New record of nucleopolyhedroviruses in tea looper caterpillars in India

Tea production in North-East India hit a record loss due to the widespread severe outbreak of a mixed brood of three species of looper caterpillar pests of geometrid moths (Lepidoptera) in 2008-2010. In addition to Buzura suppressaria, two newly recorded geometrids, viz., Hyposidra infixaria and Hyposidra talaca have caused widespread severe damage in recent years. In the present study we report the nucleopolyhedroviruses (NPV) isolated from the tea looper caterpillar from North-East India. We identified and characterized the NPV by cloning and sequencing a partial segment of polyhedrin gene of virus infected larvae of B. suppressaria, H. talaca and H. infixaria. A comparison of deduced amino acids of polyhedrin gene among H. talaca, H. infixaria and B. suppressaria showed that same strain was found to infect all the three loopers in India, which show high sequence identity with B. suppressaria Chinese isolates. Based on the polyhedrin sequence homology, it is predicted that a variant of B. suppressaria Chinese isolate of NPV found to infect H. talaca, H. infixaria and B. suppressaria in India.     

Evolution of longevity in primates

Evolution of maximum lifespan potential (MLP) and its possible relation to the evolution of extra-brain functions are investigated in the primates. MLP is related to a species' characteristic aging rate and is considered a basic biological property of an organism. MLP may be of more importance than realized in the past in determining the evolutionary success of a species. It is related to the postnatal development rate, the length of time general vigor is maintained, the length of reproductive period, generation time and the time available for learning and teaching behavior. Three other parameters are considered to be importantly related to MLP: MLP calorie consumption (MCC), encephalization quotient (EQ) and extra number of cortical neurons (N_c). MCC is calculated as the product of MLP and specific metabolic rate (SMR) and is considered to represent total “life-capacity” of an organism. It is of potential value in studying the biological mechanisms involved in the evolution of MLP. Brain function is estimated by the Jerison EQ and N_c parameters. These parameters estimate the “extra” brain capacity involved in functions beyond normal body requirements. The rate of change in MLP and N_c per unit time occurring during an ancestral-descendant sequence is used to estimate the biological complexity of the genetic processes which have evolved in governing the rate of expression of the general aging process and increasing brain function. The average rate of change of MLP during the emergence of the primates was analyzed by the difference between MLP in closely related living primate species and the evolutionary time of appearance of a common ancestor. MLP, SMR, MCC, EQ and N_c were estimated from fossil cranial capacity and body weight measurements. The rate of change in these values was calculated according to the time of appearance of the fossil species. MLP and N_c were found to increase together and reached their highest rate of increase approximately 200,000 years ago along the hominid ancestral-descendant sequence leading to modern man. The high rate of increase of these parameters suggests that few genetic changes were responsible. The general increase in MLP during the evolution of the primate species indicates that a corresponding general decrease in mutation rate may have occurred. The high levels of MLP, MCC, EQ and N_c represented in the living primates, as compared to other mammals, are considered to represent a major characteristic determining their evolutionary success.     

Longitudinal TprK profiling of in vivo and in vitro-propagated Treponema pallidum subsp. pallidum reveals accumulation of antigenic variants in absence of immune pressure

