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.