Common features of segregation distortion in plants and animals

Segregation distortion is increasingly recognized as a potentially powerful evolutionary force. This runs counter to the perception that non-Mendelian genes are rare genetic curiosities, a view that seems to be supported by the near ubiquity of the Mendelian system of inheritance. There are several reasons why segregation distortion may be more important than is evidenced by known empirical examples. One possibility is that the types of segregation distorters we have found are only a subset of a broader range of non-Mendelian systems, many of which go undetected. In this paper, we review what is known about the sex-linked meiotic drive system in the plant, Silene latifolia, and present some data on the mechanism of segregation distortion. We outline the general features that segregation distorters in plants and animals have in common. In some cases, such as the paucity of systems that directly alter meiotic segregation, there are likely to be inherent constraints on the range of systems that can possibly occur. Other generalities, however, support the notion that many forms of meiotic drive are possible, and that the known examples of segregation distortion are likely to be only subset of those that can possibly occur. Non-Mendelian genes may therefore have greater evolutionary importance than their current abundance in nature would suggest.     

Behaviour of DNA-induced inositol-independent transformants of Neurospora crassa in sexual crosses

Summary Treatment of inositolless (inl) strains of Neurospora crassa with DNA from the wild type (allo-DNA) gives rise to inositol-independent (inl ⁺) colonies. Some of these DNA-induced inl ⁺ strains (transformants) are sterile in sexual crosses on minimal medium that selects for the maintaining of the inl ⁺ character. The same inl ⁺ transformants, when crossed with an inl standard strain, are fertile on complete (inositol-containing) medium. There are, however, an increased number of unusual non-Mendelian tetrads (24%) among the progeny. The inl ⁺ and inl progeny from these complete non-Mendelian tetrads were further examined for the inheritance of the inl ⁺ trait. Several inl ⁺ progeny of these tetrads segregate inl conidia if growing on inositol-containing medium. The number of inl ⁺ conidia in certain inl ⁺ cultures decreases quickly under non-selective conditions. In transformants carrying mutant markers in linkage groups III, IV and VI non-Mendelian segregation of these traits can also be detected.The mechanism of the development of sterility and of the aberrant segregation is discussed.     

Nonrandom segregation during meiosis: the unfairness of females

Most geneticists assume that chromosome segregation during meiosis is Mendelian (i.e., each allele at each locus is represented equally in the gametes). The great majority of reports that discuss non-Mendelian transmission have focused on systems of gametic selection, such as the mouse t-haplotype and Segregation distorter in Drosophila, or on systems in which post-fertilization selection takes place. Because the segregation of chromosomes in such systems is Mendelian and unequal representation of alleles among offspring is achieved through gamete dysfunction or embryonic death, there is a common perception that true disturbances in the randomness of chromosome segregation are rare and of limited biological significance. In this review we summarize data on nonrandom segregation in a wide variety of genetic systems. Despite apparent differences between some systems, the basic requirements for nonrandom segregation can be deduced from their shared characteristics: i) asymmetrical meiotic division(s); ii) functional asymmetry of the meiotic spindle poles; and iii) functional heterozygosity at a locus that mediates attachment of a chromosome to the spindle. The frequency with which all three of these requirements are fulfilled in natural populations is unknown, but our analyses indicate that nonrandom segregation occurs with sufficient frequency during female meiosis, and in exceptional cases of male meiosis, that it has important biological, clinical, and evolutionary consequences. Received: 28 December 2000 / Accepted: 23 January 2001     

Efficient Method of Agrobacterium-mediated Transformation for Triticale (x Triticosecale Wittmack)

