Sexual Generation Research Articles

Repression, Revolution and Ambivalence: The Sexual Life of Three Generations

Attitudes toward unconventional and casual forms of sexuality and toward extramarital relations as well as the appearance of parallel (extra) sexual relationships are studied in three sexual generations. These are constructed on the basis of sexual norms and practices prevailing at the time of the sexual initiation of their members. The following sexual generations are distinguished: the sexual generations of repression born 1917-36, revolution born 1937-56, and ambivalence born 1957-73. The persistence of the sexual morals and behaviour patterns adopted in youth clearly emerges from large national surveys conducted in Finland in 1971 and 1992. The generation of sexual repression appears as the most restrictive in its attitudes toward unconventional and casual forms of sexuality. It has least often engaged in parallel relationships. The generation of sexual revolution is consistently most permissive in its sexual attitudes and behaviour. The youngest cohort, the sexually ambivalent generation, is permissive toward unconventional and casual forms of sexuality but family-oriented in attitudes toward marital fidelity. Nevertheless, it is free in its own sexual behaviour. Compared to young people twenty years ago, its members more often report parallel sexual relations. The findings are interpreted by referring to current sociological discussions on individualization in (post)modern society.

Gene conversion as a focusing mechanism for correlated mutations: a hypothesis.

Ways of producing complex mutational events without substantially raising the primary mutation rate are explored. If the small amount of DNA that is resynthesised through the action of the mismatch DNA repair system is not subject to further repair, the incidence of double mutations can increase by a factor of 100, while single mutations would increase by only 30%. Such a boost in the incidence of double mutations seems insufficient to meet the needs of higher organisms. For them, an alternative strategy would be to produce complex events by a succession of single mutations occurring in a correlated manner over several sexual generations. It is proposed that gene conversion may fulfill this role. Assuming that the resynthesis of DNA that occurs during gene conversion produces mutations in the conversion tract, one predicts a tendency for close mutations in corresponding sequences in the two homologous chromosomes, to promote, during conversion, further mutations in their vicinity. Semiquantitative calculations suggest that such a mechanism can be quite effective, provided the divergence between two paired chromosomes is around 10(-4) or less. Such a mechanism might constitute an adaptive mutation strategy acting at the population level.

A Mendelian Mutation Affecting Mating-Type Determination Also Affects Developmental Genomic Rearrangements in Paramecium tetraurelita

In Paramecium tetraurelia, mating type is determined during the differentiation of the somatic macronucleus from a zygotic nucleus genetically competent for both types, O and E. Determination of the developing macronucleus is controlled by the parental macronucleus through an unknown mechanism resulting in the maternal inheritance of mating types. The pleiotropic mutation mtFE affects macronuclear differentiation. Determination for E is constitutive in mutant homozygotes; a number of unrelated mutant characters are also acquired during development. We have examined the possibility that the mutation causes a defect in the developmental rearrangements of the germ-line genome. We show that the excision of an IES (internal eliminated sequence) interrupting the coding sequence of a surface antigen gene is impaired in the mutant, resulting in an alternative macronuclear version of the gene. Once established, the excision defect is indefinitely transmitted across sexual generations in the cytoplasmic lineage, even in a wild-type genetic context. Thus, the processes of mating-type determination and excision of this IES, in addition to their common sensitivity to the mtFE mutation, show a similar maternal inheritance of developmental alternatives in wild-type cells, suggesting a molecular model for mating-type determination.

Transformation of peas (Pisum sativum L.) using immature cotyledons

A reliable Agrobacterium tumefaciens-mediated transformation method has been developed for peas (Pisum sativum) using immature cotyledons as the explant source. Transgenic plants were recovered from the four cultivars tested: Bolero, Trounce, Bohatyr and Huka. The method takes approximately 7 months from explant to seed-bearing primary regenerant. The binary vector used carried genes for kanamycin and phosphinothricin resistance. Transformed pea plants were selected on 10 mg/l phosphinothricin. The nptII and bar genes were shown to be stably inherited through the first sexual generation of transformed plants. Expression of the phosphinothricin-resistance gene in the transformed plants was demonstrated using the 'Buster' (='Basta') leaf-paint test and the phosphinothricin acetyl transferase enzyme assay.

The adaptive advantage of phenotypic memory in changing environments.

