Diverse programs of ascus development in pseudohomothallic species of Neurospora, Gelasinospora, and Podospora

Abstract

Meiosis and ascospore development in the four-spored pseudohomothallic ascomycetes Neurospora tetrasperma, Gelasinospora tetrasperma, Podospora anserina, and P. tetraspora have been reexamined, highlighting differences that reflect independent origins of the four-spored condition in the different genera. In these species, as in the heterothallic eight-spored N. crassa, fusion of haploid nuclei is followed directly by meiosis and a postmeiotic mitosis. These divisions take place within a single unpartitioned giant cell, the ascus, which attains a length of > 0.1 mm before nuclei are enclosed by ascospore walls. Two basically different modes underlie the delivery of opposite mating type nuclei into each of the four ascospores in the different genera. In N. tetrasperma on the one hand, the mating type locus is closely centromere-linked. Mating types therefore segregate at the first meiotic division. The second-division spindles of N. tetrasperma overlap and are usually parallel to one another, in contrast to the their tandem arrangement in N. crassa. As a result, nonsister nuclei of opposite mating type are placed close together in each half-ascus and a pair is enclosed in each ascospore. In the Podospora and Gelasinospora species on the other hand, the second-division spindles are in tandem, with sister nuclei of opposite mating type associated as a pair in each half-ascus. It is established for P. anserina and inferred for P. tetraspora and G. tetrasperma that a single reciprocal crossing over almost always occurs in the mating type-centromere interval, ensuring that mating types segregate at the second meiotic division and that nuclei of opposite mating type are enclosed in each ascospore. Other differences are also seen that are less fundamental. Neurospora tetrasperma differs from the other species in the orientation of chromosomes and spindle pole body plaques at interphase II. Third-division spindles are oriented parallel to the ascus wall in Gelasinospora but across the ascus in Podospora and Neurospora. The two Podospora species differ from one another in nuclear behavior following mitosis in the young ascospores. In P. tetraspora, two of the four nuclei migrate into the tail cell, which degenerates, leaving one functional nucleus of each mating type. In P. anserina, by contrast, only one of the four nuclei moves into the tail cell, leaving the germinating ascospore with two functional nuclei of one mating type and one of the other. The pseudohomothallic condition with its heterokaryotic vegetative phase has significant consequences for both the individual organism and the breeding system. Genetic controls of development and recombination are complex.(ABSTRACT TRUNCATED AT 400 WORDS)

A new yeast genetic resource for analysis and breeding

“…Neurospora crassa (Herskowitz, 1988;Davis, 2000;Metzenberg and Glass, 1990), Podospora anserina (Zickler et al, 1995;Coppin et al, 1997;Raju and Perkins, 1994), Schizosaccharomyces pombe (Herskowitz, 1988;Kelly et al, 1988) and Ustilago maydis (Kahmann et al, 1995;Martinez-Espinosa et al, 1993), so that mating between individuals can be controlled. Development of the genetic system of S. cerevisiae has largely depended on the inactivation of HO so that strains are functionally heterothallic, i.e.…”

Maintaining Two Mating Types: Structure of the Mating Type Locus and Its Role in Heterokaryosis inPodospora anserina

“…In the absence of nuclear mixing, mathematical modeling showed that heterokaryotic thalli should often break down into homokaryons (Roper et al 2013). Pseudohomothallic Ascomycota, such as Podospora anserina and Neurospora tetrasperma, produce mat+/mat2 (or mat A/mat a) dikaryotic ascospores, which upon germination yield a self-fertile heterokaryotic mycelium (Raju and Perkins 1994). In addition to being able to engage in selfreproduction, this mycelium is also able to mate with any partner it may encounter.…”

Degeneration in Codon Usage within the Region of Suppressed Recombination in the Mating-Type Chromosomes of Neurospora tetrasperma

“…The filamentous ascomycete N. tetrasperma is a self-fertile (pseudohomothallic) organism presumed to have evolved from a self-incompatible (heterothallic) ancestor (15,50). As with most filamentous ascomycetes, the N. tetrasperma mat chromosomes contain the mat locus, which comprises two dissimilar alleles (mat a and mat A idiomorphs) that regulate mating and sexual reproduction (10,53).…”