Diploid Forms Research Articles

Hybridity and autopolyploidy increase the expressivity of apospory in diploid Paspalum rufum

Paspalum rufum Nees is a perennial grass that forms a multiploid complex. Natural populations are composed mainly of diploid (2n = 2x = 20) and tetraploid (2n = 4x = 40) cytotypes. The diploid form is sexual and highly self-sterile, while the tetraploid is pseudogamous aposporous apomict and self-fertile. Diploids can also develop aposporous sacs, and some of them complete apomixis. The objective of this work was to analyse apospory expressivity (a gametophytic apomixis component) in diploid hybrids and colchicine-induced autotetraploids of the species. One F1 family was created by crossing two diploid individuals (R6#45 × R5#49) carrying aposporous sacs in 5.8 and 13 % of their ovules, respectively. Moreover, two synthetic autotetraploids were obtained by colchicine treatment of mature seeds from R6#45. The hybrid origin of the F1s was confirmed by segregation analyses of a morphological trait and molecular markers, and the ploidy level of experimental plants was determined by flow cytometry. Apospory expressivity was estimated by embryo sacs observation at anthesis. Out of the 39 hybrids analysed, 38 showed aposporous embryo sacs. Expressivity of the trait ranged from 0 to 36 %, and some individuals differed significantly for both progenitors’ values. Both doubled-diploid plants showed 25 and 32 % of apospory expressivity, which was significantly higher than that observed in R6#45. Results presented in this work revealed a high variability in apospory expressivity of diploid hybrids and suggested that more than one allele are controlling the trait. Moreover, the new induced doubled-diploid plants showed that apospory expressivity is highly ploidy dependent.

Autotriploid origin of Carassius auratus as revealed by chromosomal locus analysis.

In the Dongting water system, the Carassius auratus (Crucian carp) complex is characterized by the coexistence of diploid forms (2n=100, 2nCC) and polyploid forms. Chromosomal and karyotypic analyses have suggested that the polyploid C. auratus has a triploid (3n=150, 3nCC) and a tetraploid origin (4n=200), respectively. However, there is a lack of direct genetic evidence to support this conclusion. In this paper, analysis of the 5S rDNA chromosomal locus revealed that the 3nCC is of triploid origin. Analysis of the species-specific chromosomal centromere locus revealed that 3nCC individuals possess three sets of C. auratus-derived chromosomes. Our results provide direct cytogenetic evidence suggesting that individuals with 150 chromosomes are of autotriploid origin within the C. auratus complex. It marks an important contribution to the study of polyploidization and the evolution of vertebrates.

Morphological and cytological characters for defining Stevia ‘Honeyleaf’ tetraploids

Purpose. Determination of peculiarities of anatomical and morphological characters of stevia plants to accelerate the identification of tetraploid forms. Methods. Field, laboratory and analysis-of-variance methods. Results. It was established that larger leaf area index (19–38%), fewer stomata per unit area (by 39–48%), larger diameter of pollen grains (by 42–92%) is a characteristic feature of anatomical and morphological characters of stevia tetraploid forms as compared to diploid ones. Conclusions. The parameters of leaf plate, the size of stomata and diameter of pollen grains are auxiliary selection criteria (rapid test). For tetraploid forms of stevia, leaf area index (the ratio of leaf blade length to width), size of stomata and diameter of pollen grains is truly larger as compared to diploid forms. Chromosomes number determination based on cytological analysis is the major and the most accurate criterion for selection of tetraploid forms, that seems advisable to use it at later stages of selection.

Temporal fluctuations in the environment and intra-specific polymorphism: A model simulating the flowering phenology of gorse (Ulex europaeus)

The onset and conservation of genetic polymorphism is a major question in evolutionary ecology. The influence of temporal fluctuations in the environment was invoked by early theorists such as J.B.S. Haldane and S. Jayakar in a controversial article published in, 1963, but their frequently cited model has almost never been used with empirical evidence. In this paper, we present a simulation model inspired by the biology of common gorse (Ulex europaeus), a species which shows polymorphism of flowering phenology: long flowering plants produce flowers from winter to spring and short flowering plants only flower in the spring. The early fruits of the former run the risk of frost, but largely escape seed predation, while those of the latter escape the risk of frost but are subject to a strong risk of seed predation. These two selection pressures vary unpredictably from year to year, making this flowering phenotype a good candidate to test Haldane and Jayakar’s model. Assuming that both flowering types are determined by a single major locus, we devise a simulation model firstly in a diploid form, and secondly by taking into account the hexaploid characteristic of gorse. Our results show that the combination of the two selective pressures acting on gorse flowering phenology can lead to fitness values meeting the Haldane and Jayakar’s conditions on geometric and arithmetic means, and to long term maintenance of polymorphism. In addition, the values of the parameters allowing polymorphism persistence and the relative proportions obtained are in agreement with values observed in natural populations. We also show that hexaploidy strongly increases the range of parameters in which polymorphism is self sustaining. These results are discussed in the context of climatic change, where increases of both mean temperature and its variance are predicted.

