Reinforced postmating reproductive isolation barriers in Neurospora, an Ascomycete microfungus.

Maladaptive hybridization promotes reinforcement, selection for stringent reproductive isolation barriers during speciation. Reinforcement is suspected when barriers between sympatric populations are stronger than allopatric barriers, and particularly when stronger barriers evolve in the species and sex suffering the greatest costs of hybridization. Canonically, reinforcement involves premating barriers. Selection for postmating barriers is controversial, but theoretically possible. We examined geographical patterns in reproductive isolation barriers between Neurospora crassa and Neurospora intermedia, fungi with pheromone-mediated mate recognition and maternal care. We find that isolation is stronger between sympatric populations than allopatric populations, and stronger barriers are associated with the species (N. crassa) and mating role (maternal) suffering the greater costs of hybridization. Notably, reinforced isolation involves a postmating barrier, abortion of fruitbodies. We hypothesize that fruitbody abortion is selectively advantageous if it increases the likelihood that maternal Neurospora individuals successfully mate conspecifically after maladaptive hybrid fertilization.

Phylogenetic and biological species diversity within the Neurospora tetrasperma complex.

The objective of this study was to explore the evolutionary history of the morphologically recognized filamentous ascomycete Neurospora tetrasperma, and to reveal the genetic and reproductive relationships among its individuals and populations. We applied both phylogenetic and biological species recognition to a collection of strains representing the geographic and genetic diversity of N. tetrasperma. First, we were able to confirm a monophyletic origin of N. tetrasperma. Furthermore, we found nine phylogenetic species within the morphospecies. When using the traditional broad biological species recognition all investigated strains of N. tetrasperma constituted a single biological species. In contrast, when using a quantitative measurement of the reproductive success, incorporating characters such as viability and fertility of offspring, we found a high congruence between the phylogenetic and biological species recognition. Taken together, phylogenetically and biologically defined groups of individuals exist in N. tetrasperma, and these should be taken into account in future studies of its life history traits.

Pseudohomothallism and evolution of the mating-type chromosome in Neurospora tetrasperma.

Ascospores of Neurospora tetrasperma normally contain nuclei of both mating-type idiomorphs (a and A), resulting in self-fertile heterokaryons (a type of sexual reproduction termed pseudohomothallism). Occasional homokaryotic self-sterile strains (either a or A) behave as heterothallics and, in principle, provide N. tetrasperma with a means for facultative outcrossing. This study was conceived as an investigation of the population biology of N. tetrasperma to assess levels of intrastrain heterokaryosis (heterozygosity). The unexpected result was that the mating-type chromosome and autosomes exhibited very different patterns of evolution, apparently because of suppressed recombination between mating-type chromosomes. Analysis of sequences on the mating-type chromosomes of wild-collected self-fertile strains revealed high levels of genetic variability between sibling A and a nuclei. In contrast, sequences on autosomes of sibling A and a nuclei exhibited nearly complete homogeneity. Conservation of distinct haplotype combinations on A and a mating-type chromosomes in strains from diverse locations further suggested an absence of recombination over substantial periods of evolutionary time. The suppression of recombination on the N. tetrasperma mating-type chromosome, expected to ensure a high frequency of self fertility, presents an interesting parallel with, and possible model for studying aspects of, the evolution of mammalian sex chromosomes.

Suppressed recombination and a pairing anomaly on the mating-type chromosome of Neurospora tetrasperma.

Neurospora crassa and related heterothallic ascomycetes produce eight homokaryotic self-sterile ascospores per ascus. In contrast, asci of N. tetrasperma contain four self-fertile ascospores each with nuclei of both mating types (matA and mata). The self-fertile ascospores of N. tetrasperma result from first-division segregation of mating type and nuclear spindle overlap at the second meiotic division and at a subsequent mitotic division. Recently, Merino et al. presented population-genetic evidence that crossing over is suppressed on the mating-type chromosome of N. tetrasperma, thereby preventing second-division segregation of mating type and the formation of self-sterile ascospores. The present study experimentally confirmed suppressed crossing over for a large segment of the mating-type chromosome by examining segregation of markers in crosses of wild strains. Surprisingly, our study also revealed a region on the far left arm where recombination is obligatory. In cytological studies, we demonstrated that suppressed recombination correlates with an extensive unpaired region at pachytene. Taken together, these results suggest an unpaired region adjacent to one or more paired regions, analogous to the nonpairing and pseudoautosomal regions of animal sex chromosomes. The observed pairing and obligate crossover likely reflect mechanisms to ensure chromosome disjunction.

