DEVELOPMENT AND CYTOLOGY
The only essentially complete developmental study
of a Hypoxylon is that on H.
fuscum (Rogers, 1967a). Development is of the Xylaria-type
sensu Luttrell (1951). Stromata initially bear conidia. As conidial production
ceases, ascogonia are initiated within the stroma and soon separated from it by hyphae
that eventually become differentiated into a peridium. Growth of the perithecium involves
the expansion of the ascocarp wall along with the production of space-making hyphae
(paraphyses). Asci develop from ascogenous hyphae via croziers and push into the
space-making hyphae, lysing some of them. At the same time an ostiolar canal develops by
activity of the opposing growth of periphyses. Mature ascospores are forcibly ejected
through the ostiole. Jensen's elegant study (1981) of Hypoxylon serpens--now
placed in Nemania--shows a very similar developmental pattern.
Rogers (1979 and refs. therein) did cytological studies-including chromosome counts-of four taxa that are now considered to be Hypoxylon species. Hypoxylon multiforme and H. fuscum were believed to have n = 4 (Rogers, 1965,1967a), and H. rubiginosum and H. cohaerens were said to have n = 5 (Rogers, 1969, 1972). Rogers (1979) suggested that the primitive number in the Xylariaceae, and perhaps many other pyrenomycetes, is n = 7, higher and lower numbers being derived. Wittmann-Meixner et al. (1989), using cytofluorometric determinations of relative DNA content, tried to establish or estimate chromosome numbers. In their opinion, chromosome numbers in Hypoxylon (in the present sense) were probably n = 7 or higher, using Nemania serpens genomic DNA as a standard reference. There is not, however, necessarily a correlation between DNA content and number of linkage groups, i.e., chromosomes. Moreover, studies utilizing electric pulse fields to separate chromosomes in gels (CHEF gel phenomena) indicate that chromosome numbers within certain taxa and subtaxa are variable. At the present time the most reliable chromosome counts in fungi are based on the elaborate reconstruction and counting of synaptonemal complexes by electron microscopy and by genetic linkage group studies. For example, the rust fungus Puccinia graminis Pers.: Pers. had been widely considered to have n = 6 and Wittmann-Meixner et al. (1989) used these and other data to correlate with their DNA data to establish haploid numbers. However, the haploid number of P. graminis is n = 18, based upon reconstruction of pachytene nuclei (Boehm et al., 1992). In any case, it is our present belief that the primitive chromosome number of the ancestral Hypoxylon is n = 7, very probably with higher and lower derived numbers.
An important cytological finding is that the ascospore of Hypoxylon, as the genus is delimited here, is uninucleate at maturity. Limited studies indicate that ascospores are uninucleate from formation to maturity or become binucleate following formation via a mitosis. One nucleus then disintegrates, leaving the spore uninucleate. The possible significance of these observations is discussed in EVOLUTION (and see Rogers, 1979).