An intragenic map of the brlA locus of Aspergillus nidulans

Chromosome rearrangements in Neurospora and other filamentous fungi

Knowledge of fungal chromosome rearrangements comes primarily from N. crassa, but important information has also been obtained from A. nidulans and S. macrospora. Rearrangements have been identified in other Sordaria species and in Cochliobolus, Coprinus, Magnaporthe, Podospora, and Ustilago. In Neurospora, heterozygosity for most chromosome rearrangements is signaled by the appearance of unpigmented deficiency ascospores, with frequencies and ascus types that are characteristic of the type of rearrangement. Summary information is provided on each of 355 rearrangements analyzed in N. crassa. These include 262 reciprocal translocations, 31 insertional translocations, 27 quasiterminal translocations, 6 pericentric inversions, 1 intrachromosomal transposition, and numerous complex or cryptic rearrangements. Breakpoints are distributed more or less randomly among the seven chromosomes. Sixty of the rearrangements have readily detected mutant phenotypes, of which half are allelic with known genes. Constitutive mutations at certain positively regulated loci involve rearrangements having one breakpoint in an upstream regulatory region. Of 11 rearrangements that have one breakpoint in or near the NOR, most appear genetically to be terminal but are in fact physically reciprocal. Partial diploid strains can be obtained as recombinant progeny from crosses heterozygous for insertional or quasiterminal rearrangements. Duplications produced in this way precisely define segments that cover more than two thirds of the genome. Duplication-producing rearrangements have many uses, including precise genetic mapping by duplication coverage and alignment of physical and genetic maps. Typically, fertility is greatly reduced in crosses parented by a duplication strain. The finding that genes within the duplicated segment have undergone RIP mutation in some of the surviving progeny suggests that RIP may be responsible for the infertility. Meiotically generated recessive-lethal segmental deficiencies can be rescued in heterokaryons. New rearrangements are found in 10% or more of strains in which transforming DNA has been stably integrated. Electrophoretic separation of rearranged chromosomal DNAs has found useful applications. Synaptic adjustment occurs in inversion heterozygotes, leading progressively to nonhomologous association of synaptonemal complex lateral elements, transforming loop pairing into linear pairing. Transvection has been demonstrated in Neurospora. Beginnings have been made in constructing effective balancers. Experience has increased our understanding of several phenomena that may complicate analysis. With some rearrangements, nondisjunction of centromeres from reciprocal translocation quadrivalents results in 3:1 segregation and produces asci with four deficiency ascospores that occupy diagnostic positions in linear asci. Three-to-one segregation is most frequent when breakpoints are near centromeres. With some rearrangements, inviable deficiency ascospores become pigmented. Diagnosis must then depend on ascospore viability. In crosses between highly inbred strains, analysis may be handicapped by random ascospore abortion. This is minimized by using noninbred strains as testers.

Polarity of meiotic gene conversion is 5' to 3' within the niaD gene of Aspergillus nidulans

We have examined polarity of meiotic gene conversion in the niiA-niaD gene cluster of Aspergillus nidulans in two-point crosses. The type and position of the mutations represented by the niaD alleles and the correlation between the relative frequency of gene conversion and the physical position of these mutations were determined. We show that polarity of meiotic gene conversion is 5' to 3' (transcribed strand) within the niaD gene. Additional crosses involving a niiA allele and a niaD allele show little polarity of gene conversion, which suggests that the recombination events leading to restoration of the niaD gene are initiated upstream of the coding region of the niaD gene but within the niiA-niaD gene cluster, possibly within the intergenic promoter region.