Vegetative incompatibility in the het-6 region of Neurospora crassa is mediated by two linked genes.

Non-self-recognition during asexual growth of Neurospora crassa involves restriction of heterokaryon formation via genetic differences at 11 het loci, including mating type. The het-6 locus maps to a 250-kbp region of LGIIL. We used restriction fragment length polymorphisms in progeny with crossovers in the het-6 region and a DNA transformation assay to identify two genes in a 25-kbp region that have vegetative incompatibility activity. The predicted product of one of these genes, which we designate het-6(OR), has three regions of amino acid sequence similarity to the predicted product of the het-e vegetative incompatibility gene in Podospora anserina and to the predicted product of tol, which mediates mating-type vegetative incompatibility in N. crassa. The predicted product of the alternative het-6 allele, HET-6(PA), shares only 68% amino acid identity with HET-6(OR). The second incompatibility gene, un-24(OR), encodes the large subunit of ribonucleotide reductase, which is essential for de novo synthesis of DNA. A region in the carboxyl-terminal portion of UN-24 is associated with incompatibility and is variable between un-24(OR) and the alternative allele un-24(PA). Linkage analysis indicates that the 25-kbp un-24-het-6 region is inherited as a block, suggesting that a nonallelic interaction may occur between un-24 and het-6 and possibly other loci within this region to mediate vegetative incompatibility in the het-6 region of N. crassa.

Natural history of prostatism: high American Urological Association Symptom scores among community-dwelling men and women with urinary incontinence.

OBJECTIVES: The American Urological Association Symptom Index (AUASI) has been reported to be nonspecific for benign prostatic hyperplasia. Comparable scores occur in patients with lower urinary tract symptoms of varying etiology, and even among women. The purpose of this study was to determine the association between the AUASI and urinary incontinence among community-dwelling men and women. METHODS: In 1994, a randomly selected cohort of 1540 men and women aged 50 years or older from Olmsted County, Minnesota completed a self-administered questionnaire that assessed urinary symptom severity (AUASI score) and estimated the prevalence of urinary incontinence in the previous 12 months. The association between moderate to severe urinary symptom (AUASI score greater than 7) and urinary incontinence was investigated using multiple logistic regression, with adjustments for gender and age. RESULTS: Mean and median AUASI scores increased with age, and were higher among those respondents with urinary incontinence and among men. All seven urologic items in the AUASI were more prevalent among respondents with urinary incontinence and among men. Nocturia was highly prevalent among respondents with and without urinary incontinence, but urgency, frequency, and weak urinary stream were more prevalent among respondents with incontinence than among respondents without incontinence. When urinary incontinence, gender, and age were considered simultaneously, the respondents with urinary incontinence (odds ratio [OR] = 4.4, 95% confidence interval [CI] = 3.4, 5.5), men (OR = 1.9, 95% CI = 1.5, 2.5), and respondents 65 years or older (OR = 1.5, 95% CI = 1.2, 1.9) were more likely to have moderate to severe urinary symptoms. CONCLUSIONS: Our findings suggest that men and women with urinary incontinence and older men and women are significantly more likely to have moderate to severe urinary symptoms. These findings suggest that urinary incontinence may contribute to a high AUASI score in both sexes. Thus, these data indicate that the similarity in the distribution of the AUASI in men and women is, in part, an artifact introduced by the confounding effects of continence status.

Distribution of the Brief Male Sexual Inventory in community men.

