ASCOSPORES

Home
Genus Hypoxylon
Key to Taxa of Hypoxylon
Accepted Taxa
List of Names
Bibliography


Jack
Yu-Ming Ju
Michael J. Adams

 More than two-third of the Hypoxylon taxa have ellipsoid-inequilateral ascospores as exemplified by H. fragiforme, H. rubiginosum, and H. shearii. Inequilateral ascospores in most cases bear the germ slit on the convex side. Only a few exceptions exist, such as H. brevirimum, H. munkii, and a few others. Moreover, inequilateral ascospores are consistently associated with dehiscent perispores. A hypothesis that explains the coordinated evolution among these three characteristics of ascospores, i.e., the inequilateral ascospores with the germ slit on the convex side and with a dehiscent perispore, is discussed in EVOLUTION. Ascospore size ranges are a major character used by Miller (1961) to delimit Hypoxylon taxa. In fact, this appears to be the only character that he used to differentiate taxa of section Hypoxylon with orange to red granules inside the stromata. Usually, taxa with small ascospores tend to have narrower size ranges. Taxa with ascospores less than 20 mm long rarely have a size range greater than 10 mm in length. Nonetheless, the range in H. fuscum is from 8 to 20 mm. This might be explained with the hypothesis that a species complex is actually being dealt with. Germ slits of ascospores are usually constant within a taxon. Spore-length, straight germ slits which sometimes tend to be somewhat sigmoid are most commonly encountered in the genus. Several Hypoxylon have peculiar germ slits. Hypoxylon curvirimum has spiral germ slits. Hypoxylon leptascum and its large-ascospore variety, macrosporum, and H. urceolatum have very short germ slits extending from only one end of the ascospores. In H. rectangulosporum, the germ slits are absent, as demonstrated by Rogers et al. (1992). Ornamentation of ascospores can be found on the perispore, the epispore, or/and beneath the epispore. Perispore ornamentation is evident in a number of taxa that have dehiscing perispores. They fall into two major patterns which are used as one of the three major characters to delimit the two sections of Hypoxylon. Transversely oriented, coil-like ornamentation can be found in section Hypoxylon, whereas a thickening situated toward one end of the perispore is almost universal in section Annulata. The conspicuousness of the coil-like ornamentation in section Hypoxylon is acknowledged as an important character at species level. Examples can be found in the separation of species pairs such as H. anthochroum and H. duranii, and H. fendleri and H. retpela. Interestingly, taxa with very conspicuous coil-like ornamentation tend to have smaller conidium sizes in general. Epispore ornamentation appears to be rare in Hypoxylon. Shallow pits can be found in H. rubellum (Rogers et al., 1987), striations in H. californicum, and pleated folds in H. rectangulosporum (Rogers et al., 1992) and H. thouarsianum (Miller, 1961). Dots that lie under the epispore cover the entire ascospore of H. macrosporum but are restricted to a stripe along the germ slit in H. carneum, H. vogesiacum, and several other taxa. Conspicuously flattened ascospores are rare and have only been observed in H. piceum and H. pouceanum.