The adaptive value of carry-over effects, the persistence of induced phenotypes for several generations despite the change in the conditions that first induced these phenotypes, is studied in the framework of a simple model. Three different organismal strategies-non-inducible (genetic), completely inducible (plastic), and intermediate (carry-over)-are compared in fitness terms within three different environments. Analytical results and numerical simulations show that carry-over effects can have an advantage in stochastic environments even over organisms with high adaptive plasticity. We argue that carry-over effects represent an adaptive mechanism on the ecological timescale that fills the gap between short-term individual adaptations and long-term evolutionary adaptations. An extension of the concept of plasticity to incorporate the time dimension and include the stability of induced phenotypes through both clonal and sexual generations, is suggested.

Geographic and between-generation variation in the parasitoid communities associated with an invading gallwasp, Andricus quercuscalicis (Hymenoptera: Cynipidae).

The knopper gallwasp Andricus quercuscalicis Burgsdorf 1783 (Hymenoptera: Cynipidae) has invaded western and northern Europe from southern and eastern Europe over the last 400 years. A. quercuscalicis has two alternating generations, which differ in phenology, structure, and host oak species. This study describes geographic variation in the community in the tiny catkin galls of the sexual generation on Turkey oak, Quercus cerris, and compares the patterns obtained with those in the community attacking the alternate agamic generation. As predicted from considerations of parasitoid recruitment to the communities of invading phytophagous insects (Cornell and Hawkins 1993), in its native range the sexual generation shows (1) higher parasitoid community species richness, (2) higher total mortality due to parasitoid attack and (3) a higher ratio of specialist to generalist parasitoid species than is evident in the invaded range. Counter to predictions, there is no indication that parasitoid community richness in the invaded range has increased with time since the arrival of the new host. Higher host mortality in the native range is due principally to a single specialist, Aulogymnus obscuripes Mayr 1877 (Hymenoptera: Eulophidae), and is not distributed evenly among parasitoid species which attack the gall-former only in this area. This contrasts with the community in Britain, where three principal generalist parasitoids cause approximately equal mortalities. The agamic gall contains a taxonomically and structurally diverse guild of parasitoid and inquiline species, associated with the changing resource provided by a large, long-lived, complex gall. In contrast, the sexual community includes a taxonomically and structurally narrow guild, associated with a resource which is structurally simple, small in size and short-lived. No parasitoid species attacks the gall-former in both generations. Surprisingly, in spite of these differences in the nature of the gall resource in the two generations, over their entire range (native and invaded) the parasitoid guilds of the two are equally species rich.

Colchicine-induced heritable modification of chiasma frequency and distribution in perennial ryegrass (Lolium perenne)

One-week-old seedlings of inbred lines of perennial ryegrass (Lolium perenne L.) were given a 3-h period of treatment with 0.2% colchicine, and this treatment was found to have heritable effects on chromosome behaviour at meiosis. The mean pollen mother cell chiasma frequency was reduced, and there was a corresponding increase in the number of univalents per cell and also changes in cell and bivalent variances. Patterns of chiasma distribution within bivalents were also modified. The effects were seen in the generation in which the treatment was given and they were also observed in the following sexual generation.

Long-range effects of colchicine sensitivity on meiosis in Lolium multiflorum L. (Italian ryegrass)

A 3-h treatment period with 0.2 per cent colchicine on 1-week-old seedlings of inbred lines of Italian ryegrass (Lolium multiflorum L.) has long-range effects on chromosome behaviour at meiosis. There is a reduction of mean pollen mother cell chiasma frequency, an increase in univalents and in cell and bivalent variances, and also changes in the pattern of chiasma distribution within bivalents. The effects are seen in the generation in which the treatment is given, and in the following sexual generation.

Genetics of life history in Daphnia magna. I. Heritabilities at two food levels

Broad- and narrow-sense heritabilities for several life-history traits were estimated from 23 mother-daughter pairs of Daphnia magna at two food levels. Sexually produced daughter clones were obtained from a field collection of ephippial females (mothers) and the subsequent hatching of their ephippial eggs (daughters). Mother and daughter clones were maintained by parthenogenetic reproduction. Heritabilities of adult body-length of eight successive instars were the highest estimates of all in good food, but the lowest of all in poor food. For clutch size and body-length of offspring from the first six clutches, narrow- and broad-sense heritabilities were about equal and lower in poor food than in good food. The amount of genetic variation present would allow a response to selection on clutch size and offspring length in both environments but adult length only in good food. For age at maturity we found no additive genetic variance. We found no difference in broad-sense heritabilities between mother clones, representing the last generation after a period of asexual reproduction, and their sexually produced offspring. This suggests that genetic variance does not increase after one sexual generation or that it was not reduced before. Differences in heritabilities between environments are discussed with reference to the enlarged phenotypic variances that result from variation in juvenile instar number. Targeted growth could explain the pronounced differences in the heritabilities of adult length between environments.