Multiple lines of evidence clarify limits of Allocasuarina thalassoscopica, A. defungens and A. littoralis (Allocasuarina sect. Cylindropitys, Casuarinaceae)

Uncertainty in identifying Allocasuarina shrubs of the North Coast botanical region of New South Wales and south-eastern Queensland prompted investigation of species limits using morphological and cytological data. PATN analysis using 36 morphometric characters, seedling morphology and cytogenetic analysis supported the existing circumscription of A. defungens and A. simulans. Trees of A. littoralis were observed throughout the study area and the distinct morphology of the diploid and polyploid forms is noted. Further work is required to establish whether these differences are consistent over the full range of the species. Shrubby plants of A. sect. Cylindropitys, other than the above species, growing along the NSW North Coast are currently known by several names, including A. defungens×A. littoralis. These plants were found to be morphologically most similar to A. emuina and A. thalassoscopica from south-eastern Queensland and to have similar phenotypes. Allocasuarina thalassoscopica is recircumscribed on morphological characters to encompass these shrubby plants in New South Wales and A. emuina and A. thalassoscopica from south-eastern Queensland. A key is provided for identification of plants of A. sect. Cylindropitys. Counts of chromosomes found that recircumscribed A. thalassoscopica is tetraploid and A. defungens is triploid at Nabiac but tetraploid at Khappinghat Nature Reserve. The results of the present study are consistent with a hypothesis of parallel evolution, which is possibly due to edaphic factors and requires further study.

Chromosomal Characterization of the Three Subgenomes in the Polyploids of Hordeum murinum L.: New Insight into the Evolution of This Complex

Hordeum murinum L. is a species complex composed of related taxa, including the subspecies glaucum, murinum and leporinum. However, the phylogenetic relationships between the different taxa and their cytotypes, and the origin of the polyploid forms, remain points of controversy. The present work reports a comparative karyotype analysis of seven accessions of the H. murinum complex representing all subspecies and cytotypes. The karyotypes were determined by examining the distribution of the repetitive Triticeae DNA sequences pTa71, pTa794, pSc119.2, pAs1 and pHch950, the simple sequence repeats (SSRs) (AG)10, (AAC)5, (AAG)5, (ACT)5, (ATC)5, and (CCCTAAA)3 via in situ hybridization. The chromosomes of the three subgenomes involved in the polyploids were identified. All tetraploids of all subspecies shared the same two subgenomes (thus suggesting them to in fact belong to the same taxon), the result of hybridization between two diploid ancestors. One of the subgenomes present in all tetraploids of all subspecies was found to be very similar (though not identical) to the chromosome complement of the diploid glaucum. The hexaploid form of leporinum came about through a cross between a tetraploid and a third diploid form. Exclusively bivalent associations among homologous chromosomes were observed when analyzing pollen mother cells of tetraploid taxa. In conclusion, the present results identify all the individual chromosomes within the H. murinum complex, reveal its genome structure and phylogeny, and explain the appearance of the different cytotypes. Three cryptic species are proposed according to ploidy level that may deserve full taxonomic recognition.

Estimation of resistance of breeding numbers of sugar beet to leaf diseases

The article deals with the resistance of CMS hybrids, tetraploid and diploid forms of sugar beet to cercosporosis and erysiphosis. There was determined that diploid materials are less resistant than tetraploid ones, and variability in diseases degree was estimated during several years. Perspective materials for hybrids which can be used in bio-adaptive technology were selected.