The objectives of the study were to characterize male sexual functioning as related to age in community-dwelling older men. In 1989, a random sample of men aged 40-79 y (n=2115) without prior prostate surgery, prostate cancer, or other conditions known to affect voiding function (except benign prostatic hyperplasia) was invited (55% agreed) to participate in the Olmsted County Study of Urinary Symptoms and Health Status Among Men. In 1996, a previously validated male sexual function questionnaire was administered to the cohort. The questionnaire has 11 questions measuring sexual drive (two questions); erectile function (three) and ejaculatory function (two), as well as assessing problems with sex drive, erections, or ejaculation (three); and overall satisfaction with sex life (one). Each question is scored on a scale of 0-4, with higher scores indicating better functioning. Cross-sectional age-specific means (+/-s.d.) for drive, erections, ejaculation, problems, and overall satisfaction declined from 5.2 (+/-1.5), 9.8 (+/-2.5), 7.4 (+/-1.4), 10.7 (+/-2.2), and 2.6 (+/-1.0), respectively, for men in their 40s to 2.4 (+/-1.6), 3.3 (+/-3.4), 3.6 (+/-3.2), 7.7 (+/-3.8), and 2.1 (+/-1.2) for men 70 y and older (all P<0.001). The cross-sectional decline in function with age was not constant, with age-related patterns differing by domain. The percentage of men reporting erections firm enough to have intercourse in the past 30 days declined from 97% (454/468) among those in their 40s to 51% (180/354) among those in their 80s (P&<0.001). In age-adjusted analyses, men reporting regular sexual partners had statistically significantly higher levels of sex drive, erectile function, ejaculatory function, and overall satisfaction than those who did not report regular sexual partners. Sexual drive, erectile functioning, ejaculatory functioning, and overall sexual satisfaction in men show somewhat differing cross-sectional patterns of decline with advancing age. Active sexual functioning is maintained well into the 80s in a substantial minority of community-dwelling men.

Care of a parturient with preeclampsia, morbid obesity, and Crouzon's syndrome.

We present the case of a 31-year-old woman with severe preeclampsia, morbid obesity, and a craniofacial syndrome who developed respiratory failure necessitating intubation and delivery by cesarean section. Her airway management was complicated by supraglottic edema and macroglossia. Fiberoptic intubation was difficult but successful. After delivery of the infant, tracheostomy was performed to provide a secure airway until the supraglottic edema resolved over the subsequent two weeks. The airway implications of preeclampsia and Crouzon's syndrome are reviewed.

Control of mating type heterokaryon incompatibility by the tol gene in Neurospora crassa and N. tetrasperma.

The mating-type of Neurospora crassa (A and a) have a dual function: A and a individuals are required for sexual reproduction, but only strains of the same mating type will form a stable vegetative heterokaryon. Neurospora tetrasperma, in contrast, is a naturally occurring A+a heterokaryon. It was shown previously that the mating-type genes of both species are functionally the same and are not responsible for this difference in heterokaryon incompatibility. This suggests that a separate genetic system determines the heterokaryon incompatibility function of mating type. The mutant tolerant (tol) in N. crassa, unlinked to mating type, acts as a specific suppressor of A+a heterokaryon incompatibility. In the present study, the wild-type alleles at the tol locus were introgressed reciprocally, from N. crassa into N. tetrasperma and from N. tetrasperma into N. crassa, to investigate the action of these alleles in the A+a heterokaryon incompatibility systems of these species. The wild-type allele from N. tetrasperma (tolT) acts as a recessive suppressor of A+a heterokaryon incompatibility in N. crassa. Furthermore, the wild-type allele from N. crassa (tolC) causes A and a to become heterokaryon incompatible in N. tetrasperma, while having no effect on the sexual reproduction. Therefore, the tol gene plays a major role in determining the heterokaryon compatibility of mating type in these species: tolC is an active allele that causes incompatibility and tolT an inactive allele that suppresses incompatibility by its inactivity.

Allelic diversity at the het-c locus in Neurospora tetrasperma confirms outcrossing in nature and reveals an evolutionary dilemma for pseudohomothallic ascomycetes.