Immune evasion by Treponema pallidum subspecies pallidum (T. pallidum) has been attributed to antigenic variation of its putative outer-membrane protein TprK. In TprK, amino acid diversity is confined to seven variable (V) regions, and generation of sequence diversity within the V regions occurs via a non-reciprocal segmental gene conversion mechanism where donor cassettes recombine into the tprK expression site. Although previous studies have shown the significant role of immune selection in driving accumulation of TprK variants, the contribution of baseline gene conversion activity to variant diversity is less clear. Here, combining longitudinal tprK deep sequencing of near clonal Chicago C from immunocompetent and immunosuppressed rabbits along with the newly developed in vitro cultivation system for T. pallidum, we directly characterized TprK alleles in the presence and absence of immune selection. Our data confirm significantly greater sequence diversity over time within the V6 region during syphilis infection in immunocompetent rabbits compared to immunosuppressed rabbits, consistent with previous studies on the role of TprK in evasion of the host immune response. Compared to strains grown in immunocompetent rabbits, strains passaged in vitro displayed low level changes in allele frequencies of TprK variable region sequences similar to that of strains passaged in immunosuppressed rabbits. Notably, we found significantly increased rates of V6 allele generation relative to other variable regions in in vitro cultivated T, pallidum strains, illustrating that the diversity within these hypervariable regions occurs in the complete absence of immune selection. Together, our results demonstrate antigenic variation in T. pallidum can be studied in vitro and occurs even in the complete absence of immune pressure, allowing the T. pallidum population to continuously evade the immune system of the infected host.     

Efficacy of Pseudomonas syringae in the management of potato tuber diseases in storage

Silver scurf caused by Helminthosporium solani and dry rot caused by Fusarium spp. are tuber diseases of economic importance in potato-growing areas worldwide. Recently, the two pathogens have developed resistance to thiabendazole (TBZ), a post-harvest fungicide commonly used for their control. Therefore, alternative disease control strategies are needed. The present study assessed the efficacy of the biopesticides Bio-Save 10LP (Pseudomonas syringae-strain ESC-10; Ps10) and Bio-Save 11LP (P. syringae-strain ESC-11; Ps11) against silver scurf and dry rot. Approximately 30 isolates representing the genus Fusarium were obtained from symptomatic potato specimens with dry rot from New Brunswick (NB), Nova Scotia (NS), Prince Edward Island (PE) and Alberta (AB), Canada. Species isolated were Fusarium sambucinum, Fusarium tumidum, Fusarium coeruleum, Fusarium culmorum, and Fusarium avenaceum. H. solani isolated from AB, NB and PE was included in the study as the causal agent of silver scurf. The efficacy of P. syringae against F. sambucinum and H. solani was tested in vitro. Ps10 and Ps11 inhibited the growth of H. solani up to 68% (NB isolate) and 73% (PE isolate), respectively and the inhibition was more or less comparable with that of TBZ. F. sambucinum was not significantly inhibited by Ps10; however Ps11 significantly inhibited AB, PE and NB isolates by 43%, 28% and 54%, respectively. Conversely, TBZ inhibited AB, PE and NB isolates of Fusarium spp. in vitro by 86%, 88% and 100%, respectively. TBZ in combination with either Ps10 or Ps11 did not always reduce the growth of H. solani or Fusarium spp. in vitro compared to that of TBZ alone. Storage trials conducted in NB and PE assessed the efficacy of P. syringae against H. solani or Fusarium spp. in vivo and confirmed that the application of P. syringae or TBZ alone or in combination significantly reduced the incidence and/or severity of silver scurf and Fusarium dry rot. Ps11 alone or in combination with TBZ was significantly more effective than Ps10 in controlling silver scurf disease severity. The reduction in disease severity of dry rot and silver scurf in storage due to Ps10, Ps11, or TBZ or their combinations was consistently comparable. Results indicate that the use of P. syringae (strains ESC-11 or ESC-10) as a post-harvest treatment can contribute to the management of both silver scurf and Fusarium dry rot in potato storages.     