Transgenic plants of triticale cv. Wanad were obtained after transformation using three combinations of strain/vectors. Two of them were hypervirulent Agrobacterium tumefaciens strains (AGL1 and EHA101) with vectors containing bar under maize ubiquitin 1 promoter (pDM805), and both hpt under p35S and nptII under pnos (pGAH). The third one was a regular LBA4404 strain containing super-binary plasmid pTOK233 with selection genes the same as in pGAH. The efficiency of transformation was from 0 to 16% and it was dependent on the selection factor, auxin pretreatment, and the strain/vector combination. The highest number of transgenic plants was obtained after transformation with LBA4404(pTOK233) and kanamycin selection. Pretreatment of explants with picloram led to the highest number of plants obtained after transformation with both Agrobacterium/vector systems LBA4404(pTOK233) and EHA101(pGAH) and selected with kanamycin. Transgenic character of selected plants was examined by PCR using specific primers for bar, gus, nptII, and hpt and confirmed by Southern blot hybridization analysis. There was no GUS expression in T₀ transgenic plants transformed with gus under p35S. However the GUS expression was detectable in the progeny of some lines. Only 30% of 46 transgenic lines showed Mendelian segregation of GUS expressing to GUS not expressing plants. In the remaining 70% the segregation was non-Mendelian and the rate was much lower than 3:1. Factors that might effect expression of transgenes in allohexaploid monocot species are discussed.     

Selective Embryo Abortion Hypothesis Revisited - A Molecular Approach

Abstract: Many plant species abort a large fraction of their embryos. It has often been suggested that embryos of genotypes that would perform worse later in life are preferentially aborted. Such selective embryo abortion would lead to investment of resources only in the offspring with the highest potential fitness. Many studies have shown that otherwise viable embryos are aborted. However, only few manipulative studies have indeed shown a correlation between the level of abortion and offspring quality and these studies have been challenged for their experimental design. Molecular techniques open new opportunities to study selective embryo abortion. Non-random abortion at the level of molecular markers can be observed as a deviation from Mendelian segregation: over- or under-representation of markers in the offspring. Subsequently, the over- or under-represented markers can be related to offspring quality later in life. We reviewed the literature on the genetic maps of intraspecific crosses of wild plant species and the selection of cultivated species. The level of non-Mendelian segregation we found in these maps is high. On average, 11.5 % of the tested markers in the genetic maps of wild species and 14.6 % in the cultivated ones, show a departure from Mendelian segregation. From six studies, providing sufficient data, it was calculated that in 68 % of loci segregating in non-Mendelian fashion post-fertilization selection is involved. We propose that the deviation from Mendelian segregation can be partly explained by selective embryo abortion. We describe an experimental design that allows for attributing selective embryo abortion to the non-Mendelian segregation that is found in a genetic map.     

Analyses of Phaseolus vulgaris L. and P. coccineus Lam. hybrids by RFLP: preferential transmission of P. vulgaris alleles

Summary Restriction fragment length polymorphism (RFLP) was determined among P. vulgaris genotypes and Phaseolus species using 19 probes. The incidence of polymorphism was high (70–86%) between species, but relatively low (22–26%) between genotypes of P. vulgaris. Suitable probes were identified for the analysis of P. vulgaris and P. coccineus hybrids. The segregation pattern in F₂ populations was Mendelian for two probes (LHB and VEE20) and non-Mendelian for GS-g, CHS, and CHI. Statistical analyses indicated gametic selection with preferential transmission of the P. vulgaris alleles, which may account for the selective recovery of P. vulgaris progeny types observed earlier. The available hybrids of P. vulgaris and P. coccineus and the high degree of interspecific RFLP will facilitate the construction of a linkage map for Phaseolus.     

Impact of the stringency of cell selection on plastid segregation in protoplast fusion-derived Nicotiana regenerates

Summary Vegetative segregation of a mixed plastid population in protoplast fusion-derived cell lines can be directed by a selection favouring the multiplication of one of the parental plastid types. This report defines some of the critical conditions leading to a homogeneous plastid population in cybrid plants generated by protoplast fusion between Nicotiana plumbaginifolia and an albino and streptomycin-resistant N. tabacum plastid mutant. Light (1,500 lx) conferred a strong selective advantage to chloroplasts versus albino plastids, while the lack of this effect in dim light (300 lx) indicated that a sufficient light intensity is essential to the phenomenon. Selection on streptomycin-containing medium in the dark, however, led to the preferential multiplication of resistant plastids. Streptomycin selection of resistant chloroplasts in the light, consequently, results in a plastid selection of doubled stringency. In another experiment a definite, but leaky, selection for chloroplast recombination (selection for greening on streptomycin-containing medium in dim light) was used to reveal various recombination products. Protoplast fusion in fact resulted in cybrid plants showing only simple chimeric segregation of unchanged parental plastids. These results demonstrate the essential requirement for stringent plastid selection, as defined by cell culture conditions, to precede the formation of shoots expected to possess the desired plastid genetic composition.     