Vegetative cells of the filamentous ascomycete Neurospora tetrasperma are typically heterokaryotic, possessing haploid nuclei of both A and a mating types. As a consequence, N. tetrasperma is self-fertile. This life cycle, referred to as pseudohomothallism, clearly derives from true heterothallism of the type exhibited by related species such as N. crassa. Occasional homokaryotic, single-mating-type (heterothallic) isolates occur; in the laboratory, such strains can be outcrossed. The potential for outcrossing in N. tetrasperma raises the question of how this organism avoids heterokaryon incompatibility. Heterokaryon incompatability in vegetatively growing fungi is controlled by multiple loci. Two strains must be identical at each het locus (11 in N. crassa) to form a stable heterokaryon. Prior to the present survey, it seemed plausible that N. tetrasperma avoids heterokaryon incompatibility by maintaining compatible allele combinations through continual selfing. A survey of het-c variation among wild-type isolates in this study demonstrated that N. tetrasperma outcrosses in nature and that such matings can result in incompatible combinations of het-c alleles. Whereas individual wild-type isolates are invariably homoallelic for het-c, closely related strains may possess functionally different het-c alleles, which predate the origin of N. tetrasperma. Therefore, pseudohomothallic ascomycetes such as N. tetrasperma face an apparent evolutionary dilemma: the benefits of outcrossing must be balanced against the fact that matings can produce unstable heterokaryons and disrupt the pseudohomothallic life cycle.

The genetics of hyphal fusion and vegetative incompatibility in filamentous ascomycete fungi.

Filamentous fungi grow as a multicellular, multinuclear network of filament-shaped cells called hyphae. A fungal individual can be viewed as a fluid, dynamic system that is characterized by hyphal tip growth, branching, and hyphal fusion (anastomosis). Hyphal anastomosis is especially important in such nonlinear systems for the purposes of communication and homeostasis. Filamentous fungi can also undergo hyphal fusion with different individuals to form heterokaryons. However, the viability of such heterokaryons is dependent upon genetic constitution at heterokaryon incompatibility (het) loci. If hyphal fusion occurs between strains that differ in allelic specificity at het loci, vegetative incompatibility, which is characterized by hyphal compartmentation and cell lysis, is induced. This review covers microscopic and genetic analysis of hyphal fusion and the molecular and genetic analysis of the consequence of hyphal fusion between individuals that differ in specificity at het loci in filamentous ascomycetes.

A kalilo-like linear plasmid in Louisiana field isolates of the pseudohomothallic fungus Neurospora tetrasperma.

Two Louisiana strains of Neurospora tetrasperma contain a linear plasmid (LA-kalDNA) with a restriction map identical to the Hawaiian Neurospora intermedia senescence plasmid, kalDNA, but with termini 100 nucleotide pairs shorter. One of these strains also bore a circular plasmid similar to the Hawaiian circular plasmid Hanalei-2. One species probably acquired both plasmids from the other by horizontal transfer, at a time sufficiently distant for sequence divergence to take place. Many LA-kalDNA-bearing derivative strains senesced, but this plasmid does not guarantee senescence. Furthermore, LA-kalDNA does not insert into mtDNA. One senescent strain showed no LA-kalDNA. The plasmids are effectively transmitted via the pseudohomothallic sexual cycle. Single mating-type derivatives transmit plasmids maternally.

Heteroallelism at the het-c locus contributes to sexual dysfunction in outcrossed strains of Neurospora tetrasperma.

Neurospora tetrasperma is naturally heterokaryotic, with cells possessing haploid nuclei of both a and A mating types. As a result, isolates are self-fertile (pseudohomothallic). Occasional homokaryotic ascospores and conidia arise, however, and they produce strains that are self-sterile and must outcross to complete sexual reproduction. Invariably, laboratory crosses employing sibling a and A strains from the same parental heterokaryon restore the pseudohomothallic, heterokaryotic state. In contrast, outcrosses employing a and A strains from different wild isolates typically result in sexual dysfunction. Diverse sexual dysfunction types have been observed, ranging from complete sterility to reduced viability. We report that one type of dysfunction, characterized by spontaneous loss of the heterokaryotic state upon ascospore germination, can result from the interaction of incompatible alleles at heterokaryon incompatibility loci. Specifically, we demonstrate that homoallelism at the het-c locus in N. tetrasperma is required for heterokaryon stability. Heterokaryon incompatibility therefore provides an obstacle to outcrossing in this species, an observation with important implications for fungal life-cycle evolution.

