Mating-type differentiation in ascosporogenous yeasts on the basis of mating-type-specific substances responsible for sexual cell-cell recognition

Summary Sexual agglutination occurred only between cells of opposite mating types of the same species in all the Sacharomyces, Hansenula, Saccharomycodes, and Pichia yeasts tested. We succeeded in solubilizing the sex-specific glycoprotein, cell wall agglutination substance responsible for sexual agglutinability by briefly autoclaving these yeasts. The agglutination substances of all the above yeasts were univalent and sensitive to the enzyme pronase. The formation of complementary complexes was observed only between agglutination substances of opposite mating types of the same species. In general, the agglutination substance of one mating type was more resistant to heat treatment at 100°C in 3% acetic acid and more sensitive to 5% 2-mercaptoethanol treatment than the agglutination substance of the other mating type in these yeasts. On the basis of these results together with the pheromone response and production, we expect that almost all ascosporogenous yeasts can be classified into the two mating types corresponding to a and mating types in Saccharomyces cerevisiae, respectively.     

Sex-specific growth responses in yeasts

Growing cells of complementary mating types from two genera of yeasts,Saccharomyces andHansenula, exhibited sex-specific responses. Two kinds of responses were observed. Either one of the sexes grew preferentially toward its mate which ceased to bud and increased in size, or both mating types grew preferentially toward each other. A growth response was observed between mating types ofSaccharomyces cerevisiae andHansenula anomala presumably complementary.     

Cell-cell recognition and pheromone response of the yeast Saccharomyces globosus

Sexual agglutination and pheromone interaction between cells of two mating types, a and alpha, in the yeast Saccharomyces globosus were studied. S. globosus was shown to produce mating-type-specific factors analogs to a- and alpha-mating pheromones of Saccharomyces cerevisiae and to undergo the sexual agglutination reaction between cells of two mating types. While the sexual agglutination of cells of different species was not observed, mating type a cells of each species were shown to respond to alpha-factors produced by the other species. Thus, the mating response of S. globosus was shown to be identical to what has been observed in two other species of the same genera: S. cerevisiae and Saccharomyces kluyveri.     

Multiple allelic incompatibility factors among bifactorial strains of the yeastLeucosporidium (Candida) scottii

A biallelic, unifactorial, sexual incompatibility system, consisting of two mating types, is characteristic of the ascomycetous yeasts, such asSaccharomyces cerevisiae. In addition to the biallelic system, the basidiomycetous yeasts,Rhodosporidium andLeucosporidium have a multiple allelic bifactorial system with 2 A and 2B factors for a total of four mating types. This report presents evidence for an increase in this number of mating types inLeucosporidium scottii due to the presence of a minimum of 5 A and 3 B specificities with linked loci at each factor. These results indicate that sexual incompatibility system in basidiomycetous yeasts is similar to that found in other basidiomycetes such asSchizophyllum commune.     

Some properties ofSaccharomyces kluyveri

Four isolates of aSaccharomyces species which differed fromS. kluyveri by their ability to use cellobiose were analyzed genetically in relation to the latter species. Isolated single spores had low viability. Spore tetrads segregated mating types 2 2, with sexual agglutination occurring between complementary mating types. All single-spore isolates assimilated cellobiose indicating that these isolates were not naturally occurring hybrids betweenS. kluyveri and a cellobiose assimilatingSaccharomyces species.Two cell types were exhibited by single-spore cultures ofS. kluyveri, one granulated (G-type) and one vacuolated (g-type). G-type cultures formed fertile hybrids with complementary mating types of both G- and g-type cultures. Hybrids between two g-type cultures were sterile. They would, however, give fertile hybrids when mixed with G-type cultures.Sporulating hybrids betweenSaccharomyces sp. andS. kluyveri were produced. However the percentage spore germination was low. Single-spore cultures examined had cell types atypical of either parent. The ability to assimilate cellobiose was dominant and appeared to segregate with mating type and cell type.Weak mating reactions occurred when the (+) and (-) mating types ofSaccharomyces sp. were mixed with (a) and () mating types ofS. cerevisiae, respectively.The species ofSaccharomyces isolated from the Pacific Coast are designated as strains ofS. kluyveri.     

