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PubMed2002

Directional bias during mating type switching in Saccharomyces is independent of chromosomal architecture.

Haploid Saccharomyces cells have the remarkable potential to change mating type as often as every generation, a process accomplished by an intrachromosomal gene conversion between an expressor locus MAT and one of two repositories of mating type information, HML or HMR. The particular locus selected as donor is dictated by the mating type of the cell, a bias that ensures productive mating type interconversion. Here we use green fluorescent protein tagging of the expressor and donor loci on chromosome III to show that this preference for donor locus does not result from a predetermined organization of chromosome III: HML and MAT as well as HMR and MAT remain separated in cells of both mating types. In fact, cells in which the inappropriate donor locus is artificially tethered to MAT still predominantly select the correct donor. We find, though, that initiation of switching leads to a rapid association of the correct donor locus with MAT. Thus, in mating type switching in Saccharomyces, donor preference is imposed at commitment to recombination rather than at physical contact of interacting DNA strands.

Chromosomes, Fungal↗

PubMed1993

Directionality of fission yeast mating-type interconversion is controlled by the location of the donor loci.

Cells of homothallic strains of Schizosaccharomyces pombe efficiently switch between two mating types called P and M. The phenotypic switches are due to conversion of the expressed mating-type locus (mat1) by two closely linked silent loci, mat2-P and mat3-M, that contain unexpressed information for the P and M mating types, respectively. In this process, switching-competent cells switch to the opposite mating type in 72-90% of the cell divisions. Hence, mat2-P is a preferred donor of information to mat1 in M cells, whereas mat3-M is a preferred donor in P cells. We investigated the reason for the donor preference by constructing a strain in which the genetic contents of the donor loci were swapped. We found that switching to the opposite mating type was very inefficient in that strain. This shows that the location of the silent cassettes in the chromosome, rather than their content, is the deciding factor for recognition of the donor for each cell type. We propose a model in which switching is achieved by regulating accessibility of the donor loci, perhaps by changing the chromatin structure in the mating-type region, thus promoting an intrachromosomal folding of mat2 or mat3 onto mat1 in a cell type-specific fashion. We also present evidence for the involvement of the Swi6 and Swi6-mod trans-acting factors in the donor-choice mechanism. We suggest that these factors participate in forming the proposed folded structure.

Base Sequence↗

PubMed1995

Mutations affecting donor preference during mating type interconversion in Saccharomyces cerevisiae.

Homothallic strains of Saccharomyces cerevisiae can convert mating type from a to alpha or alpha to a as often as every generation, by replacing genetic information specifying one mating type at the expressor locus, MAT, with information specifying the opposite mating type. The cryptic mating type information that is copied and inserted at MAT is contained in either of two loci, HML or HMR. The particular locus selected as donor during mating type interconversion is regulated by the allele expressed at MAT. MATa cells usually select HML, and MAT alpha cells usually select HMR, a process referred to as donor preference. To identify factors required for donor preference, we isolated and characterized a number of mutants that frequently selected the nonpreferred donor locus during mating type interconversion. Many of these mutants were found to harbor chromosome rearrangements or mutations at MAT or HML that interfered with the switching process. However, one mutant carried a recessive allele of CHL1, a gene previously shown to be required for efficient chromosome segregation during mitosis. Homothallic strains of yeast containing a null allele of CHL1 exhibited almost random selection of the donor locus in a MATa background but were normal in their ability to select HMR in a MAT alpha background. Our results indicate that Chl1p participates in the process of donor selection and are consistent with a model in which Chl1p helps establish an intrinsic bias in donor preference.

Alleles↗

PubMed1996

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.

Chromosome Mapping↗

PubMed2004

A pheromone receptor gene, pre-1, is essential for mating type-specific directional growth and fusion of trichogynes and female fertility in Neurospora crassa.

