Download Eight-spored asci in certain genotypes of Neurospora tetrasperma

Eight-spored asci in certain genotypes of Neurospora tetrasperma
Asci of N. tetrasperma are normally four-spored, and each ascospore encloses two
nuclei of opposite mating type (mat A and mat a). Consequently, single-ascospore
cultures are self-fertile; such species are said to be pseudohomothallic or secondarily
homothallic (Dodge 1927). The pseudohomothallic behavior of N. tetrasperma results
from the first-division segregation of mating types, followed by spindle overlap at second
and third divisions, and pair-wise alignment of nuclei prior to spore delimitation (Figs. 1,
2; Raju 1992; Gallego et al. 2000). At least three genotypes are known in N.
tetrasperma that perturb the four-spored ascus program, resulting in 5- to 8-spored asci
(Figs. 3, 4, 24).
1. Dodge’s gene E (Eight spore). Eight spore originated in a N. tetrasperma wild strain
from Texas (Dodge 1939). When heterozygous, E caused a majority (up to 80%) of asci
to produce 5-8 spores, with 5 and 6-spored asci being more frequent than 7 and 8spored asci. The remaining 20% of asci were four-spored. In the 5-8 spored asci, one
or more large, heterokaryotic, ascospores were replaced by pairs of small,
homokaryotic, ascospores (Fig. 3). The alignment of spindle at the first division is
normal in almost all asci (Figs. 5-7). However, the alignment of spindles at the second
and third divisions, and pair-wise association of nuclei were altered in asci that are
destined to produce 5 to 8-spores. In many asci, the second and/or third-division
spindles failed to overlap, and their orientation relative to other spindles was often
abnormal (see Figs. 8-13). Consequently, the nuclei failed to associate in pairs, and two
or more single nuclei became sequestered into smaller ascospores (Figs. 16-20; Raju
1992). The smaller ascospores matured normally, but produced homokakryotic, selfsterile cultures (Fig. 22). We have used Dodge’s E gene to examine the behavior of Sk2 and Sk-3 in the larger heterokaryotic, and in the smaller homokaryotic ascospores of
N. tetrasperma (Raju and Perkins 1991).
2. Spore killer-3 augments eight-spore phenotype in E background. In crosses of E x
Sk-3 or wild type x E Sk-3, over 90% of asci contained 6 to 8 ascospores (Fig. 4). We
have used young developing asci from these two crosses, for correlating 8-spore
phenotype with abnormal spindle alignment and behavior. As expected, the spindles at
the second division were aligned in tandem and failed to align in pairs across the ascus
at the first postmeiotic mitosis. Instead, they were aligned equidistant and oriented
irregularly relative to one another (Figs. 8-13). This abnormal spindle behavior resulted
in failed pair-wise association of nuclei prior to spore delimitation, and cutting of mostly
6 to 8-spored asci (Figs. 14, 15, 21). In contrast to Sk-3, Spore killer-2 did not augment
the eight-spore phenotype of E, however.
3. Production of eight-spored asci in certain out crosses of N. tetrasperma. When single
mating-type strains from diverse wild-collected heterokaryons were isolated and
intercrossed, several crosses produced 8-spored asci (Jacobson 1995). In certain
combinations, all asci were four-spored when one parent was used as protoperithecial
parent, but they were all eight-spored when the parents were reversed in reciprocal
crosses (Figs. 23, 24). In the eight-spored crosses, developing asci showed non-
overlapping spindles at second and third divisions. The nuclei failed to associate in
pairs, lined up in single file, and cutout eight uninucleate ascospores, similar to the
behavior in the eight-spored asci of N. crassa (Raju 1992, Jacobson 1995).