Production of transgenic maize plants by direct DNA uptake into embryogenic protoplasts

Fertile transgenic maize plants were regenerated after direct transfer of a chimeric gene into maize protoplasts. Plasmid DNA containing mutant dihydrofolate reductase (DHFR) mouse gene, that confers methotrexate (MTX) resistance, under the control of the CaMV 35S promoter was introduced into maize embryogenic protoplasts by polyethylene glycol (PEG) treatment. Transformation was also carried out with a modified plasmid in which the selective marker gene casette was cloned into the BstBI site of the Ds 1 maize transposable element. Resistant callus tissues grown in the presence of 10 −6 or 10 −7 M MTX were selected and shoot or plant regeneration was achieved under hormone-free culture conditions. The presence of the introduced DHFR gene in DNA isolated from the selected colonies and the primary regenerants (T 0) was shown by Southern hybridization and PCR analysis. PCR primers for the 35S promoter and for two regions of the coding sequence of the DHFR gene were used for amplification of the foreign sequence present in maize genomic DNA. The PCR products were hybridized with a mouse DHFR gene specific probe. Synthesis of the mouse DHFR in MTX resistant maize tissues was detected by staining for enzyme activity after native PAGE. The in vitro regenerated plants could be grown up to maturity in the greenhouse. Cross pollination has resulted in seeds and the F 1 progenies were also analyzed. In addition to the segregation of MTX-resistant and-sensitive offsprings, molecular evidences based on Southern data and PCR analysis have indicated that the introduced gene was transferred in the first sexual generation. This report provides a new example for potentials in the use of embryogenic cereal protoplasts for production of fertile transgenic crop plants.

Transformation of peas

The lateral cotyledonary meristems present in germinating seed were inoculated with a non-oncogenic strain of A. tumefaciens carrying a gene conferring kanamycin resistance as a selectable marker and a β-glucuronidase sequence as a reporter gene. Kanamycin resistant plants were derived from the meristems and shown to be transformed on the basis of Southern blots, polymerase chain reaction analysis and tests for β-glucuronidase activity. The plants were fertile and tests of their progeny confirmed the transmission of integrated sequences through a sexual generation. This transformation method has the merit of an unlimited supply of material for inoculation and a relatively short time scale from inoculation to the production of rooted plants.

Mosses and the production of Chinese gallnuts

Chinese gallnuts are galls that form on tfie-leaves of sumac trees (Rhus spp) in response to attack by aphids in the Eriosomatinae, for which mosses form obligate winter hosts. Walls of the gallnuts may comprise up to 70% tannins and yield compounds of considerable commercial value, notably tannic acid and gallic acid. Gallnut production can therefore have a major impact on the local economy in rural areas of China.At least 14 types of gallnut have been described, each caused by a different species of aphid. A wide range of mosses act as winter hosts. Of greatest importance commercially is the true-horned gallnut which forms on R. chinensis in response to attack by Schlechtendalia chinensis, an aphid that overwinters on members of the Mniaceae, particularly Plagiomnium acutum. There are six generations of S. chinensis in each annual cycle. Reproduction is principally viviparous and parthenogenetic, except for a single sexual generation produced after spring migrants fly from mosses to the sumac trees in April and early May.True-horned gallnuts are traditionally harvested from semi-natural woodland on rocky hillsides. Approximately 1-2% cover of P. acutum is adequate to give high production of gallnuts provided that it is uniformly distributed, and favourable distribution and abundance of the moss is achieved by artificial transplantation. Attempts are being made to produce gallnuts by growing sumac trees in agricultural fields, but success is lirnited by the difficulty of establishing Plagiomnium spp in the fields. One solution is to cultivate the moss in plastic bowls which are kept for most of the year in sheds. In October each year the bowls are placed outside under the sumac trees for several weeks and autumnmigrant aphids fly into the moss colonies. The bowls are kept in the shed during the winter, the moss mat is stripped from the bowls and placed under the trees in April, and spring-migrant aphids then leave the moss and fly to the trees. The moss regenerates in the bowls during the summer. We are investigating the growth and reproductive ecology of P. acutum as an approach towards increasing gallnut production.