Neurospora crassa is a heterothallic filamentous fungus with two mating types, mat a and mat A. Its mating involves differentiation of female reproductive structures (protoperithecia) and chemotropic growth of female-specific hyphae (trichogynes) towards a cell of the opposite mating type in a pheromone-mediated process. In this study, we characterize the pre-1 gene, encoding a predicted G-protein-coupled receptor with sequence similarity to fungal pheromone receptors. pre-1 is most highly expressed in mat A strains under mating conditions, but low levels can also be detected in mat a strains. Analysis of pre-1 deletion mutants showed that loss of pre-1 does not greatly affect vegetative growth, heterokaryon formation or male fertility in either mating type. Protoperithecia from Deltapre-1 mat A mutants do not undergo fertilization; this defect largely stems from an inability of their trichogynes to recognize and fuse with mat a cells. Previous work has demonstrated that the Galpha subunit, GNA-1, and the Gbeta protein, GNB-1, are essential for female fertility in N. crassa. Trichogynes of Deltagna-1 and Deltagnb-1 mutants displayed severe defects in growth towards and fusion with male cells, similar to that of Deltapre-1 mat A strains. However, the female sterility defect of the Deltapre-1 mat A mutant could not be complemented by constitutive activation of gna-1, suggesting additional layers of regulation. We propose that PRE-1 is a pheromone receptor coupled to GNA-1 that is essential for the mating of mat A strains as females, consistent with a role in launching the pheromone response pathway in N. crassa.

Fungal Proteins↗

PubMed2006

Can non-directional male mating preferences facilitate honest female ornamentation?

Recent studies have demonstrated male mate choice for female ornaments in species without sex-role reversal. Despite these empirical findings, little is known about the adaptive dynamics of female signalling, in particular the evolution of male mating preferences. The evolution of traits that signal mate quality is more complex in females than in males because females usually provide the bulk of resources for the developing offspring. Here, we investigate the evolution of male mating preferences using a mathematical model which: (i) specifically accounts for the fact that females must trade-off resources invested in ornaments with reproduction; and (ii) allows male mating preferences to evolve a non-directional shape. The optimal adaptive strategy for males is to develop stabilizing mating preferences for female display traits to avoid females that either invests too many or too few resources in ornamentation. However, the evolutionary stability of this prediction is dependent upon the level of error made by females when allocating resources to either signal or fecundity.

Animals↗

PubMed2006

Cryptococcus neoformans population includes hybrid strains homozygous at mating-type locus.

Recent attempts to characterise the hybrid strains of Cryptococcus neoformans have led to the identification of a cryptic population of hybrid strains ('H strains') with double DNA content but only a single mating-type allele. To verify a set of hypotheses concerning their origin, we investigated 14 previously isolated H strains and ten F1-progeny strains arising from H99 and JEC20 mating. The double DNA content was tested by flow cytometry; the presence of only one mating type was tested by amplifying 12 mating-type-specific genes and one gene unlinked with the mating-type locus (URA5). Analysis of the F1 progeny identified two H strains, and electrophoretic karyotyping confirmed the occurrence of genetic recombination. The simultaneous presence of the homozygous and heterozygous loci, and the fact that all of the F1-progeny strains presented a recombinant karyotype, suggest that the H strains originated from the post-meiotic random fusion of two of the four recombinant nuclei. Further studies are required to elucidate the role of the homozygous mating-type loci in the virulence of C. neoformans.

Crosses, Genetic↗

PubMed1996

Tetrapolar fungal mating types: sexes by the thousands.

In order to achieve genetic rearrangement in a sexual cycle, eukaryotes go through the processes of meiosis and mating. Different mating types assure that mating is only possible between two genetically diverse individuals. Basidiomycetous fungi display thousands of different mating types that are determined by two genetically unlinked loci. One locus is multiallelic and contains genes for homeodomain transcription factors which are able to form heterodimers. The activation of target genes is dependent on heterodimers formed from the monomeric transcription factor proteins originating from different alleles of this genetic locus. The interactions between the two monomeric transcription factors and the activation of target genes by the heterodimeric proteins make this regulatory system both complex and interesting. The second locus contains a pheromone receptor system: the pheromone receptor is similar to the G protein-linked serpentine receptors in Saccharomyces cerevisiae that activate the pheromone response via a phosphorylation signal transduction cascade in S. cerevisiae. This pheromone perception is a trigger of sexual development and not, as with yeast, itself under control of mating type genes. Rather it directly senses diversity at the mating type loci. Whereas heterobasidiomycetes display a bi-allelic structure for this locus with recognition between one receptor and the opposite pheromone, homobasidiomycetes contain multiple specificities for pheromone receptors and pheromones.

