Xylaria apiculata

Xylaria apiculata Cooke, from Reserva Los Cedros, Ecuador. Given the paucity of functional illustrations of this taxon, I wanted to create one of my own. This was the first Xylaria that I figured, back in 2012. While my illustration technique has certainly improved, I still remain proud of the functional beauty of this illustration. (scale bars: a = 2 mm; b = 1 mm (including stromatal section); c = 10 µm; d = 50 µm).

This is one of the more common of the tropical/subtropical Xylaria that one is likely to encounter, and represents the typical form for a large species complex that includes many similar taxa, largely distinguished from one another by microscopic characteristics of the ascospores. I’d like to spend some time considering this taxon, the history of its taxonomy, and the larger group of which it is a part. Members of the “Xylaria apiculata group” will typically have delicate stromata with a distinctly pointed (acute!) sterile apex (sterile, meaning that there are no perithecia) and an ectostromal layer, generally remaining well into maturity, which has been split into vertical striations by the expansion of the perithecia.

The stromata of this fungus are slender and relatively tall compared to the other species in the complex, ranging from about 0.5 cm to 3-4 cm tall by 1-3 mm wide. The fertile (peritheciate) portion ranges from about 0.3- 1.5 cm, and begins and ends abruptly. The perithecial contours are just visible beneath the surface, and there is a pale coating on the outer surface that is split into vertical striations or scales by the perithecia developing underneath. The interior tissues are white, and softer than the black outer coating; the stipes are often (but not always) densely tomentose. Mature stromata are not often branched, though they may be; non-peritheciate (conidial/anamorphic) stromata may be highly branched, or not branched at all. (Remember: Xylaria species are nearly always impossible to identify from non-peritheciate, conidial stromata).

The perithecia are globose, 0.3-0.6 mm diam. The ostioles are very finely papillate.

The asci are monoserrate, with stipes shorter than spore-bearing parts, tipped with a I+ (reactive in Melzer’s reagent) apical apparatus 3-4.5 µm tall by 3-4 µm broad, inverted-hat shaped (flaring at the outer rim), sometimes with a swelling in the center as well.

The ascospores are brown to olivaceous-brown, unicellular, ellipsoid-inequilateral (like a flattened football) to navicular, with narrowly to broadly rounded ends. They measure 20-28 × 7-12 µm (length:width ration is usually between 2.2 and 2.8), with straight, spore-length (or nearly so) germ slit on the flattened (less curved) side of the spore.

Part of the large Xylaria arbuscula species complex (also called the Xylaria apiculata group), this taxon is distinguished by its larger ascospores and spore-length germ slit. These are the commonly encountered species that might be confused for this taxon: X. pseudoapiculata, X. venosula, X. arbuscula, X. xylarioides (= X. schreuderiana), X. pseudoapiculata, and X. smilacicola. They are distinguished mostly by ascospore characteristics — ascospore length, germ slit morphology, and other distinguishing characters are given below to help separate these similar taxa.

X. apiculata: 20-28 µm; germ slit nearly spore length, straight
X. venosula: 15-19 µm; germ slit conspicuously less than spore length, straight
X. arbuscula: 12-15 µm; germ slit conspicuously less than spore length, straight
X. xylarioides: 15-21 µm; germ slit almost spore length, straight; penzigioid
X. pseudoapiculata: 15-19 µm; germ slit spiraling germ slit
X. smilacicola: 15-21 µm; germ slit almost spore length, straight; host-specific for plants in the monocot family Smilacaceae, known from New Zealand

This was a fungus originally described by Mordecai C. Cooke, back in 1879, from New Zealand. He had exactly this to say about it, without even a simple figure:

Cooke, M.C. 1879. New Zealand fungi. Grevillea. 8(46):54-68.

Folks were a bit more laconic about describing new species back then; the purpose of the publication was mostly just to point people to the type specimen (which has since been transferred to Kew, along with most of the rest of Cooke’s herbarium).

The species is described in a bit more depth by other folks, however. Richard William George (R.W.G.) Dennis (who deserves a post of his own at some point), provided a lot of the foundational work on the Xylariaceae of tropical America. He wrote the papers that are still, to this day, the first places to which anyone working on tropical Xylaria go. In 1956, R.W.G. Dennis published “Some Xylarias of tropical America”, and then in 1957, he followed up with “Further notes on tropical American Xylariaceae”, both published in the Kew Bulletin. His description of Xylaria apiculata is certainly the basis of my concept of the species, though he did not recognize several features as taxonomically informative that I find absolutely essential to consider, including the germ slit and the ascus apical plug morphology. In his 1956 paper reads:

