Isthmosporella Shearer & J.L. Crane, Mycologia 91(1): 141 (1999).
Index Fungorum number: IF 28249; Facesoffungi number: FoF xxx, 1 morphological species (Species Fungorum 2022), molecular data unavailable.
Saprobic on host. Sexual morph: Ascomata on natural substrate immersed becoming superficial, oblique, black, membranous, rostrate. Ascomatal wall 2-layered, inner layer of compressed, subhyaline to light brown, fusoid cells, outer layer of brown, fusoid-cylindric pseudoparenchyma cells. Beak short, cylindrical, periphysate. Hamathecium of sparse, interascal, septate pseudoparaphyses. Asci in a broad basal layer, fissitunicate, ventricose to clavate, broadly rounded at apex, short-stalked, nonamyloid apex, without an apical apparatus or oculus, 8- spored, persistent. Ascospores hyaline, cylindrical to fusoid, isthmoid at center, sometimes bent at isthmus and becoming u- or v-shaped, end cells tapering, phragmoseptate, constricted at septa, frequently fragmenting at isthmus to form part spores, surrounded by a gelatinous sheath. Asexual morph: Unknown (Adapted from Shearer and Crane 1999).
Type species: Isthmosporella pulchra Shearer & J.L. Crane
Notes: Isthmosporella was introduced by Shearer and Crane (1999) with I. pulchra as type species. Isthmosporella is characterized by superficial, oblique, black, ascomata, subhyaline to light brown, fusoid cells in the peridium, ventricose to clavate asci and hyaline, cylindrical to fusoid ascospores surrounded by a gelatinous sheath. Shearer and Crane (1999) placed Isthmosporella in Phaeosphaeriaceae based on morphology namely small, globose ascomata, fairly undistinguishable peridium of small pseudoparenchymatic cells, and few pseudoparaphyses (Barr 1987; Shearer and Crane 1999). Isthmosporella resembles Bricookea in having hyaline, phragmoseptate ascospores but differs in that the latter forms locules in stromatic tissues which open via a slit and in the acute to obtuse, inequilateral, 3-septate ascospores (Shearer and Crane 1999). Isthmosporella differs from Nodulosphaeria in lacking setae surrounding and covering the apical papilla and differs from Paraphaeosphaeria in having an enlarged submedian cell in the ascospores (Shoemaker and Babcock 1985; Shearer and Crane 1999). Isthmosporella differs from Phaeosphaeria in ascomal anatomy, having few pseudoparaphyses, and in ascospore size and septation. Isthmosporella has relatively thick peridium comprising an inner layer of 5-7 rows of fusoid cells and an outer layer of 3-4 fusoid-cylindric cells and is only known from submerged wood while Phaeosphaeria has thin peridium of 2-4 polygonal cells, and can be found in monocotyledonous substrates (Shoemaker and Babcock 1989). Isthmosporella differs from Kalmusia in having thin ascomatal wall and isthmoid ascospore while the latter genus is stromatic, has thick peridium with 3-septate, brown ascospores. Isthmosporella is unique in having isthmoid ascospore. Some genera such as Vialaea has isthmoid ascospore but members of this genus have unitunicate asci (Cannon 1995; Shearer and Crane 1999). Isthmosporella is currently placed in Pleosporales, genera incertae sedis and the familial position is still uncertain. Sequence data is lacking for Isthmosporella. Fresh collections with DNA sequence data is needed to confirm the taxonomic placement of the genus.
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