Sex pheromones of the yeast Hansenula wingei and their relationship to sex pheromones in Saccharomyces cerevisiae and Saccharomyces kluyveri

The yeast, Hansenula wingei has two mating types designated 5 and 21. Cells of each mating type were found to produce mating type-specific sex pheromone which induces sexual agglutinability of the opposite mating type. Crude fractions of these pheromones were prepared by using an Amberlite CG 50 (H⁺ type) column. The agglutinability-inducing action of the pheromones required glucose as carbon source, but no external nitrogen source. The action of the pheromones was inhibited by 5 g/ml cycloheximide. The optimum pH for the pheromone action was 4.0. Pheromones of Saccharomyces cerevisiae and Saccharomyces kluyveri induced sexual agglutinability of 5 mating type cells but did not that of 21 mating type cells. a Pheromones of the Saccharomyces yeasts had no effect on both 5 and 21 mating type cells. The sex pheromones of H. wingei had no effect on the sexual agglutinability of inducible a cells of S. cerevisiae. From the experimental results obtained so far, we propose to call 5 and 21 mating types in H. wingei a and mating types, respectively.     

Sex ratio and gamete size across eastern North America in Dictyostelium discoideum,a social amoeba with three sexes

Theory indicates that numbers of mating types should tend towards infinity or remain at two. The social amoeba, Dictyostelium discoideum, however, has three mating types. It is therefore a mystery how this species has broken the threshold of two mating types, but has not increased towards a much higher number. Frequency‐dependent selection on rare types in combination with isogamy, a form of reproduction involving gametes similar in size, could explain the evolution of multiple mating types in this system. Other factors, such as drift, may be preventing the evolution of more than three. We first looked for evidence of isogamy by measuring gamete size associated with each type. We found no evidence of size dissimilarities between gametes. We then looked for evidence of balancing selection, by examining mating type distributions in natural populations and comparing genetic differentiation at the mating type locus to that at more neutral loci. We found that mating type frequency varied among the three populations we examined, with only one of the three showing an even sex ratio, which does not support balancing selection. However, we found more population structure at neutral loci than the mating type locus, suggesting that the three mating types are indeed maintained at intermediate frequencies by balancing selection. Overall, the data are consistent with balancing selection acting on D. discoideum mating types, but with a sufficiently weak rare sex advantage to allow for drift, a potential explanation for why these amoebae have only three mating types.     

A SEX DETERMINING MECHANISM IN THE CLOSTERIUM EHRENBERGII (CHLOROPHYTA) SPECIES COMPLEX1

Homozygous mt^?/mt^? diploid clones of the Closterium ehrenbergii Menegh. ex Ralfs species complex were obtained by hypertonic treatment from minus vegetative cells, and mating type segregation ratios in the F₁ progeny of “triploid” zygospores between wild type mt⁺ haploid and mt^?/mt^? homozygous diploui were analyzed. The ratio of plus to minus individuals was 1:4.8, and the ratio of the pairs of opposite mating types to those of minus mating type was 1:2.1. The results clearly show that mt^? is dominant to mt⁺ and that the mating type inheritance in these zygospores follows the triploid-like pattern. The validity of our assumption that the two mating types are determined by one genetic factor (mt^? allele dominant) was confirmed in B₁ progeny analyses as well. The results suggest that this sex determining mechanism is working effectively in the C. ehrenbergii species complex, in which several biological species have evolved through polyploidization.     

Sex in a cyclical parthenogen: mating behaviour of Chydorus sphaericus (Crustacea; Branchiopoda; Anomopoda)

1. We describe the interactions during mating in Chydorus sphaericus, a cyclical parthenogenetic anomopod. Mating behaviour is more complex than previously assumed, with evidence for a diffusible chemical to which males react at the onset of mating, for reproductive isolation, and for postcopulatory mate guarding. 2. During mating, the male and female form a ‘mating cross’ that may be maintained for several hours, while copulation itself typically lasts less than a minute. Furthermore, males invariably attach to the right valve of females. Copulation involves intromission of the postabdomen between the valves, so that the gonopores approach the left ovarium. 3. This behaviour is reflected in the morphology of both sexes: males have a specialised anterior valve margin, postabdomen, first limb and rostrum, under selective pressure for successful mate guarding and copulation, while gamogenetic females have asymmetric ovaries, and a species‐specific setulation of the valves. Males of the structurally related Chydorus ovalis react to the presence of C. sphaericus, but fail to dock to females, suggesting a lock‐antilock element in the reproductive isolation of both species. 4. The morphological and ethological adaptations in C. sphaericus suggest that there is a strong selective pressure on mating behaviour in this cyclical parthenogen and specifically towards the formation of the ‘mating cross’.     

Mate choice assays and mating propensity differences in natural yeast populations

In sexual microbes, mating occurs by fusion of individual cells. This complete fitness investment suggests that cell behaviour could potentially mediate prezygotic isolation between microbial species, a topic about which very little is known. To investigate this possibility, we conducted individual cell mate choice trials and mass-culture mating propensity assays with isolates from sympatric natural populations of the closely related yeasts Saccharomyces cerevisiae and Saccharomyces paradoxus. Although we found no evidence for active species recognition in mate choice, we observed a marked difference in mating propensity between these two species. We briefly discuss the possibility that this mating propensity difference may contribute to reproductive isolation between S. cerevisiae and S. paradoxus in nature.     