Regulated inactivation of homologous gene expression in transgenic Nicotiana sylvestris plants containing a defense-related tobacco chitinase gene

The class I chitinases are vacuolar proteins implicated in the defense of plants against pathogens. Leaves of transgenic Nicotiana sylvestris plants homozygous for a chimeric tobacco (Nicotiana tabacum) chitinase gene with Cauliflower Mosaic Virus (CaMV) 35S RNA expression signals usually accumulate high levels of chitinase relative to comparable leaves of non-transformed plants. Unexpectedly, some transgenic plants accumulated lower levels of chitinase than nontransformed plants. We call this phenomenon silencing. The incidence of silencing depends on the early rearing conditions of the plants. When grown to maturity in a greenhouse, approximately 25% of plants raised as seedlings in closed culture vessels were of the silent type; none of the plants raised from seed in a greenhouse showed this phenotype. Silencing is also developmentally regulated. Plants showed three patterns of chitinase expression: uniformly high levels of expression in different leaves, uniformly low levels of expression in different leaves, and position-dependent silencing in which expression was uniform within individual leaves but varied in different leaves on the same plant. Heritability of the silent phenotype was examined in plants homozygous for the transgene. Some direct descendants exhibited a high-silent-high sequence of activity phenotypes in successive sexual generations, which cannot be explained by simple Mendelian inheritance. Taken together, the results indicate that silencing results from stable but potentially reversible states of gene expression that are not meiotically transmitted. Gene-specific measurements of chitinase and chitinase mRNA showed that silencing results from co-suppression, i.e. the inactivation of both host and transgene expression in trans. The silent state was not correlated with cytosine methylation of the transgene at the five restriction sites investigated.

Inducing the sexual forms and hatching the eggs of pea aphids

AbstractIn temperate climates, pea aphids (Acyrthosiphon pisum) produce a single sexual generation each year in response to declining photoperiod and temperature. Mating occurs in the fall and the eggs have an obligatory winter diapause. Genetic recombination during the sexual phase is thought to be an important source of genetic variability within cyclically parthenogenetic aphid populations. Methods for reliably producing sexual forms and hatching the eggs of aphids are therefore central not only to the study of evolutionary change in aphid populations, but also for a general understanding of the origin of agriculturally important variation in destructiveness within pest species.Here, sexual forms of six pea aphid clones were induced in the laboratory and eggs were successfully hatched by creating conditions that closely mimicked those found in field situations. A declining photoperiod was produced by controlling artificial lighting using a timer with variable cycle length. Using these conditions, sexual forms were successfully produced for all six clones tested, which were then mated in all combinations. Eggs were exposed to a daily cycle of freezing and thawing in an incubator under a short‐day photoperiod. Egg hatch averaged 60%, but was as high as 89% for some crosses. These methods will permit testing of evolutionary hypotheses and execution of detailed genetic studies of sources of variability within pea aphid populations. They are thus important tools for both evolutionary and agricultural studies.

Spatial Density Dependence in Populations of a Cynipid Gall-Former Andricus Quercuscalicis