Parasexuality and Ploidy Change in Candida tropicalis

Candida species exhibit a variety of ploidy states and modes of sexual reproduction. Most species possess the requisite genes for sexual reproduction, recombination, and meiosis, yet only a few have been reported to undergo a complete sexual cycle including mating and sporulation. Candida albicans, the most studied Candida species and a prevalent human fungal pathogen, completes its sexual cycle via a parasexual process of concerted chromosome loss rather than a conventional meiosis. In this study, we examine ploidy changes in Candida tropicalis, a closely related species to C. albicans that was recently revealed to undergo sexual mating. C. tropicalis diploid cells mate to form tetraploid cells, and we show that these can be induced to undergo chromosome loss to regenerate diploid forms by growth on sorbose medium. The diploid products are themselves mating competent, thereby establishing a parasexual cycle in this species for the first time. Extended incubation (>120 generations) of C. tropicalis tetraploid cells under rich culture conditions also resulted in instability of the tetraploid form and a gradual reduction in ploidy back to the diploid state. The fitness levels of C. tropicalis diploid and tetraploid cells were compared, and diploid cells exhibited increased fitness relative to tetraploid cells in vitro, despite diploid and tetraploid cells having similar doubling times. Collectively, these experiments demonstrate distinct pathways by which a parasexual cycle can occur in C. tropicalis and indicate that nonmeiotic mechanisms drive ploidy changes in this prevalent human pathogen.

Analysis of Chromosome Number in Some Allium and Silene Wild Species with Ornamental Use

The present study analyses the number of somatic chromosomes in plant species with ornamental value, in Romanian indigenous flora, Allium (A. flavum L., A. saxatile Bieb.) and Silene (S. compacta Fischer., S. supina M.Bieb.). The biological material was identified and harvested in the South-Eastern part of Dobrogea (Tulcea and Constanţa counties), area in the South-Eastern part of Romania, situated between the Danube and the Black Sea. Individuals from two populations of Allium flavum and Allium saxatile, respectively from Tulcea county (Turcoaia town) and Constanţa county (from Cheile Dobrogei) were analyzed. In the case of Silene compacta and Silene supina, plants of one population in Tulcea county, Turcoaia town, were used. The aim of the analysis was the quantification and comparison of the somatic chromosomes of plants from the same species but different populations, as is the case of the two Allium species, but also to relate them to results from specialty literature, reported by others. The chromosome number was determined in the roots of the small plants that were obtained from seeds collected in their natural habitat. Except the case of Silene supina species, where only diploid forms were identified (2n=24), when in specific literature tetraploid forms are mentioned (2n=48), found in other areas, all other analyzed taxa register similar data to that reported in other geographic regions. Respectively, 2n= 16 in the case of Allium flavum and Allium saxatile and 2n= 24 in the case of Silene compacta.

Geographical distribution of cytotypes in the Chrysanthemum indicum complex as evidenced by ploidy level and genome‐size variation

AbstractA detailed knowledge of the geographical distribution of cytotypes within and between species comprising a polyploid complex is critical to our understanding of the history and evolution of such complexes. In the present study we examined the geographical distributions of cytotypes within six tentatively delimited species comprising the Chrysanthemum indicum complex in China. We determined the ploidy of 188 individuals sampled from 47 populations, based on DNA content using flow cytometry. In addition, chromosome counts were made on samples of each taxon. We confirmed that all samples of C. rhombifolium and C. lavandulifolium were diploid (2n = 18), those of C. hypargyrum and C. potentilloides were tetraploid (2n = 36), and those of C. vestitum were hexaploid (2n = 54). In contrast, we confirmed that C. indicum contained both diploid and tetraploid cytotypes. We found that in addition to marked differences in genome size between ploidy levels, there was a variation in genome size between species of the same ploidy level. Although the diploid, tetraploid, and hexaploid taxa of the complex, as well as the diploid form of C. indicum, occurred only in central and northern China, the tetraploid form of C. indicum was widespread both north and south of the Yangtze River. We suggest that the tetraploid form of C. indicum may have expanded its range southward during recent Quaternary glacial periods when forests retreated in south China as conditions became drier.

The ‘obligate diploid’ Candida albicans forms mating-competent haploids

Candida albicans, the most prevalent human fungal pathogen, is considered to be an obligate diploid that carries recessive lethal mutations throughout the genome. Here we demonstrate that C. albicans has a viable haploid state that can be derived from diploid cells under in vitro and in vivo conditions, and that seems to arise through a concerted chromosome loss mechanism. Haploids undergo morphogenetic changes like those of diploids, including the yeast-hyphal transition, chlamydospore formation and a white-opaque switch that facilitates mating. Haploid opaque cells of opposite mating type mate efficiently to regenerate the diploid form, restoring heterozygosity and fitness. Homozygous diploids arise spontaneously by auto-diploidization, and both haploids and auto-diploids show a similar reduction in fitness, in vitro and in vivo, relative to heterozygous diploids, indicating that homozygous cell types are transient in mixed populations. Finally, we constructed stable haploid strains with multiple auxotrophies that will facilitate molecular and genetic analyses of this important pathogen.