Molecular and functional analyses of incompatibility genes at het-6 in a population of Neurospora crassa.

Two closely linked genes, un-24 and het-6, associated with the het-6 heterokaryon incompatibility functional haplotype were examined in 40 Neurospora crassa strains from a Louisiana sugarcane field. Partial diploid analyses were used to determine that half of the strains were functionally Oak Ridge (OR) and half were non-OR and indistinguishable from the standard Panama (PA) form. PCR-based markers were developed to identify polymorphisms within both un-24 and het-6. Two common forms of each gene occur based on these molecular markers. Rare forms of both un-24 and het-6 were identified as variants of the non-OR form by a DNA transformation assay. The heterokaryon incompatibility function of haplotypes, based on partial diploid analyses, was perfectly correlated with the PCR-based markers at both loci. This correlation indicates that the two loci are in severe linkage disequilibrium in this population sample and may act as an incompatibility gene complex. Southern hybridizations using OR- and PA-derived cloned probes from the region that spans un-24 and het-6 showed that the apparent absence of recombination in this approximately 25-kbp region is associated with low levels of overall sequence identity between the PA and OR forms.

An osmotic-remedial, temperature-sensitive mutation in the allosteric activity site of ribonucleotide reductase in Neurospora crassa.

An osmotic-remedial, temperature-sensitive conditional mutant (un-24) was generated by Repeat Induced Point mutation (RIP) from a cross between a wild-type N. crassa strain and a strain carrying a approximately 250-kb duplication of the left arm of linkage group II (LGII). The mutation was mapped to the duplicated segment, within 2.6 map units of the heterokaryon incompatibility locus het-6. DNA transformation identified a 3.75-kb fragment that complemented the temperature-sensitive phenotype. A large ORF within this fragment was found to have a high degree of sequence identity to the large subunit of ribonucleotide reductase (RNR) from diverse organisms. Conserved amino acids at the active site and the allosteric activity sites are also evident. An unusual feature of the Neurospora sequence is a large insertion near the C-terminus relative to otherwise homologous sequences from other organisms. Three transition mutations, indicative of RIP, were identified in the N-terminal region of the temperature-sensitive mutant allele. One of these mutations results in a non-conservative amino acid substitution within the four-helix bundle that is important in the allosteric control of ribonucleotide reductase activity. This substitution appears to disrupt proper folding of the allosteric activity site during synthesis of the protein.

Longitudinal changes in peak urinary flow rates in a community based cohort.

PURPOSE: We describe longitudinal changes in peak urinary flow rates in community dwelling men in Olmsted County, Minnesota. MATERIALS AND METHODS: A cohort of 2,115 men 40 years old or older was randomly selected from the Olmsted County, Minnesota population. Peak urinary flow rates and the American Urological Association symptom index were assessed in all men at baseline and biennially, and in a 25% random subsample prostate volume was determined by transrectal ultrasonography. The annualized percentage change in peak urinary flow rate (slope) was assessed for 492 men in the subsample during 6 years of followup. RESULTS: Median peak urinary flow rate slope was -2.1% per year (25th percentile -4.0, 75th percentile -0.6). Peak urinary flow rate declined more rapidly with decreasing baseline rate, and increasing baseline age, prostate volume and symptom severity (all p = 0.001). When the variables were simultaneously adjusted for each other, a rapid decline (negative slope 4.5% or greater per year) was more likely in men 70 years old or older (odds ratio 46.4, 95% confidence intervals 16.8, 127.7) and those with a rate less than 10 ml. per second (42.0, 14.1, 125.3) at baseline compared to those 40 to 49 years old and those with a rate of 15 ml. or greater, respectively. Prostate volume and symptom severity were not statistically significant predictors of a rapid decline in peak urinary flow rate when variables were considered simultaneously. CONCLUSIONS: Despite variability in measurement of peak urinary flow rate, a consistent decline was observed when measured longitudinally in a community based cohort. Furthermore, this decline was associated with impairments in other physiological and anatomical measures of lower urinary tract function in an unselected cohort of men.