Role of Antibiosis in Competition of Erwinia Strains in Potato Infection Courts

Erwinia carotovora subsp. betavasculorum strains produced a bactericidal antibiotic in vitro that inhibited a wide spectrum of gram-negative and gram-positive bacteria. The optimum temperature for production was 24°C, and the addition of glycerol to culture media enhanced antibiotic production. Antibiotic production by these strains in the infection court of potato was the principal determinant enabling it to gain ascendancy over competing antibiotic-sensitive Erwinia carotovora subsp. carotovora strains. There was a complete correlation between antibiotic production by E. carotovora subsp. betavasculorum in vitro and inhibition of competing E. carotovora subsp. carotovora strains in planta. Inhibition of the latter by the former was apparent after 10 h of incubation in potato tuber wounds. Population densities of sensitive E. carotovora subsp. carotovora strains in mixed potato tuber infections with E. carotovora subsp. betavasculorum were approximately 10⁶-fold lower after 48 h of incubation than in corresponding single sensitive strain infections. E. carotovora subsp. carotovora were not inhibited in tuber infections that were incubated anaerobically. This correlated with the absence of antibiotic production during anaerobic incubation in vitro. Antibiotic-resistant strains of E. carotovora subsp. carotovora were not inhibited in planta or in vitro by E. carotovora subsp. betavasculorum. Moreover, isogenic antibiotic-negative (Ant⁻) mutant E. carotovora subsp. betavasculorum strains were not inhibitory to sensitive E. carotovora subsp. carotovora strains in tuber infections.     

Comparison of the in vivo and in vitro genotoxicity of glyphosate isopropylamine salt in three different organisms

There is considerable controversy with regard to the genotoxicity of glyphosate, with some reports stating that this compound is non-toxic for fish, birds and mammals. In this work, we used the comet assay to examine the genotoxicity of glyphosate isopropylamine (0.7, 7, 70 and 700 μM) in human lymphocytes, erythrocytes of Oreochromis niloticus and staminal nuclei of Tradescantia (4430) in vitro and in vivo. Cells, nuclei and fish that had and had not been exposed to 5 mM N-nitrosodiethylamine (NDEA) were used as positive and negative controls, respectively. Significant (p < 0.01) genetic damage was observed in vivo and in vitro in all cell types and organisms tested. Human lymphocytes and Tradescantia hairs showed lower genetic damage in vivo compared to in vitro, possibly because of efficient metabolization of the herbicide. In O. niloticus erythrocytes, significant (p < 0.001) genotoxicity was observed at ≥ 7 μM, whereas in vitro, glyphosphate was genotoxic in human lymphocytes and Tradescantia hairs at ≥ 0.7 μM. These results indicate that glyphosate is genotoxic in the cells and organisms studied at concentrations of 0.7–7 μM.     

Role of Tellurite Resistance Operon in Filamentous Growth of Yersinia pestis in Macrophages

Background

Yersinia pestis initiates infection by parasitism of host macrophages. In response to macrophage infections, intracellular Y. pestis can assume a filamentous cellular morphology which may mediate resistance to host cell innate immune responses. We previously observed the expression of Y. pestis tellurite resistance proteins TerD and TerE from the terZABCDE operon during macrophage infections. Others have observed a filamentous response associated with expression of tellurite resistance operon in Escherichia coli exposed to tellurite. Therefore, in this study we examine the potential role of Y. pestis tellurite resistance operon in filamentous cellular morphology during macrophage infections.

Principal Findings

In vitro treatment of Y. pestis culture with sodium tellurite (Na₂TeO₃) caused the bacterial cells to assume a filamentous phenotype similar to the filamentous phenotype observed during macrophage infections. A deletion mutant for genes terZAB abolished the filamentous morphologic response to tellurite exposure or intracellular parasitism, but without affecting tellurite resistance. However, a terZABCDE deletion mutant abolished both filamentous morphologic response and tellurite resistance. Complementation of the terZABCDE deletion mutant with terCDE, but not terZAB, partially restored tellurite resistance. When the terZABCDE deletion mutant was complemented with terZAB or terCDE, Y. pestis exhibited filamentous morphology during macrophage infections as well as while these complemented genes were being expressed under an in vitro condition. Further in E. coli, expression of Y. pestis terZAB, but not terCDE, conferred a filamentous phenotype.

Conclusions

These findings support the role of Y. pestis terZAB mediation of the filamentous response phenotype; whereas, terCDE confers tellurite resistance. Although the beneficial role of filamentous morphological responses by Y. pestis during macrophage infections is yet to be fully defined, it may be a bacterial adaptive strategy to macrophage associated stresses.     