Summary 1. Patterns of spatial density dependence in mortality factors affecting the sexual generation of Andricus quercuscalicis (Burgsdorf 1783) on Quercus cerris (L.) were investigated as part of a long-term study of the population dynamics of the gall wasp. Experimental enhancement of gall density was used to increase the range of densities observed in the field. 2. We present a new application of logistic regression for detecting spatial density dependence in single-sample collections of galls or leaf-mines. The test, first described by Cox (1970) uses GLIM with binomial errors. The performance of the test and the frequency of Type I and Type II errors were assessed by computer simulation of a variety of different ecological circumstances including density independence, mild and severe, direct and inverse density dependence, and proved satisfactory in most respects. 3. In our field data, statistically significant spatial density dependence was detected in about 15% of cases. Out of 62 cases of significant density dependence, 66% were of inverse density dependence. In particular, the gall wasp data demonstrate a preponderance of inverse density dependence in rates of parasitism. Only predation of galls by birds exhibited a consistent pattern of direct density dependence. There was marked variation in the intensity of all mortality factors from tree to tree. 4. As spatial scale is increased from catkins, through buds, shoots and twigs, so the sample size declines and the range of densities increases. Since reducing the sample size reduces the probability of detecting significant density dependence, but increasing the range of densities increases the likelihood of detecting density dependence, the overall effect of changing spatial scale on the probability of detecting density dependence is difficult to predict. Even when density dependence was detected at the smallest scale, the frequency of detection at higher scales declined, and this may be due in part to spatial heterogeneity in the action of density dependence. Thus, it is not obvious whether changes in the rate of detection of density dependence at different spatial scales are statistical artefacts or reflect real changes in the ecology of the organisms involved. 5. There are two potential problems in applying logistic regression to such data sets. First, overdispersion may occur if p (the probability of mortality) varies significantly within the sample. One way of dealing with this is to rescale the model, according to the degree of aggregation. Second, apparent underdispersion may occur if both p and n are low because, with small sample sizes, variation in p may be undetectable. The most obvious solution in this case is to increase the sample size n, though this may be difficult at larger spatial scales. 6. Enhancing gall densities by caging agamic females directly onto the branches of the host tree led to an increase in mean gall density and in the range of gall densities. This, in turn, led to an increase in the detection of inverse density dependence (mainly through parasitoid attack) but had no significant impact on the detection of direct density dependence. The use of experimentally enhanced gall densities appears to hold considerable potential for demonstrating the importance of spatial density dependence, so long as care is taken to ensure that the highest densities are not unrealistic. 567

The Culture, Sexual and Asexual Reproduction, and Growth of the Sea Anemone Nematostella vectensis.

Nematostella vectensis, a widely distributed, burrowing sea anemone, was raised through successive sexual generations at room temperature in non-circulating seawater. It has separate sexes and also reproduces asexually by transverse fission. Cultures of animals were fed Artemia sp. nauplii every second day. Every eight days the culture water was changed, and the anemones were fed pieces of Mytilus spp. tissue. This led to regular spawning by both sexes at eight-day intervals. The cultures remained reproductive throughout the year. Upon spawning, adults release either eggs embedded in a gelatinous mucoid mass, or free-swimming sperm. In one experiment, 12 female isolated clonemates and 12 male isolated clonemates were maintained on the 8-day spawning schedule for almost 8 months. Of the female spawnings, 75% occurred on the day following mussel feeding and water change, and 64% of the male spawnings were similarly synchronized under this regime. Fertilization and development occur when gametes from both sexes are combined in vitro. At 20°C, the embryos gastrulate within 12-15 hours. Spherical ciliated planulae emerge from egg massess 36-48 hours post-fertilization. The planulae elongate and form the first mesenteric couple, as well as four tentacle buds, by day five. By day seven, they metamorphose and settle as 250-500 μm long, four-tentacled juvenile anemones. More tentacles and all eight macrocnemes are present at 2-3 weeks. Individuals may become reproductively mature in as few as 69 days. Nematostella vectensis has the potential to become an important model for use in cnidarian developmental research.

The Evolution of Aphid Life Cycles

The life cycles of aphids are among the most remarkable of any animal group. They include parthenogenetic and sexual generations, elaborate polyphen­ isms, and obligate shifting between unrelated host-plant taxa. These and other unusual life-cycle traits occur in a variety of combinations among the approx­ imately 4000 extant species within the Aphidoidea (46). Although the complexity and the diversity found in aphid life cycles are often daunting, the study of these insects repeatedly draws both entomologists and evolutionary biologists, for several reasons. First, a few aphid species are agricultural pests, and studies of life cycles can be essential to effective control measures (e.g. 71). Second, aphids, especially their life cycles, are intrinsically fascinating. Finally, aphids are good study organisms for addressing outstanding problems in evolutionary biology. For example, the partially clonal reproductive mode of aphids is useful for testing hypotheses for advantages of sexuality (18, 21, 36, 115, 124, 186) and also facilitates estimation of the genotypic component of fitness traits (e.g. 54, 102, 124, 129, 131, 148, 166, 167, 176-179). The occurrence of several other unusual phenomena, including extensive polyphenism (129), complex life cycles with seasonal alternation between two disjunct sets of hosts (29, 82, 89, 90, 109, 126, 128), soldier castes (3-5, 86), and sex-ratio control (130, 190) have

Reactivation of a silent Ac following tissue culture is associated with heritable alterations in its methylation pattern.

