The cytology of Aspergillus nidulans

Abstract

In haploid strains of Aspergillus nidulans, asci arise from croziers. The two nuclei of the young ascus (the penultimate cell of the crozier) fuse, and the zygote nucleus immediately undergoes meiosis. At diakinesis and first metaphase eight bivalents are seen: three large (one, Chromosome 2, with a satellite), two medium sized (Chromosomes 4 and 5), two small (Chromosomes 6 and 7) and one very small (Chromosome 8). The perithecia of haploid strains are packed with asci and have very few sterile hyphae.Diploid strains (heterozygotes made by Roper's technique) are very different from haploids in that the perithecia contain many sterile hyphae with little cytoplasm and coiled hyphae with dense cytoplasm made up mainly of uninucleate cells, and there are few asci in a perithecium. Croziers are absent: some of the cells of the coiled hyphae become asci. At diakinesis and first metaphase, the same chromosome configurations are seen as in a cross between the haploid strains from which the diploid was synthesized; the chromosomes are bivalents. The young ascus thus has a single (diploid) nucleus which undergoes meiosis, and asci develop apogamously. No evidence of nuclear fusion in the young ascus and of a tetraploid meiosis was obtained. Cultures from ascopores isolated by micro-manipulation from perithecia of diploids were all haploid. Meiosis appears to proceed normally to first metaphase, but typical later stages, and asci with spores, are rarely seen. First anaphase frequently fails to occur, and the chromosomes clump together.In the strain ad2 y, obtained from a normal strain by X-irradiation, nine bivalents are present, one a very small fragment. In crosses between ad2 y and a normal strain, and in a diploid made from them, the fragment pairs with Chromosome 6. Asci in the cross between ad2 y and a normal haploid are highly irregular, often with less than the usual eight spores.

Electrophoretic karyotype of Aspergillus nidulans.

“…cerevisiae chromosome IV, which has an approximate size of 1.6 mb (9,20). Since (19). The estimated total genome size based on CHEF analysis is 31 mb, which agrees with the 26 mb estimate of Timberlake (12), obtained using DNADNA reassociation analysis.…”

“…The relative intensities of ultraviolet fluorescence of the bands after ethidium bromide staining suggested that two of the bands were doublets, indicating the chromosome number in Aspergillus to be eight. This cytological determination of eight LGs for this organism (11,19). The three Sc.…”

“…Cytological identi-fication designated the chromosomes on the basis of decreasing length using arabic numerals (19), whereas genetic LGs were assigned Roman numerals in chronological order of their discovery (11 Often DNA sequences from Aspergillus are isolated from genomic libraries by selecting for complementation of auxotrophic mutations in either homologous or heterologous systems. Hybridization analysis using cloned DNA sequences as probes to the electrophoretic karyotype can be used in conjunction with genetic analysis to determine the chromosomal origin and genetic location of these sequences.…”

Diverse data supports the transition of filamentous fungal model organisms into the post-genomics era

“…In keeping with progress in diverse research areas, fungal chromosome were targeted for analysis early in the classical genetics era (Subramaniam & Ranganathan 1946) and revealed a similarity between bacterial and fungal chromosomes (Lindegren 1948) ultimately leading to characterisation of fungal karyotypes in Sordaria (Carr & Olive 1958) and Ceratocystis (Aist 1969). Fungal model organisms soon came to the attention of classical cytologists and led to the observation of Neurospora (McClintock 1945) and Aspergillus (Elliott 1960) chromosomes. At this time, there were few data points for each organism: genus, species, host, morphology, colour, culture requirements, mating type, number of chromosomes, and applied use.…”

CHPA, a Cysteine- and Histidine-Rich-Domain-Containing Protein, Contributes to Maintenance of the Diploid State in Aspergillus nidulans

“…In this case, however, the hyphae are differentiated into other types of cells comprising the reproductive structure (conidiophore) and uninucleate conidia (28). In contrast to the transient diploid phase in sexual development, somatic diploids formed from haploid nuclear fusion are stable and undergo asexual reproduction cycles similar to those of haploids, producing isomorphic developmental structures (10). However, little is known of the genes that control the formation and maintenance of diploids during the sexual or asexual life cycle.…”