Basidiomycota↗

PubMed2006

Pheromones are essential for male fertility and sufficient to direct chemotropic polarized growth of trichogynes during mating in Neurospora crassa.

Neurospora crassa is a self-sterile filamentous fungus with two mating types, mat A and mat a. Its mating involves chemotropic polarized growth of female-specific hyphae (trichogynes) toward male cells of the opposite mating type in a process involving pheromones and receptors. mat A cells express the ccg-4 pheromone and the pre-1 receptor, while mat a strains produce mRNA for the pheromone mfa-1 and the pre-2 receptor; MFA-1 and CCG-4 are the predicted ligands for PRE-1 and PRE-2, respectively. In this study, we generated Deltaccg-4 and Deltamfa-1 mutants and engineered a mat a strain to coexpress ccg-4 and its receptor, pre-2. As males, Deltaccg-4 mat A and Deltamfa-1 mat a mutants were unable to attract mat a and mat A trichogynes, respectively, and consequently failed to initiate fruiting body (perithecial) development or produce meiotic spores (ascospores). In contrast, Deltaccg-4 mat a and Deltamfa-1 mat A mutants exhibited normal chemotropic attraction and male fertility. Deltaccg-4 Deltamfa-1 double mutants displayed defective chemotropism and male sterility in both mating types. Heterologous expression of ccg-4 enabled mat a males to attract mat a trichogynes, although subsequent perithecial differentiation did not occur. Expression of ccg-4 and pre-2 in the same strain triggered self-stimulation, resulting in formation of barren perithecia with no ascospores. Our results indicate that CCG-4 and MFA-1 are required for mating-type-specific male fertility and that pheromones (and receptors) are initial determinants for sexual identity during mate recognition. Furthermore, a self-attraction signal can be transmitted within a strain that expresses a pheromone and its cognate receptor.

DNA, Fungal↗

PubMed1990

Courtship in Saccharomyces cerevisiae: an early cell-cell interaction during mating.

During conjugation in Saccharomyces cerevisiae, two cells of opposite mating type (MATa and MAT alpha) fuse to form a diploid zygote. Conjugation requires that each cell locate an appropriate mating partner. To investigate how yeast cells select a mating partner, we developed a competition mating assay in which wild-type MAT alpha cells have a choice of two MATa cell mating partners. We first demonstrated that sterile MAT alpha 1 cells (expressing no a- or alpha-specific gene products) do not compete with fertile MATa cells in the assay; hence, wild-type MATa and MAT alpha cells can efficiently locate an appropriate mating partner. Second, we showed that a MATa strain need not be fertile to compete with a fertile MATa strain in the assay. This result defines an early step in conjugation, which we term courtship. We showed that the ability to agglutinate is not necessary in MATa cells for courtship but that production of a-pheromone and response to alpha-pheromone are necessary. Thus, MATa cells must not only transmit but must also receive and then respond to information for effective courtship; hence, there is a "conversation" between the courting cells. We showed that the only alpha-pheromone-induced response necessary in MATa cells for courtship is production of a-pheromone. In all cases tested, a strain producing a higher level of a-pheromone was more proficient in courtship than one producing a lower level. We propose that during courtship, a MAT alpha cell selects the adjacent MATa cell producing the highest level of a-pheromone.

Blotting, Northern↗

PubMed1989

Yeast pheromone response pathway: characterization of a suppressor that restores mating to receptorless mutants.