On the relation between the instability of ESS in discrete dynamics and segregation distortion

There is a simple correspondence between discrete dynamical systems associated with evolutionary game dynamics and general locus multiallele selection models with non-Mendelian segregation. When interpreted properly the payoff matrix has two components, a fitness matrix component and a segregation matrix component. The presence of segregation distortion which corresponds to a non-symmetric payoff matrix, is a source of instability. With non-symmetric payoff an ESS does not usually correspond to a stable equilibrium. It is always externally stable but does not necessarily have an internally stable equilibrium.     

Transgene inheritance in plants

The patterns of transgene inheritance in plants and the possible explanations for non-Mendelian transmission are reviewed. The non-Mendelian inheritance of a transgene has been recorded with a frequency between 10% and 50% in transgenic plants produced either by Agrobacterium-mediated transformation or through particle bombardment. Different effects such as deletion, duplication, rearrangement, repeated sequence recombination as well as gene interaction have been observed for transgenic loci. The nature of the recipient genome, nature of the transgene and the interactions between them seem to contribute to the non-Mendelian segregation of transgenes.     

Stochastic selection in both haplophase and diplophase

Summary A population genetic model incorporating the evolutionary forces of zygotic selection, gametic selection and non-Mendelian segregation has been analyzed for the case in which all selection coefficients and the segregation parameter are assumed to be random variables that are uncorrelated from generation to generation. The diffusion approximation of the model is developed, and the subsequent analysis shows that one of four limiting outcomes of the stochastic process may obtain — an allele may be fixed or lost almost surely and irrespective of the initial gene frequency, the gene frequency may converge to a unique stationary distribution, or an allele may be fixed or lost with probabilities depending on the initial gene frequency. These outcomes correspond rather closely with the possible outcomes of the deterministic model — fixation or loss of an allele, convergence to a stable equilibrium, or the existence of an unstable equilibrium.Work supported by N. I. H. grants GM21732 and GM21623. The author is supported by Research Career Award GM2301.     

Induction of streptomycin resistance in the wild tomato Lycopersicon peruvianum

Summary A protoplast mutagenesis and cell selection system was used for the isolation of streptomycin resistant Lycopersicon peruvianum colonies. Protoplasts were treated with the mutagen N-nitroso-methylurea and could be regenerated into fertile plants, carrying the streptomycin resistant character. Several classes of streptomycin resistance could be distinguished. Reciprocal crosses between streptomycin resistant and sensitive plants showed a non-Mendelian transmission of the resistance trait. Streptomycin resistance is the first selectable and maternally inherited cell organelle marker described in tomato.     

Evidence of Positive Darwinian Selection in Putative Meningococcal Vaccine Antigens

Meningococcal meningitidis is a life-threatening disease. In Europe and the United States the majority of cases are caused by virulent meningococcal strains belonging to serogroup B. Presently there is no effective vaccine against serogroup B strains, as traditional vaccine antigens such as polysaccharide capsules are unusable as they lead to autoimmunity. The year 2000 saw the publication of the complete genome of Neisseria meningitidis MC58, a virulent serogroup B bacterium. Working in conjunction with the sequencing project, researchers endeavored to locate highly conserved membrane-associated proteins that elicit an immune response. It is hoped that these proteins will provide a basis for novel vaccines against serogroup B strains. A number of potential vaccine antigens have been located and are presently in phase I clinical trials. Recently many reports pertaining to the evidence of positive Darwinian selection in membrane proteins of pathogens have been reported. This study utilized in silico methods to test for evidence of historical positive Darwinian selection in seven such vaccine candidates. We found that two of these proteins show signatures of adaptive evolution, while the remaining proteins show evidence of strong purifying selection. This has significant implications for the design of a vaccine against serogroup B strains, as it has been shown that vaccines that target epitopes that are under strong purifying selection are better than those that target variable epitopes.[Reviewing Editor: Rasmus Nielsen]     