Width and surface of leaf blades of Lolium multiflorum Lam.

Differences in the shape of ribs can be observed in transverse sections of leaf blades of Italian ryegrass (<i>Lolium multiflorum</i> Lam.) var. 'Szelejewska', a diploid, has ribs similar to triangles, and 'Kroto', a tetraploid, has ribs which are more rounded, dome-like. A coefficient expressing width and surface increase of the leaf blade in relation to unribbed leaves has been determined. This coefficient (W) was calculated from the ratio of upper, wavy part of the leaf blade (g), to its width W=g/l. For Italian ryegrass the coefficient is from 1.5-1.46 and thus the width and the surface of the upper leaf blade is increased on the average by about 30%. With basic fertilization (1N) this coefficient is smaller for the diploid form as compared to the tetraploid. An increased dose of nitrogen (3N) causes increase of the coefficient in the diploid and a decrease in the tetraploid. Thus the joint action of the variety and fertilization in respect to the examined character is not equally significant to the two varieties.

High male incidence and evolutionary implications of triploid form in northeast Asia Carassius auratus complex

Carassius auratus complex is believed to originate from East Eurasia and consist of diploid and triploid forms. Diploid form reproduces sexually, whereas triploid form possesses mixture modes of unisexual gynogenesis and sexual reproduction, which makes it a unique case to study evolutionary issues among vertebrates. In this study, we identified 337 triploid individuals from 386 specimens of Carassius auratus complex sampled from 4 different sites of Xingkai Lake and Suifen River on the northeast Asia transboundary areas of Russia and China, and found that triploids were ubiquitous, whereas diploids existed only in SII site of Suifen River. Triploid males were detected in all surveyed sites, and an unusually high triploid male incidence (23%) was found in the Chinese reach of Suifen River. Then, nuclear and cytoplasmic markers were used to analyze their genetic diversity and phylogenetic relationship. A total of 61 distinct tf alleles and 35 mtDNA CR haplotypes were revealed. Higher genetic diversity and divergence were confirmed in triploids than in diploids, and identical genetic background between triploid males and females was demonstrated. Moreover, evolutionary implications and roles of triploid males were suggested in population proliferation and diversity creation of the triploid form.

Interspecific hybridization in the genus Paeonia (Paeoniaceae): Polymorphic sites in transcribed spacers of the 45S rRNA genes as indicators of natural and artificial peony hybrids

The ITS1-5.8S rDNA-ITS2 regions of 33 accessions belonging to 16 species and five natural and garden interspecific hybrids of the genus Paeonia L. were sequenced. Chromatograms of the peony hybrids demonstrated the presence of the signals, corresponding to two different nucleotides at the positions differing in the parents, indicating that in the hybrids, no rDNA isogenization usually occurred, and they preserved rDNA of both parents. Analysis of these polymorphic sites (PS) showed that P. x majkoae was interspecific hybrid between P. tenuifolia and P. caucasica. The ITS of P. hybrida differs from ITS of P. x majkoae in 19 mutations. Because of this, P. x majkoae is definitely not synonymous to P. hybrida. Comparative analysis of ITS 1-5.8S rDNA-ITS2 showed that species diversity in section Paeonia was based on recombination as a result of intraspecific hybridization of three haplotype families. Specifically, haplotypes A, typical of the P. tenuifolia and P. anomala genomes, haplotypes B, typical of P. mlokosewitschii and P. obovata, and haplotypes of family C, currently represented in rDNA of diploid and tetraploid forms of some Caucasian and Mediterranean species. The ITS regions many diploid peonies contain no dimorphic sites, while P. oreogeton, P. cambessedesii, P. rhodia, and P. daurica carry from ten to 17 PS, and supposed to be the interspecific hybrids. Most of the tetraploid peonies contain from six to 18 PS in the ITS regions. These are alloploids with one of the parental genomes similar to that of P. mlokosewitschii (B1), or P. obovata (B3). The second parental genome in P. banatica, P. peregrina, and P. russii is represented by the genome, close to that of P. tenuifolia (A). P. macrophylla, P. mascula, P. coriacea, P. wittmanniana, and P. tomentosa carry genome of series B and genome of series C, which slightly resembles genome A.