Variation in cuticular lipid profiles of black butterflies of the genus Papilio in Japan

Insect (epi)cuticular lipids characterize sex and species and often play an important role in mating behavior. We previously revealed that two black-colored swallowtail butterflies, Papilio polytes and Papilio protenor, show sexual dimorphism and species specificity in cuticular lipid composition and that P. polytes males use specific monoene components for mate discrimination. These findings suggest that their closely related species may have different profiles of cuticular lipids. We examined the cuticular lipid compositions of five Papilio species (P. bianor, P. maackii, P. helenus, P. macilentus, and P. memnon), closely related and sympatric to P. polytes and P. protenor, and discussed whether it is possible to discriminate between sexes, and between species based on their chemical profiles. The cuticular lipids consist mainly of C23–C31 aliphatic hydrocarbons, in which n-tricosane, n-heptacosane, and n-nonacosane are predominant. Several aliphatic ketones, aliphatic acids, and oxygenated terpenoids were also identified as major components shared by several species. There were no components exclusive to a particular species. Conspecific males and females shared most of the components but were mostly distinguishable based on their composition. Moreover, P. helenus males, P. polytes females, and P. protenor females had two different phenotypes of lipid composition. Although unrelated to the genetic lineage, the seven species were classified into four clusters based on their lipid profiles. The first cluster was composed of only P. memnon. The other six species were broadly classified into three clusters consisting of subclusters for each species: 1) P. polytes, P. helenus, P. macilentus, and several P. protenor females; 2) P. bianor and P. protenor; and 3) P. maackii and several P. helenus males. These results indicate that cuticular lipid profiles characterize the species and sex of the Papilio species and may be responsible for mate discrimination among them.     

Involvement of Androgen Receptor in Sex Determination in an Amphibian Species

In mice and humans, the androgen receptor (AR) gene, located on the X chromosome, is not known to be involved in sex determination. In the Japanese frog Rana rugosa the AR is located on the sex chromosomes (X, Y, Z and W). Phylogenetic analysis shows that the AR on the X chromosome (X-AR) of the Korean R. rugosa is basal and segregates into two clusters: one containing W-AR of Japanese R. rugosa, the other containing Y-AR. AR expression is twice as high in ZZ (male) compared to ZW (female) embryos in which the W-AR is barely expressed. Higher AR-expression may be associated with male sex determination in this species. To examine whether the Z-AR is involved in sex determination in R. rugosa, we produced transgenic (Tg) frogs carrying an exogenous Z-AR. Analysis of ZW Tg frogs revealed development of masculinized gonads or ‘ovotestes’. Expression of CYP17 and Dmrt1, genes known to be activated during normal male gonadal development, were up-regulated in the ZW ovotestis. Testosterone, supplied to the rearing water, completed the female-to-male sex-reversal in the AR-Tg ZW frogs. Here we report that Z-AR is involved in male sex-determination in an amphibian species.     