Tissue cultures were initiated from embryos with an inactive form of Ac in the wx-m9 Ds-cy allele. Plants regenerated from the cultures showed a high frequency of activation of Ac. That activation was shown to be associated with reduced methylation of cytosine residues at the 5' end of the transposable element. An examination of Ac activity and methylation status of the Ac element in progenies of the regenerant plants demonstrated transmission of the altered epigenotype through two sexual generations. In these progenies no evidence for trans activation of inactive, partially methylated, Ac elements was obtained. These results confirm that in certain instances altered methylation patterns can be inherited through the germ line.

The Population Dynamics of an Alien Insect: Andricus quercuscalicis (Hymenoptera: Cynipidae)

SUMMARY (1) We describe the results of the first 10 years of a long-term study of the population biology of a cynipid gall wasp, Andricus quercuscalicis (Burgsdorf 1783), an alien herbivorous insect that appears to have reached the limits of its geographical distribution within England. (2) We document the annual fluctuations in abundance of the sexual generation on Quercus cerris (L.), the Turkey oak, and the agamic (parthenogenetic) generation on Q. robur (L.), the English oak, as measured by gall densities per shoot of the host tree. The sexual generation has fluctuated 6-fold and the agamic generation has fluctuated 10-fold over the 10-year period. (3) Resource use and rates of parasitism vary substantially between the two generations. The agamic generation on Q. robur uses 35-60% of the acorn crop in any given year, whereas the sexual generation on Q. cerris uses only 2-8% of the male flowers. There was virtually no parasitism of the agamic generation, while the sexual generation suffers 25-35% mortality from a group of five native parasitoids. (4) The two generations appear to be loosely coupled. It is impossible to predict the abundance of one generation from the numbers in the previous generation, either within or between years. Three features of the system contribute to this uncoupling: (i) the insect is food limited in the agamic generation; (ii) acorn production fluctuated 20-fold, independent of gall densities; (iii) there were two large variable migration mortalities between each generation. (5) There was extremely high tree-to-tree variation in the rate of galling on both species of oak, and these differences were consistent from year to year. Some trees have never been galled, while others support predictably high gall densities losing over 90% of their acorns every year. The possible causes and consequences of this heterogeneity are discussed. (6) The invasion of A. quercuscalicis appears to have had less impact on the native guild of oak-galling cynipids than might have been predicted. Competitive effects appear to be negligible, as the wasp attacks tissues on the two oak species that are not utilized by other cynipids in the U.K. The effect of adding an abundant new host on the dynamics of shared natural enemies was reduced because virtually all the parasitoids emerging from the sexual galls on Q. cerris were males, and the agamic galls were almost parasite-free.

Inheritance of a mutable phenotype that is activated in alfalfa tissue culture

Mutable alleles, typically regulated by transposable elements, have been identified in a number of plant species. Tissue culture induced genomic shocks may activate such elements. A mutable recessive condition arose in tissue culture of alfalfa and was highly unstable in subsequent cycles of culture. The mutable allele, designated c2-m4, is allelic to the C2 locus, a basic color factor locus involved in anthocyanin synthesis. Current research has focused on the inheritance and instability of c2-m4 in new genetic backgrounds as well as on dosage effects of the allele. We have confirmed a previous report that c2-m4 reverts to function at much higher frequencies in vitro (reversion frequency ca. 0.23) than in planta (reversion frequency < 0.001). Over sexual generations c2-m4 continues to be unstable. Transmission of the mutable phenotype to selfed, testcross, and F2 populations demonstrated monofactorial inheritance patterns (P > 0.25). In populations expected to have some plants carrying two or more c2-m4 alleles, individuals were found that reverted in vitro at significantly higher frequencies than their parent (0.67 ± 0.04 and 0.44 ± 0.08 versus 0.20 ± 0.09). In planta reversion also increased with increasing c2-m4 dosage. Preliminary evidence indicates that as c2-m4 dosage increases, each allele maintains its original capacity to revert.Key words: somaclonal variation, transposable element, tissue culture, mutable allele.