Engineering properties of red clover (Trifolium pratense L.) seeds

Selected engineering properties of seeds of four red clover varieties were studied. The seeds of diploid and tetraploid forms were statistically different in terms of true and bulk density, porosity, volume, 1000 seed weight, equivalent diameter and yellowness index. The volume, 1000 seed weight and equivalent diameter were found to be statistically higher for tetraploid than for diploid forms. The accuracy of varietal classification based on the physical properties of seeds was found to be most satisfactory. Varieties of Bona and Jubilatka were identified with 100% accuracy based on the physical properties of seeds. Bona was the most conducive variety for the classification of red clover seeds based on four groups of engineering properties. The relationships between groups of properties described by total redundancy did not exceed 50% for any of variable pairs.

Allopolyploid speciation of Calypogeia sphagnicola (Jungermanniopsida, Calypogeiaceae) based on isozyme and DNA markers

Calypogeia sphagnicola is one of nine species of the genus Calypogeia known in Europe. Occurrence of the species is closely connected with peat bogs. Nowadays, two forms of this species are distinguished—C. sphagnicola f. sphagnicola and C. sphagnicola f. paludosa. The results of the present study, based on two classes of markers—isozymes and sequences of chloroplast genom (trnH-psbA, rpoC1)—unanimously support the genetic differentiation within the taxon and show that the present-day forms represent genetically distinct species. Phylogenetic analysis resolved two lineages that correspond with the present-day forms with high bootstrap support, which differ in ploidy level: C. sphagnicola f. sphagnicola is haploid, whereas C. sphagnicola f. paludosa is a diploid form. Allopolyploid origin of the diploid form was revealed by the isozyme pattern. Nei’s genetic distance between the two present-day forms of C. sphagnicola was 0.472. The forms in Poland have an allopatric pattern of geographic distribution: C. sphagnicola f. sphagnicola occurs exclusively in the lowlands of the northern part of the country on raised peat bogs, whereas C. sphagnicola f. paludosa is found only in the mountains of southern Poland, mainly in the subalpine zone, where it grows on Sphagnum-Polytrichum hummocks on the upper part of north-facing slopes. Plants regarded in this study as C. sphagnicola f. sphagnicola morphologically correspond to the syntype specimen of C. sphagnicola.

Prolamin storage proteins and alloploidy in wild populations of the small grass Brachypodium distachyon (L.) P. Beauv.

This paper reports the glutenin diversity of a collection of 23 wild populations of the grass Brachypodium distachyon collected in the Mediterranean and southern areas of the Iberian Peninsula. The plant material studied included the three different cytotypes of this species: 2n = 10, 2n = 20 and 2n = 30. A specific method of extraction was used to isolate the glutenin subunits from the caryopsis. Separation by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) showed them to correspond to wheat low-molecular-weight glutenin subunits (LMW-GS). Twenty-two LMW-GS–like were identified that showed great diversity within and between populations. All the populations investigated were polymorphic for the endosperm proteins studied. The 2n = 30 forms had the largest number of subunits; these were also more diverse than those of the 2n = 10 or 2n = 20 forms. The 2n = 10 forms, the most common in the higher, interior areas of the Iberian Peninsula, showed the smallest subunit variation. In fact, negative correlations were found between subunit diversity and altitude and longitude. In contrast, a positive correlation was detected with the annual average temperature and the indices of thermicity and Mediterraneity. The similarity between the populations was estimated using the Sorensen–Dice coefficient, calculated on the basis of the presence/absence of the 22 LMW-GS proteins. The similarity indices were used to produce a dendrogram using the unweighted pair-group method with arithmetic means (UPGMA). This method produced three main groups corresponding to the three cytotypes. An analysis was made of the possible correlation between eco-geographic and climatic factors and the gene diversity and polymorphism shown in the populations. The diversity correlated positively with the number of chromosomes (2n), annual mean temperature, index of thermicity and index of Mediterraneity. In contrast, diversity correlated negatively with altitude, longitude and index of precipitation during the summer. The number of chromosomes correlated negatively with altitude, longitude and precipitation during summer and positively with all the other climatic indices. These results are coherent with the fact that diploid forms were more common in areas of the interior and at higher altitude, where the climate is more extreme.