Cytoplasmic inheritance of antimycin A resistance in Saccharomyces cerevisiae

Summary Three antimycin resistant mutants of Saccharomyces cerevisiae are characterized genetically. The mutations have been shown to be cytoplasmically inherited by four criteria. The phenotype persists in diploids formed by a cross with a ⁰ strain of yeast of the opposite mating type. Diploids heterozygous for the antimycin marker, however, show segregation of the resistance and sensitivity during mitosis. Tetrad analysis indicated a non-Mendelian segregation (4:0 and 0:4) of the mutations. The antimycin marker can be eliminated by ethidium bromide treatment under conditions that should have deleted all of the mitochondrial DNA.     

A quantitative genetics model for viability selection

Viability selection will change gene frequencies of loci controlling fitness. Consequently, the frequencies of marker loci linked to the viability loci will also change. In genetic mapping, the change of marker allelic frequencies is reflected by the departure from Mendelian segregation ratio. The non-Mendelian segregation of markers has been used to map viability loci along the genome. However, current methods have not been able to detect the amount of selection (s) and the degree of dominance (h) simultaneously. We developed a method to detect both s and h using an F2 mating design under the classical fitness model. We also developed a quantitative genetics model for viability selection by proposing a continuous liability controlling the viability of individuals. With the liability model, mapping viability loci has been formulated as mapping quantitative trait loci. As a result, nongenetic systematic environmental effects can be easily incorporated into the model and subsequently separated from the genetic effects of the viability loci. The quantitative genetic model has been verified with a series of Monte Carlo simulation experiments.     

Induction of streptomycin-resistant plantlets in <Emphasis Type="Italic">Solanum surattense</Emphasis> through <Emphasis Type="Italic">in vitro</Emphasis> mutagenesis

The species Solanum surattense Burm.f. has importance in ayurvedic medicine and also as vegetable. Streptomycin-resistant plantlets were induced showing chloroplast encoded mutants in S. surattense from mutagenised (ethyl methane sulphonate and gamma-rays) cotyledon explants. Chloroplast encoded – streptomycin resistant – shoots were developed from green (unbleached) sectors of the cotyledons. The streptomycin-resistant plants were similar to parental plants in morphology and ploidy level (2n=2x=24). Reciprocal crosses between streptomycin-resistant and the original streptomycin sensitive plants have shown the non-Mendelian transmission under the control of chloroplast – DNA. These antibiotic resistant plants are useful in designing biochemical selection schemes aimed at somatic hybrid/cybrid recovery in S. surattense.     

Artificial selection for 18-day pupa weight and opposing simulated naturel selection in Tribolium castaneum

Summary The effect of simulated opposing natural selection on the response to mass selection for 18-day pupa weight of Tribolium castaneum was studied for 10 generations of selection. Natural selection was simulated in replicated treatment lines by imposing a negative relationship between mid-parent genetic value for pupa weight and fertility. Responses to selection and realized heritabilities were smaller (P < 0.05 and P < 0.10, respectively) for the treatment lines than for control lines under selection for pupa weight only. One treatment, line E3G1, reached an intermediate selection plateau by generation 10, and responded linearly to 4 generations of artificial selection after natural selection had been discontinued. Possible explanations for the different behaviors of the replicate lines E3G1 and E3G2 were also discussed.     

