Shear and Dodge (1927) described the sexual biology of two eight-spored heterothallic species (N.crassa and N. sitophila) and one four-spored apparently homothallic species (N. tetrasperma, later called pseudohomothallic). They named the genus Neurospora because of the nerve-like ornamentation on developing ascospore walls (Fig 1). Dodge (1927) described the nuclear behavior responsible for heterothallism in the eight-spored species and for pseudohomothallism in the four-spored species (Fig 2; see Raju 1980, 1992).
Several hundred Neurospora photos are shown here, organized into various chapters. In addition to the series on normal ascus development in wild type N. crassa and N. tetrasperma, other series illustrate ascus and ascospore development in several mutants, chromosome rearrangements, and Spore killers. Other series illustrate ascus development in related ascomycetes – for example, Coniochaeta tetraspora and Cochliobolus heterostrophus.
Staining. McClintock (1945) showed that meiosis and chromosome behavior in Neurospora are very similar to those of higher plants and animals (see Singleton 1953). McClintock, Singleton, and E.G. Barry have used an aceto-orcein staining procedure for studying meiotic chromosome behavior. I use an iron-hematoxylin procedure, which is effective not only for staining chromosomes, but also for nucleolus, spindles, spindle pole bodies, and the ascus apical pore (Raju 1980). I have also used the DNA-specific acriflavin for detailed meiotic chromosome analysis (Raju 1986). A ten-fold dilution of ferric acetate and hematoxylin solutions has been used for lightly staining unfixed rosettes of maturing asci (Figs 3-6).
Sizes. Perithecia of N. crassa measure ~400 x 500 µm (Fig 9), and produce well over 200 asci. Mature asci are 18-20 µm x 175-200 µm, and mature ascospores (14-15 x 28-30 µm) contain up to 64 nuclei. Although the nuclei in mature ascospores and hyphae do not exceed ~2 µm in diameter, the meiotic nuclei in asci are much larger. At pachytene, the nuclei can reach 12-15 mm in diameter, and the chromosomes measure from 6 to 18 mm long (Figs 3, 4). The highly condensed metaphase/anaphase chromosomes during meiosis are no larger than 0.5-2.0 µm in diameter, however (Figs 5, 7, 8).
Times. For studies of ascus development, we routinely grow the protoperithecial parent (female) on Petri plates containing synthetic crossing medium for 5 days and fertilize with conidia (or mycelial fragments) from a strain of opposite mating type. The developing perithecia contain ascogenous hyphae and young asci at 3 days after fertilization, various meiotic division stages at 4-5 days, spore delimitation and maturation at 5-8 days (Fig 10). Ascospores from individual asci are ejected forcefully, as discrete groups, shortly thereafter.
Permission and Copyright
You may use any of these photos for educational purposes (not for profit). We ask that you use a credit line citing the original reference and the photographer. High-resolution versions of these photos are available upon request from N.B. Raju’s photo collection, see Contacts. If previously published photos are to be used in publication, permission will, of course, be required from the publisher (see the captions of enlarged photos).
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