Coexistence of diploid, triploid and tetraploid crucian carp (Carassius auratus) in natural waters

BackgroundCrucian carp (abbreviated CC) belongs to the genus of Carassius within the family of Cyprinidae. It has been one of the most important freshwater species for Chinese aquaculture and is especially abundant in the Dongting water system of Hunan province. CC used to be considered as all diploid forms. However, coexistence of diploid (abbreviated 2nCC), triploid (abbreviated 3nCC) and tetraploid crucian carp (abbreviated 4nCC) population of the Dongting water system was first found by our recently researches.ResultsWe examined the ploidy level and compared biological characteristics in different ploidy CC. In reproductive mode, 2nCC was bisexual generative and 4nCC generated all-female offspring by gynogenesis. However, 3nCC generated progenies in two different ways. 3nCC produced bisexual triploid offspring fertilized with 3nCC spermatozoa, while it produced all-female triploid offspring by gynogenesis when its ova were activated by heterogenous spermatozoa. The complete mitochondrial DNA of three different ploidy fishes was sequenced and analyzed, suggesting no significant differences. Interestingly, microchromosomes were found only in 3nCC, which were concluded to be the result of hybridization. Allogenetic DNA fragments of Sox genes were obtained in 3nCC and 4nCC, which were absent in 2nCC. Phylogenetics analysis based on Sox4 gene indicated 3nCC and 4nCC formed a separate group from 2nCC.ConclusionsIn summary, this is the first report of the co-existence of three types of different ploidy crucian carps in natural waters in China. It was proved that the coexistence of different ploidy CC was reproductively maintained. We further hypothesized that 3nCC and 4nCC were allopolyploids that resulted from hybridization. The different ploidy CC population we obtained in this study possesses great significance for the study of polyploidization and the evolution of vertebrates.

Influence of environmental parameters on growth pattern and population structure of Carassius auratus gibelio in Eastern Ukraine

Population structure and growth parameters of Prussian carp (Carassius auratus gibelio) were studied in 12 freshwater ecosystems of the Donbass region (Eastern Ukraine). These ecosystems differed significantly with respect to their surface area, water transparency and annual concentrations of phosphorus and nitrogen. Amongst the studied ecosystems, diploid males and females as well as a smaller percentage of triploid females were found. The population structure of C. a. gibelio varied significantly in terms of the percentage of triploids and sex ratio amongst diploids. A considerable proportion of triploid females (>10%) was found in four ecosystems with intermediate surface area (38–50 ha) and relatively high growth rate of specimens. The sex ratio amongst diploids was significantly female-biased in seven of 12 ecosystems, including those where triploids were present in considerable numbers. The growth parameters of Prussian carp were significantly correlated with ecosystem characteristics, but the growth patterns of diploids and triploids were not significantly different from each other. The restricted distribution of triploid specimens suggests that the triploid form might counter diverse environmental challenges, whereas the diploid form of Prussian carp seems to be efficiently adapted to a wide range of ecosystem conditions.

Degradation of the Saccharomyces cerevisiae Mating-Type Regulator α1: Genetic Dissection of Cis-determinants and Trans-acting Pathways

Mating phenotype in the yeast Saccharomyces cerevisiae is a dynamic trait, and efficient transitions between alternate haploid cell types allow the organism to access the advantageous diploid form. Mating identity is determined by cell type-specific transcriptional regulators, but these factors must be rapidly removed upon mating-type switching to allow the master regulators of the alternate state to establish a new gene expression program. Targeted proteolysis by the ubiquitin-proteasome system is a commonly employed strategy to quickly disassemble regulatory networks, and yeast use this approach to evoke efficient switching from the alpha to the a phenotype by ensuring the rapid removal of the alpha2 transcriptional repressor. Transition to the a cell phenotype, however, also requires the inactivation of the alpha1 transcriptional activator, but the mechanism by which this occurs is currently unknown. Here, we report a central role for the ubiquitin-proteasome system in alpha1 inactivation. The alpha1 protein is constitutively short lived and targeted for rapid turnover by multiple ubiquitin-conjugation pathways. Intriguingly, the alpha-domain, a conserved region of unknown function, acts as a degradation signal for a pathway defined by the SUMO-targeted ligase Slx5-Slx8, which has also been implicated in the rapid destruction of alpha2. Our observations suggest coordinate regulation in the turnover of two master regulatory transcription factors ensures a rapid mating-type switch.