: Conidiophores. C: Conidia. D, E: Hyphal appressoria. Bars: 20 µm.

Fig. 7– Phyllactinia japonica on Fraxinus sieboldiana (TNS-F-91391, holotype). A–C: Chasmothecia. D–H: Penicillate cells. I–R: Asci. Bars: A–C 200 µm; D–H 20 µm; I–R 40 µm.

Fig. 8– Phyllactinia fraxini-longicuspidis on Fraxinus longicuspis (TSU-MUMH7227). A, B: Conidiophores. C–E: Conidia. F, G: Appressoria. Bars: A–E, G 20 µm; F 10 µm.

Fig. 9– Phyllactinia fraxini-longicuspidis on Fraxinus longicuspis (TSU-MUMH5421). A–C: Chasmothecia. D–M: Asci. N–S: Penicillate cells. Bars: A–C 200 µm; D–M 40 µm; N–S 15 µm.

Fig. 10– Comparison of host ranges of Phyllactinia and Erysiphe occurring on Fraxinus.

* Fraxinus apertisquamifera has not been recorded as a host of Phyllactinia.