黑龙江省药用植物根际土壤真菌多样性

为了解黑龙江省药用植物根际土壤真菌的种群结构和区系分布特点,于2010年7月份和10月份,在黑龙江省的伊春、铁力、绥化、哈尔滨、牡丹江和佳木斯6个中药材产区共采集土壤样品220份,所采集的药用植物种类主要有五味子、平贝母、刺五加、党参、防风、柴胡、桔梗、黄芩等14种.经稀释平板法和土壤颗粒平板法分离共获得1016株真菌,经形态鉴定归为35属86种,其中接合菌7属10种,占7.78％;子囊菌1属2种,占0.69％;无性型真菌27属74种,占70.76％,其余20.77％的菌株为不产孢真菌.试验结果表明,黑龙江省药用植物根际土壤真菌的种群多样性丰富,其中青霉属Penicillium、曲霉属Aspergillus、木霉属Trichoderma、镰孢菌属Fusarium是优势种群,粘帚霉属Gliocladium、金孢属Chrysosporium、毛霉属Mucor、枝孢属Cladosporium、枝顶孢属Acremonium、根霉属Rhizopus是亚优势种群.不同药用植物根际土壤真菌区系的结构和组成存在一定的差异.除无孢类群外,青霉属Penicillium、曲霉属Aspergillus、木霉属Trichoderma和镰孢菌属Fusarium是14种药用植物根际土壤真菌的优势菌群.五味子、平贝母和柴胡是黑龙江省种植的主要中药材,它们在6个采样地点间的真菌种群的多样性水平存在差异,其中伊春地区的多样性指数(H’=2.9574)和丰富度指数(R=5.6683)最高,而佳木斯地区的均匀度指数(J=0.9200)最高.不同地区的相似性水平也存在差异,其中牡丹江与绥化的药用植物根际土壤真菌种群组成之间的相似性系数最高(Cj=0.6315),牡丹江与哈尔滨的相似性最低(Cj=0.3704).     

Molecular detection of Lotmaria passim in honeybees in Japan

Crithidia mellificae (C. mellificae) and Lotmaria passim (L. passim) are trypanosomatids that infect Apis mellifera. We analyzed the prevalence of C. mellificae and L. passim in six regions of Japan from 2018 to 2019. The detection rate of C. mellificae was 0.0% in all regions, whereas L. passim was detected in 16.7%–66.7% of the honeybees. L. passim was detected at a significantly lower rate in the Cyugoku-Shikoku region than in other regions. Furthermore, phylogenetic analysis of the internal transcribed spacer 1 (ITS1) locus of related species was performed. All the samples in this study could be assigned to the L. passim clade. This study reveals that L. passim infection is predominantly prevalent in Japan. Further epidemiological surveys are needed to clarify the prevalence of C. mellificae infection in honeybees in Japan.     

Elucidation of Molecular Identity of the W3 Locus and Its Implication in Determination of Flower Colors in Soybean

The wide range of flower colors in soybean is controlled by six independent loci (W1, W2, W3, W4, Wm, and Wp). Among these loci, mutations in the W3 locus under the w4 allelic background (i.e., w3w4) produce near-white flowers, while the W3w4 genotype produces purple throat flowers. Although a gene encoding dihydroflavonol 4-reductase, DFR1, has been known to be closely associated with the W3 locus, its molecular identity has not yet been characterized. In the present study, we aimed to determine whether DFR1 is responsible for allelic variations in the W3 locus. On the basis of the sequence of a DFR probe, Glyma.14G072700 was identified as a candidate gene for DFR1, and nucleotide sequences of Glyma.14G072700 from cultivars with previously validated genotypes for the W3 locus were determined. As a result, a number of nucleotide polymorphisms, mainly single-base substitutions, between both coding and 5′-upstream region sequences of the W3 and w3 alleles were identified. Among them, an indel of 311-bp in the 5′-upstream region was noteworthy, since the Glyma.14G072700 in all the w3 alleles examined contained the indel, whereas that in all the W3 alleles did not; the former was barely expressed, but the latter was well expressed. These results suggest that Glyma.14G072700 is likely to correspond to DFR1 for the W3 locus and that its expression patterns may lead to allelic color phenotypes of W3 and w3 alleles under the w4 allelic background.     