Saccharomyces cerevisiae haploid cells, alpha and a, mate after being appropriately stimulated by the pheromone secreted by the opposite cell type (a-factor and alpha-factor, respectively). The binding of a pheromone to its receptor is a signal that initiates a series of intracellular changes that lead to the specific physiological alterations required for mating. To identify components of the signal transduction pathway, we sought pseudorevertants that restored mating competence to receptor mutants (MAT alpha ste3::LEU2). The suppressor srm1-1 was isolated as a recessive mutation that conferred temperature-sensitive growth to all strains and mating ability to MAT alpha ste3::LEU2 strains at the nonpermissive temperature. In addition, when srm1-1 mutants were shifted to the nonpermissive temperature, they exhibited two phenotypes characteristic of pheromone response, induction of FUS1 transcription and accumulation of cells in the G1 phase of the cell cycle. The srm1-1 mutation also suppressed a deletion of the alpha-factor-receptor gene in a cells. Together, these phenotypes suggest that the wild-type SRM1 product is a component of the pheromone response pathway. Deletion of STE4 or STE5, which are required in both haploid cell types for mating and response to pheromone, was not suppressed by srm1-1, suggesting that the SRM1 product may function before the STE4 and STE5 products. SRM1 is an essential gene and is expressed in both haploid cell types as well as in the product of their mating, a/alpha diploids. Homozygous srm1-1 a/alpha diploids were temperature sensitive although they did not arrest in G1. Thus, the SRM1 product may also have a role in the vegetative life cycle of cells.

Amino Acid Sequence↗

PubMed1992

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.

Crosses, Genetic↗

PubMed1997

Mating type in filamentous fungi.

Mating type genes regulate sexual compatibility and sexual reproduction in fungi. This review focuses on recent molecular analyses of well-characterized mating systems from representative ascomycete (Neurospora crassa, Podospora anserina) and basidiomycete (Ustilago maydis, Coprinus cinereus, Schizophyllum commune) fungi. These mating systems include many conserved components, such as gene regulatory polypeptides and pheromone/receptor signal transduction cascades, as well as conserved processes, like self-nonself recognition and controlled nuclear migration. The components' structures and their genetic arrangements in the mating system vary greatly in different fungi. Although similar components and processes are also found in ascomycete yeasts (Saccharomyces cerevisiae and Schizosaccharomyces pombe), the filamentous systems exhibit properties not encountered in yeast. Mating type genes act within, and control the development of, spatially differentiated fruiting bodies. The complex mating systems of basidiomycetes, unlike ascomycete systems, involve novel one-to-many specificity in both pheromone-receptor and homeodomain protein interactions.

Ascomycota↗

PubMed2005

Mating in Candida albicans and the search for a sexual cycle.

Candida albicans is a normal part of the human microflora, but it is also an opportunistic fungal pathogen that causes both mucosal infections and life-threatening systemic infections. Until recently, C. albicans was thought to be asexual, existing only as an obligate diploid. However, a mating locus was identified that was homologous to those in sexually reproducing fungi, and mating of C. albicans strains was subsequently demonstrated in the laboratory. In this review, we compare and contrast the mating process in C. albicans with that of other fungi, particularly Saccharomyces cerevisiae, whose mating has been most intensively studied. Several features of the mating pathway appear unique to C. albicans, including aspects of gene regulation and cell biology, as well as the involvement of "white-opaque" switching, an alteration between two quasi-stable inheritable states. These specializations of the mating process may have evolved to promote the survival of C. albicans in the hostile environment of a mammalian host.

Animals↗

PubMed2000

Preliminary observations on sexual behavior and the mating system in free-ranging lesser galagos. (Galago moholi).