You can see that he synonymized X. apiculata with X. venosula, H. xylarioides (= X. xylarioides), and X. Schreuderiana. I agree that Xxylarioides and X. Schreuderiana are one and the same, but the rest I tend to disagree with. However, Dennis isn’t the only one who thinks that some of these things are too similar to be considered distinct; in 2010, Hladki & Romero synonymized X. xylarioides, X. venosula, and X. smilacicola under the name X. xylarioides, citing overlapping size ranges of the ascospores and variability of stromatal morphology. Doctor Yu-Ming Ju disagrees, and believe you can sort these species effectively by ascospore size and germ slit morphology combined with stromatal features. I tend to agree with Dr. Ju, (where I have seen X. venosula and X. xylarioides, the length of the germ slit has been quite distinct) but here is their explanation:

“The types of X. smilacicola, X. venosula and H. xylarioides [i.e., X. xylarioides] share all characters that we consider of taxonomic value (e.g. ascospores of the same colour, shape and size, with the same germ slit morphology; H. xylaroides 15.5– 19.5 × 8–9 μm; X. venosula 17.5–21.5 × 7.5–9 μm; X. smilacicola 15.5–18 × 6.5–8 μm). They differ in the shape of the fertile region (from globose to subglobose to subcylindrical) and the length of the stipe (from 0.5–13 mm or occasionally absent), characters that we accept as intra- specific variation and characters that intergrade in the extensive material studied.”

Something to keep in mind when you encounter these fungi. Unfortunately, there still isn’t really enough genetic evidence to sort these taxa effectively. There is a need for increased collection, and the preservation of high-quality DNA extractions — something that is complicated by the fact that these fungi are often collected in remote regions of developing countries.

Xylaria apiculata has been included in some more modern examinations of Xylaria, however. This is what Rogers and Ju have to say about this fungus in their 2012 “The Xylariaceae of the Hawaiian Islands”, which Rogers once told me he considered the best place to start in the current state of literature for identifying xylariaceous fungi:

” Xylaria apiculata Cooke (Figs. 43 and 44) Notes: This small fungus is probably a member of the complex that includes X. arbuscula, X. bambusicola and X. partita; these taxa are discussed elsewhere herein. They are separated primarily on ascospore size and, in the case of X. bambusicola, on host.

“Hosts and Substrates: Pipturus hawaiensis, Sapindus saponaria
“Distribution: HA: Kipuka Puaulu; OA: Castle Tr.

“Reference: Dennis, 1956.”

From Rogers & Ju 2012.

I don’t find this figure all that useful, personally. Luckily, Rogers has done a more complete job illustrating this fungus elsewhere. In a treatment of the genus from New Zealand back in 1986, he and Gary Samuels provide this illustration:

Fig. 8 Xylaria apiculata A, B, C, Stromata (A: PDD 41970, B: PDD 43161, PDD 43171). D, Anamorphic stromata (PDD 44427). E, Stromal surface (PDD 41968). F, Hairs at base of stroma (PDD 41968). G, Ascal ring (Melzer’s reagent, PDD 41970). H, Ascospores with germ slits (Melzer’s reagent, PDD 41968). I, Culture on OA (PDD 45352) (Lines A, B, C, D = 1 cm; G, H = 10 µm). (From: Rogers & Samuels 1989.)It’s a little grainy, and the black-and-white that was necessary due to the printing technology available at the time is limiting, but overall, I think this is the best illustration of this taxon in the current scientific literature on Xylaria.

Interestingly, there is mounting evidence that this taxon occurs outside of the tropics. I have seen pictures of fungi in the X. apiculata group from a handful of places in the Southern US, and have been sent one collection from Central California (likely representing to the northern range limit for this taxon). This far north the stromata are stunted, but the barcode gene, though somewhat divergent, matches. Also, the germ slit and apical plug morphology agree

 

Xylaria apiculata Cooke, from Jackson State Park, California, MO 155465, collected by Sydney Glassman. Photo in a by Danny Newman. (scale bars: a = 1 cm; b = 1 mm; c,d = 5 mm; e,f,g,h = 10 µm)

Hopefully, if you find a pointy-tipped Xylaria, now you’ve got some tools to help you determine if it’s Xylaria apiculata, or one of the many other taxa in this fascinating group of fungi.

 

References:

Dr. Yu-Ming Ju, personal communication
Cooke, M. C. (1879). New Zealand fungi. Grevillea, 8(46), 54-68.
Dennis, R. W. G. (1956). Some Xylarias of tropical America. Kew Bulletin, 401-444.
Dennis, R. W. G. (1957). Further notes on tropical American Xylariaceae. Kew Bulletin, 297-332.
Hladki, A. I., & Romero, A. I. (2010). A preliminary account of Xylaria in the Tucuman Province, Argentina, with a key to the known species from the Northern Provinces. Fungal Diversity, 42(1), 79-96.
Rogers, J. D., & Samuels, G. J. (1986). Ascomycetes of New Zealand 8. Xylaria. New Zealand Journal of Botany, 24(4), 615-650.
Rogers, J. D., & Ju, Y. M. (2012). The Xylariaceae of the Hawaiian Islands. North American Fungi, 7, 1-35.

 

 

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