The genusSaccharomyces (Meyen) Reess

Twenty-nine different strains of several fermentation type II species of the genusSaccharomyces, whose main common characteristic was complete fermentation of raffinose, were studied. These can be regarded as the most highly developed species of the genusSaccharomyces. Their common and differential morphological, physiological, biochemical and serological characteristics were studied. In addition to the usual criteria, factors which played a role in the history and formation of these species were taken into account. Large species were compared with typical strains chosen on the basis of statistical typing, e.g.Saccharomyces carlsbergensis Hansen, and small species were compared with type strains, e.g.Saccharomyces logos van Laer et Den. andSaccharomyces uvarum Beijer, as nomenelatural types. It was found that the whole group could be divided into two categories, one grouped roundSaccharomyces carlsbergensis Hansen and the other roundSaccharomyces uvarum Beijer., which overlapped at the place occupied bySaccharomyces logos van Laer et Den. Another group of intermediate strains isolated from grapes came betweenSaccharomyces logos andSaccharomyces uvarum. In future, closer attention will have to be paid to strains which do not give a serological reaction with serum against the typical speciesSaccharomyces carlsbergensis, i. e. to the speciesSaccharomyces monacensis Hansen andSaccharomyces mandshuricus Saito.     

Uneven Distribution of Mating‐Type Alleles Among Togninia minima Isolates,One of the Causal Agents of Leaf Stripe Disease on Grapevines in Northwest Iran

Togninia minima is the main fungal species associated with grapevine leaf stripe disease worldwide. This species is mainly known from its asexual state in nature; nevertheless, a biallelic heterothallic mating strategy has been confirmed for this species based on in vitro crossing studies. There are no data available on the incidence of an active sexual cycle within the populations of this species in many grapevine‐producing countries as well as Iran. The possibility of a clandestine sexual cycle within the Iranian isolates of T. minima was evaluated by analysing the distribution and frequency of the mating‐type alleles on a microspatial and a macrogeographical scales. Towards this aim, a total of 90 T. minima isolates were recovered from grapevines with esca disease from the vineyards in north and north‐western Iran. A multiplex PCR method previously designed by authors was applied for simultaneous identification and determination of the mating‐type alleles in T. minima populations. The results on the screening of mating‐type alleles using multiplex PCR method revealed the mating‐type identity of 77 isolates as Mat1‐2 and 23 isolates as Mat1‐1. Our results showed that both Mat1‐1 and Mat1‐2 isolates are present in a single vineyard and even on single vines. The distribution of mating‐type alleles in the sampled area skewed from the 1 : 1 ratio (77 : 23); however, co‐occurrence of both mating types in a single vineyard and even on single vines is suggestive for the presence of an active sexual cycle for T. minima in north‐western Iran.     

Greater Mating Success of Drosophila biarmipes Males Possessing an Apical Dark Black Wing Patch

Drosophila biarmipes males possess an apical black patch on their wings. In a laboratory stock of this species, males without such a patch have been found. Mating success of these two types of male was studied in an Elcns-Wattiaux mating chamber. Results show that males possessing the dark black patch have greater mating success than those without. This finding suggests that visual stimulus plays an important role in the mating behaviour of D. biarmipes.     

Facultative pupal mating in Heliconius erato: Implications for mate choice,female preference,and speciation

Mating systems have broad impacts on how sexual selection and mate choice operate within a species, but studies of mating behavior in the laboratory may not reflect how these processes occur in the wild. Here, we examined the mating behavior of the neotropical butterfly Heliconius erato in the field by releasing larvae and virgin females and observing how they mated. H. erato is considered a pupal‐mating species (i.e., males mate with females as they emerge from the pupal case). However, we observed only two teneral mating events, and experimentally released virgins were almost all mated upon recapture. Our study confirms the presence of some pupal‐mating behavior in H. erato, but suggests that adult mating is likely the prevalent mating strategy in this species. These findings have important implications for the role of color pattern and female mate choice in the generation of reproductive isolation in this diverse genus.     

CHLAMYDOMONAS SMITHII SP. NOV.–A CHLAMYDOMONAD INTERFERTILE WITH CHLAMYDOMONAS REINHARDTIP

Chlamyclomonas smithii sp. nov. is described from two strains of Chlamyclomonas originally isolated by Dr. Gilbert M. Smith. The mating types are inter-fertile with those of Chlamyclomonas reinhardtii (Culture No. 89 and 90) maintained in the Culture Collection of Algae at Indiana University. This species is distinguished from all others by the body shape and slit chromatophore.     