Quantification of proton budgets in soils of cropland and adjacent forest in Thailand and Indonesia

Mechanisms for tolerance and avoidance of salinity at germination were examined in five self-regenerating annual pasture legumes of Mediterranean origin (Medicago polymorpha, Melilotus siculus, Trifolium subterraneum, T. michelianum and T. tomentosum). The maximum NaCl concentrations, for which no reduction in germination percentage occurred, were 300 mM for M. siculus, 240 mM for M. polymorpha and 120 mM for T. michelianum, T. subterraneum and T. tomentosum. Following 21 days exposure to 300 mM NaCl, imbibed seeds of T. subterraneum showed reduced germinability upon transfer to non-saline solution, whereas those of the four other species recovered full germinability. Following exposure to 600 mM NaCl, however, only T. michelianum and Melilotus siculus had some degree of recovery, with 38% and 31% germinability, respectively. Na⁺ in imbibed seed tissues of all species increased markedly with increasing NaCl concentration, while K⁺ decreased in all but Melilotus siculus. Seed coat impermeability (‘hard seeds’) acted as protection against the toxic effects of salinity. Melilotus siculus, Medicago polymorpha and T. tomentosum showed a delay in hard-seed breakdown (‘seed softening’) over the summer–autumn period, compared to T. subterraneum and T. michelianum, which acts to defer germination until soil surface salinity levels are likely to be lower. These three species also had higher levels of residual hard seeds. The results from this study support field observations that Melilotus siculus and Medicago polymorpha are the best adapted annual pasture legumes for highly saline soils in southern Australia, T. tomentosum and T. michelianum have some adaptation to moderately and mildly saline soils, respectively, whereas T. subterraneum has no adaptive traits for even mildly saline environments. A model for annual legume germination on saline land in southern Australia is also presented.     

Improvement of Posttranslational Bottlenecks in the Production of Penicillin Amidase in Recombinant Escherichiacoli Strains

Using periplasmic penicillin amidase (PA) from Escherichia coli ATCC 11105 as a model recombinant protein, we reviewed the posttranslational bottlenecks in its overexpression and undertook attempts to enhance its production in different recombinant E. coli expression hosts. Intracellular proteolytic degradation of the newly synthesized PA precursor and translocation through the plasma membrane were determined to be the main posttranslational processes limiting enzyme production. Rate constants for both intracellular proteolytic breakdown (k_d) and transport (k_t) were used as quantitative tools for selection of the appropriate host system and cultivation medium. The production of mature active PA was increased up to 10-fold when the protease-deficient strain E. coli BL21(DE3) was cultivated in medium without a proteinaceous substrate, as confirmed by a decrease in the sum of the constants k_d and k_t. The original signal sequence of pre-pro-PA was exchanged with the OmpT signal peptide sequence in order to increase translocation efficiency; the effects of this change varied in the different E. coli host strains. Furthermore, we established that simultaneous coexpression of the OmpT pac gene with some proteins of the Sec export machinery of the cell resulted in up to threefold-enhanced PA production. In parallel, we made efforts to increase PA flux via coexpression with the kil gene (killing protein). The primary effects of the kil gene were the release of PA into the extracellular medium and an approximately threefold increase in the total amount of PA produced per liter of bacterial culture.     

宁夏盐池荒漠草原步甲物种多样性

采用巴氏罐诱法于2009年3月到10月对盐池四墩子3种不同类型荒漠草原的步甲物种组成、数量分布进行了系统调查,共获得步甲标本1318号,分属于9属15种,其中直角通缘步甲Pterostichus gebleri、蒙古伪葬步甲Pseudotaphoxnus mongolicus、径婪步甲Harpalus salinus和短翅伪葬步甲Pseudotaphoxnus brevipennis为优势种,个体数量分别占个体总数的33.08%、19.73%、15.94%和8.04%。多样性分析表明适度干扰荒漠草地的Shannon-Wiener多样性、丰富度和均匀度均高于低干扰和强干扰荒漠草地。群落相似性分析显示猪毛蒿和甘草混交带与围栏放牧带及柠条带有较高的相似性,蒙古冰草带和苜蓿与柠条混种带相似性较高。从时间动态上看,步甲群落的个体数量在8月份达到最高。4种优势种季节性变化分别是直角通缘步甲盛发期为8月,径婪步甲发生的高峰期为8、9月,短翅伪葬步甲在5月和8月,蒙古伪葬步甲7-10月个体数量均多;不同物种在不同类型荒漠草地出现的高峰期是不一致的。典范对应分析(CCA)分析表明土壤含水量是影响步甲分布的最重要环境因子,植物生物量和植被密度次之;土壤含水量与步甲的Shannon-Wiener多样性、均匀度显著负相关,与优势度呈显著正相关。     