Age-specific reference ranges for serum prostate-specific antigen in black men.

BACKGROUND: The detection of prostate cancer by screening for prostate-specific antigen (PSA) in serum is improved when age-specific reference ranges are used, but these ranges have been derived from white populations. We determined the distribution of PSA and age-specific reference ranges in black men both with and without prostate cancer. METHODS: From January 1991 through May 1995, we measured serum PSA in 3475 men with no clinical evidence of prostate cancer (1802 white and 1673 black) and 1783 men with prostate cancer (1372 white and 411 black). We studied the data as a function of age and race to determine the usefulness of measuring PSA in diagnosing prostate cancer. RESULTS: Serum PSA concentrations in black men (geometric mean in controls, 1.48 ng per milliliter; in patients, 7.46) were significantly higher than those in white men (geometric mean in controls, 1.33 ng per milliliter; in patients, 6.28). The values in the controls correlated directly with age. The area under the receiver-operating-characteristic curve was 0.91 for blacks and 0.94 for whites. If traditional age-specific reference ranges were used in screening black men, with the test specificity kept at 95 percent, 41 percent of cases of prostate cancer would be missed. For the test to have 95 percent sensitivity among black men, the following normal reference ranges should be used: for men in their 40s, 0 to 2.0 ng of PSA per milliliter (test specificity, 93 percent); for men in their 50s, 0 to 4.0 ng per milliliter (specificity, 88 percent); for men in their 60s, 0 to 4.5 ng per milliliter (specificity, 81 percent); and for men in their 70s, 0 to 5.5 ng per milliliter (specificity, 78 percent). CONCLUSIONS: Serum PSA concentrations can be used to discriminate between men with prostate cancer and those without it among both blacks and whites. Over 40 percent of cases of prostate cancer in black men would not be detected by tests using traditional age-specific reference ranges, which maintain specificity at 95 percent. In this high-risk population, the alternative approach--maintaining sensitivity at 95 percent--may be used with acceptable decrements in specificity.

Characterization of Neurospora crassa mutants isolated following repeat-induced point mutation of the beta subunit of fatty acid synthase.

Neurospora crassa cel-2 mutants were isolated following repeat-induced point mutation using part of the gene encoding beta-fatty acid synthase. These mutants are phenotypically less leaky than cel-1, which has a defective alpha-fatty acid synthase. The cel-2 mutant had a strict fatty acid (16:0) requirement for growth, and synthesized less fatty acid de novo than cel-1. Unlike cel-1, cel-2 has impaired fertility, and homozygous crosses are infertile, suggesting a low but strict requirement for fatty acid synthesis during sexual development. Like cel-1, cel-2 synthesized unusually high levels of the polyunsaturate 18:3(Delta9,12,15), and elongated 18:2(Delta9,12 )and 18:3(Delta9,12,15 )to 20:2(Delta11,14) and 20:3(Delta11,14,17), respectively. These fatty acids are not synthesized by wild-type, except following treatment with cerulenin (a fatty acid synthase inhibitor), demonstrating that inhibition of fatty acid biosynthesis results in a relative increase in both fatty acid desaturation and elongation activity.

Phylogenetic species recognition and species concepts in fungi.

The operational species concept, i.e., the one used to recognize species, is contrasted to the theoretical species concept. A phylogenetic approach to recognize fungal species based on concordance of multiple gene genealogies is compared to those based on morphology and reproductive behavior. Examples where Phylogenetic Species Recognition has been applied to fungi are reviewed and concerns regarding Phylogenetic Species Recognition are discussed.