Modeling segregation distortion for viability selection I. Reconstruction of linkage maps with distorted markers

Molecular markers have been widely used to map quantitative trait loci (QTL). The QTL mapping partly relies on accurate linkage maps. The non-Mendelian segregation of markers, which affects not only the estimation of genetic distance between two markers but also the order of markers on a same linkage group, is usually observed in QTL analysis. However, these distorted markers are often ignored in the real data analysis of QTL mapping so that some important information may be lost. In this paper, we developed a multipoint approach via Hidden Markov chain model to reconstruct the linkage maps given a specified gene order while simultaneously making use of distorted, dominant and missing markers in an F₂ population. The new method was compared with the methods in the MapManager and Mapmaker programs, respectively, and verified by a series of Monte Carlo simulation experiments along with a working example. Results showed that the adjusted linkage maps can be used for further QTL or segregation distortion locus (SDL) analysis unless there are strong evidences to prove that all markers show normal Mendelian segregation.     

RESPONSE TO SELECTION ON AUTOGAMY IN PHLOX

Two cycles of artificial selection were performed to increase autogamous fruiting in two wild populations of the self-incompatible Phlox drummondii, to decrease autogamous fruiting in two wild populations of the self-compatible Phlox cuspidata, and to both increase and decrease autogamous fruiting in a cultivar of P. drummondii which is pseudo-self-compatible. The breeding systems were determined to be genetically quite flexible, independent of inbreeding depression and other genetic phenomena which could hinder a breeding system shift. This is especially true for increasing autogamy. Self-pollen-pistil compatibility seems to be the single character affected by selection. Based on the continuous variation in both autogamy and self-compatibility, we suggest that the change has been due to genes which modify the self-incompatibility reaction rather than to the simple segregation of alleles at the S-locus.     

In planta transformation of Arabidopsis thaliana

Transformants of Arabidopsis thaliana can be generated without using tissue culture techniques by cutting primary and secondary inflorescence shoots at their bases and inoculating the wound sites with Agrobacterium tumefaciens suspensions. After three successive inoculations, treated plants are grown to maturity, harvested and the progeny screened for transformants on a selective medium. We have investigated the reproducibility and the overall efficiency of this simple in planta transformation procedure. In addition, we determined the T-DNA copy number and inheritance in the transformants and examined whether transformed progeny recovered from the same Agrobacterium-treated plant represent one or several independent transformation events. Our results indicate that in planta transformation is very reproducible and yields stably transformed seeds in 7–8 weeks. Since it does not employ tissue culture, the in planta procedure may be particularly valuable for transformation of A. thaliana ecotypes and mutants recalcitrant to in vitro regeneration. The transformation frequency was variable and was not affected by lower growth temperature, shorter photoperiod or transformation vector. The majority of treated plants gave rise to only one transformant, but up to nine siblings were obtained from a single parental plant. Molecular analysis suggested that some of the siblings originated from a single transformed cell, while others were descended from multiple, independently transformed germ-line cells. More than 90% of the transformed progeny exhibited Mendelian segregation patterns of NPTII and GUS reporter genes. Of those, 60% contained one functional insert, 16% had two T-DNA inserts and 15% segregated for T-DNA inserts at more than two unlinked loci. The remaining transformants displayed non-Mendelian segregation ratios with a very high proportion of sensitive plants among the progeny. The small numbers of transformants recovered from individual T₁ plants and the fact that none of the T₂ progeny were homozygous for a specific T-DNA insert suggest that transformation occurs late in floral development.National Research Council of Canada Publication No. 38003     

Genetic and molecular analysis of Arabidopsis thaliana (ecotype ‘Estland’) transformed with Agrobacterium

Summary Agrobacterium-mediated transformation of Arabidopsis, ecotype ‘Estland’, was established from root explants using kanamycin selection. Continuous light during callus and shoot induction phases was promotive for shoot regeneration, as compared to light/dark cycles. Use of optimized conditions for transformation led to the formation of kanamycin-resistant calluses (up to 77%) and transformed plantlets at a frequency of up to 45%. Southern analysis showed the presence of 1.2. or more T-DNA inserts in 33%, 50%, and 17% of the primary transformants, respectively. Mendelian, as well as non-Mendelian, inheritance patterns were obtained upon screening the progeny (T1) of various transformants for the expression of gus and nptII genes; the analysis of some of these transformants at the molecular level also corroborated the Mendelian inheritance pattern. Moreover, genotypes of the T1 progeny could be predicted on the basis of T2 progeny analysis.