Sexual and associated patterns of behavior of lesser galagos (Galago moholi) were recorded during an 18-month study conducted at the Nylsvley Nature Reserve in South Africa. Animals were trapped and fitted with radio transmitter belts in order to monitor nocturnal activities during twice-yearly mating seasons. Most copulations occurred during the last week in May, while a subsidiary (post-partum) mating season occurred in late September-early October. Females came into estrus sequentially during the May season. Adult males exhibited increase in body weight and testes volume during the mating season, changes which were most pronounced among the larger males (> 226 g). Larger males also had the greatest mating success, initiating 88% of observed copulations. Sixty-seven percent of matings involved more than one male copulating with the same female during her estrus, which lasted 1-3 days. Mounts were prolonged (range 2-53 min, mean 9.0 min) and males copulated repeatedly (2-5 times) with the same partner during a single night. These observations of sexual behavior and of large relative testes size in free-ranging lesser galagos are consistent with the occurrence of a dispersed mating system involving sperm competition in this nocturnal prosimian species.

Animals↗

PubMed2004

Mating system of an exceptional primate, the olive colobus (Procolobus verus).

In the olive colobus (Procolobus verus), many groups have multiple males and the males have large testes. This indicates that even though this species lives in small groups, single males do not monopolize the groups. We investigated the strategies employed by males to secure their mating success, and sought to determine whether the lack of male monopolization was a result of female mating strategies, as indicated by the exaggerated sexual swellings of the females. Four study groups were monitored for demographic changes, and group composition was determined in six additional groups in Taï National Park, Ivory Coast, between 1994 and 1999. Social behavior was recorded by scan and focal sampling in the study groups. The almost permanent association of olive colobus with Diana monkeys (Cercopithecus diana) in effect provided males a resource at which they could expect females to visit and sometimes even permanently join them, as well as protection from predators. As alternative strategies for obtaining females, one male took over the group of another male and one male immigrated into a bisexual group. Within bi-male groups, dominant males mated most frequently and males defended their groups during intergroup interactions. Lone females that visited groups or solitary males had a swelling more often than expected, and generally mated with the males they visited. Females had long receptive periods, several consecutive receptive cycles, and some overlap in receptive periods within groups. Females mated with extragroup males, and during infertile periods. We concluded that the males used the Diana monkeys for safety reasons and to obtain mating partners, and that female reproductive biology and behavior prevented the monopolization of groups of females by single males. Our data were inconclusive as regards the benefits to females of avoiding monopolization by males.

Animals↗

PubMed1989

The spread of exclusive mating in a chacma baboon population.

Mating has progressively tended toward pair bonding in a chacma baboon (Papio cynocephalus ursinus) population during eight years of study. Both males and females have been mating with a smaller and smaller percentage of the potential mates available to them. Troop subdivision during the dry winter in the absence of predation was the original cause of mating limitation, but most members of winter subtroops only rarely mated with adults who had not belonged to the same winter subtroop during the summers as well. Following reintroduction of potential predation, both subtroop size and frequency decreased, but mating did not become less exclusive.

Animals↗

PubMed2005

Romantic love: an fMRI study of a neural mechanism for mate choice.

Scientists have described myriad traits in mammalian and avian species that evolved to attract mates. But the brain mechanisms by which conspecifics become attracted to these traits is largely unknown. Yet mammals and birds express mate preferences and make mate choices, and data suggest that this "attraction system" is associated with the dopaminergic reward system. It has been proposed that intense romantic love, a cross-cultural universal, is a developed form of this attraction system. To determine the neural mechanisms associated with romantic love we used functional magnetic resonance imaging (fMRI) and studied 17 people who were intensely "in love" (Aron et al. [2005] J Neurophysiol 94:327-337). Activation specific to the beloved occurred in the right ventral tegmental area and right caudate nucleus, dopamine-rich areas associated with mammalian reward and motivation. These and other results suggest that dopaminergic reward pathways contribute to the "general arousal" component of romantic love; romantic love is primarily a motivation system, rather than an emotion; this drive is distinct from the sex drive; romantic love changes across time; and romantic love shares biobehavioral similarities with mammalian attraction. We propose that this attraction mechanism evolved to enable individuals to focus their mating energy on specific others, thereby conserving energy and facilitating mate choice-a primary aspect of reproduction. Last, the corticostriate system, with its potential for combining diverse cortical information with reward signals, is an excellent anatomical substrate for the complex factors contributing to romantic love and mate choice.

Adolescent↗