Yeast two- and three-species hybrids and high-sugar fermentation

The dominating strains of most sugar-based natural and industrial fermentations either belong to Saccharomyces cerevisiae and Saccharomyces uvarum or are their chimeric derivatives. Osmotolerance is an essential trait of these strains for industrial applications in which typically high concentrations of sugars are used. As the ability of the cells to cope with the hyperosmotic stress is under polygenic control, significant improvement can be expected from concerted modification of the activity of multiple genes or from creating new genomes harbouring positive alleles of strains of two or more species. In this review, the application of the methods of intergeneric and interspecies hybridization to fitness improvement of strains used under high-sugar fermentation conditions is discussed. By protoplast fusion and heterospecific mating, hybrids can be obtained that outperform the parental strains in certain technological parameters including osmotolerance. Spontaneous postzygotic genome evolution during mitotic propagation (GARMi) and meiosis after the breakdown of the sterility barrier by loss of MAT heterozygosity (GARMe) can be exploited for further improvement. Both processes result in derivatives of chimeric genomes, some of which can be superior both to the parental strains and to the hybrid. Three-species hybridization represents further perspectives.     

Biology of Euplotes focardii,an Antarctic ciliate

Euplotes focardii, a ciliate species recently collected from sand sediments of Terra Nova Bay (Ross Sea, Antarctica) reproduced in the laboratory with a duplication time of approximately 72 h, at 4°C. Strains representative of two different mating types were identified and mixed together to produce mating pairs. These showed traits rather unusual for Euplotes species. The two pair members remained united for at least 8–10 days. However, only one carried out fertilization and was able to give rise to a new clone of vegetative cells; the other underwent cell body shrinking after 4–5 days of union, lost the locomotory ciliary apparatus, and eventually died. By analyses of mating pairs formed in mixtures of cell samples cytologically distinct from each other, it was ascertained that the different cell behavior is strain-specific.     

MOLECULAR DELINEATION OF SPECIES AND SYNGENS IN VOLVOCACEAN GREEN ALGAE (CHLOROPHYTA)1

Two species of the colonial green flagellate family Volvocaceae are worldwide in distribution yet exhibit contrasting species structure. Geographically disparate isolates of Gonium pectorale Mueller can interbreed while isolates of Pandorina morum Bory behave quite differently. More than 20 sexually isolated subpopulations occur within this species; these have been termed “syngens” (sensu Sonneborn). Because prezygotic barriers to mating cause intersyngen pairings to fail, breeding analyses cannot be used to estimate genetic relatedness among the syngens of P. morum. DNA comparisons provide an alternative method of assessing genetic relatedness. We compared the nucleotide sequence of the internal transcribed spacer (ITS) region of the nuclear ribosomal repeat among clones of P. morum and of G. pectorale. Members of syngens of P. morum with distribution restricted to one small geographical area show great similarity. Likewise, members of any syngen of worldwide distribution show near uniformity, even those from different continents. However, the ITS sequence of each syngen differs from that of other syngens. In contrast, G. pectorale, which has an ITS region that is remarkably uniform throughout the world, appears to consist of a single syngen within North America and Europe by mating tests. The molecular data are in complete conformity with previous syngen assignment. Because the latter is based on mating affinity, with two complementary mating types per syngen, the evolution of new mating type pairs appears to be the basis of microevolution in these algae. We infer that either P. morum is a more ancient species than G. pectorale or that P. morum has a less stable genome. In either case, the biogeographic distribution of certain syngens may reflect climatological changes of the past.     

Assortative mating and differential male mating success in an ash hybrid zone population

Background  

The structure and evolution of hybrid zones depend mainly on the relative importance of dispersal and local adaptation, and on the strength of assortative mating. Here, we study the influence of dispersal, temporal isolation, variability in phenotypic traits and parasite attacks on the male mating success of two parental species and hybrids by real-time pollen flow analysis. We focus on a hybrid zone population between the two closely related ash species Fraxinus excelsior L. (common ash) and F. angustifolia Vahl (narrow-leaved ash), which is composed of individuals of the two species and several hybrid types. This population is structured by flowering time: the F. excelsior individuals flower later than the F. angustifolia individuals, and the hybrid types flower in-between. Hybrids are scattered throughout the population, suggesting favorable conditions for their local adaptation. We estimate jointly the best-fitting dispersal kernel, the differences in male fecundity due to variation in phenotypic traits and level of parasite attack, and the strength of assortative mating due to differences in flowering phenology. In addition, we assess the effect of accounting for genotyping error on these estimations.