Prevalence of simian malaria parasites in macaques of Singapore

Plasmodium knowlesi is a simian malaria parasite currently recognized as the fifth causative agent of human malaria. Recently, naturally acquired P. cynomolgi infection in humans was also detected in Southeast Asia. The main reservoir of both parasites is the long-tailed and pig-tailed macaques, which are indigenous in this region. Due to increased urbanization and changes in land use, there has been greater proximity and interaction between the long-tailed macaques and the general population in Singapore. As such, this study aims to determine the prevalence of simian malaria parasites in local macaques to assess the risk of zoonosis to the general human population. Screening for the presence of malaria parasites was conducted on blood samples from 660 peridomestic macaques collected between Jan 2008 and Mar 2017, and 379 wild macaques collected between Mar 2009 and Mar 2017, using a Pan-Plasmodium-genus specific PCR. Positive samples were then screened using a simian Plasmodium species-specific nested PCR assay to identify the species of parasites (P. knowlesi, P. coatneyi, P. fieldi, P. cynomolgi, and P. inui) present. All the peridomestic macaques sampled were tested negative for malaria, while 80.5% of the 379 wild macaques were infected. All five simian Plasmodium species were detected; P. cynomolgi being the most prevalent (71.5%), followed by P. knowlesi (47.5%), P. inui (42.0%), P. fieldi (32.5%), and P. coatneyi (28.5%). Co-infection with multiple species of Plasmodium parasites was also observed. The study revealed that Singapore’s wild long-tailed macaques are natural hosts of the five simian malaria parasite species, while no malaria was detected in all peridomestic macaques tested. Therefore, the risk of simian malaria transmission to the general human population is concluded to be low. However, this can be better demonstrated with the incrimination of the vectors of simian malaria parasites in Singapore.     

Illegal transposition of mating-type genes in yeast

Mating-type switching in the yeast Saccharomyces cerevisiae involves the transposition of a copy of a or α information from unexpressed “library” genes, HML or HMR, to replace the sequence at the mating type locus, MAT. In normal homothallic strains, where conversions of MAT may occur as often as every cell division, the switching of MAT alleles does not alter the alleles at HML or HMR. We have discovered that several mutations within or adjacent to MAT that impair the excision of the MAT allele permit conversions of the alleles at HML or HMR in more than 1% of the cells analyzed. The two mutations within the MAT locus (MATa-inc and MATα-inc) can transpose to HML or HMR without being lost at MAT. Thus a MATα-inc HMLα HMRa HO strain can switch to MATα-inc HMLα HMRα-inc HO. Even though the α-inc and a-inc alleles prevent their own replacement at MAT, these sequences are efficiently transposed back from HMLα-inc or HMLa-inc to replace normal MAT alleles. When these alleles reappear at MAT, they are again blocked in excision. Thus the sequences used to remove an allele from MAT must differ from those used to replicate and transpose it. Two cis-acting stk mutations adjacent to MAT that block switching of MATa to MATα also induce the conversion of HMLα to HMLa. However, we have previously shown that these events do not occur in strains carrying a recessive “switch” mutant (swi1) or in strains carrying a defective allele of the HO gene. In stk1 MATa HO strains, HMLα was converted to HMLa in approximately 4% of the subclones examined. In contrast, the HMLα-inc sequence was not converted in similar stk1 MATa HO strains. Thus the excision of the α-inc sequence seems to be prevented at both MAT and HML. These results suggest that the illegal conversions of HML and HMR occur by a mechanism similar to that used for normal conversions of MAT.