Intragenic recombination in maize: pollen analysis methods and the effect of parental adh1 isoalleles

Intragenic Meiotic Crossovers Generate Novel Alleles with Transgressive Expression Levels

Meiotic recombination is an evolutionary force that generates new genetic diversity upon which selection can act. Whereas multiple studies have assessed genome-wide patterns of recombination and specific cases of intragenic recombination, few studies have assessed intragenic recombination genome-wide in higher eukaryotes. We identified recombination events within or near genes in a population of maize recombinant inbred lines (RILs) using RNA-sequencing data. Our results are consistent with case studies that have shown that intragenic crossovers cluster at the 5′ ends of some genes. Further, we identified cases of intragenic crossovers that generate transgressive transcript accumulation patterns, that is, recombinant alleles displayed higher or lower levels of expression than did nonrecombinant alleles in any of ∼100 RILs, implicating intragenic recombination in the generation of new variants upon which selection can act. Thousands of apparent gene conversion events were identified, allowing us to estimate the genome-wide rate of gene conversion at SNP sites (4.9 × 10⁻⁵). The density of syntenic genes (i.e., those conserved at the same genomic locations since the divergence of maize and sorghum) exhibits a substantial correlation with crossover frequency, whereas the density of nonsyntenic genes (i.e., those which have transposed or been lost subsequent to the divergence of maize and sorghum) shows little correlation, suggesting that crossovers occur at higher rates in syntenic genes than in nonsyntenic genes. Increased rates of crossovers in syntenic genes could be either a consequence of the evolutionary conservation of synteny or a biological process that helps to maintain synteny.

Bioenergetics of Monoterpenoid Essential Oil Biosynthesis in Nonphotosynthetic Glandular Trichomes

The commercially important essential oils of peppermint (Mentha × piperita) and its relatives in the mint family (Lamiaceae) are accumulated in specialized anatomical structures called glandular trichomes (GTs). A genome-scale stoichiometric model of secretory phase metabolism in peppermint GTs was constructed based on current biochemical and physiological knowledge. Fluxes through the network were predicted based on metabolomic and transcriptomic data. Using simulated reaction deletions, this model predicted that two processes, the regeneration of ATP and ferredoxin (in its reduced form), exert substantial control over flux toward monoterpenes. Follow-up biochemical assays with isolated GTs indicated that oxidative phosphorylation and ethanolic fermentation were active and that cooperation to provide ATP depended on the concentration of the carbon source. We also report that GTs with high flux toward monoterpenes express, at very high levels, genes coding for a unique pair of ferredoxin and ferredoxin-NADP+ reductase isoforms. This study provides, to our knowledge, the first evidence of how bioenergetic processes determine flux through monoterpene biosynthesis in GTs.

The rise of the angiosperms: a genecological factor

In the primitive angiosperms, closed carpels are believed to have evolved as protection for ovules, which would otherwise be injured by animal pollinators. The hypothesis is presented that, whatever the origin and other functions of angiosperms, insect pollination and closed carpels may, in combination, reduce the influence of random variation on pollen tube competition, thus enhancing the ability of natural selection to act on the gametophytic phase of the life cycle. The microgametophytic phase represented by vast numbers of haploid individuals can then serve, by insect pollination and closed carpels, as a screen against any genome not functioning with a high degree of metabolic vigor. Poorly balanced genomes could thus be eliminated at relatively little cost. Insect-pollinated angiosperms would therefore benefit from positive aspects of sexual recombination. Such a system may have allowed the angiosperms to undergo their rise to dominance.

Radiation-induced alcohol dehydrogenase mutants in maize following allyl alcohol selection of pollen

The alcohol dehydrogenase-1 gene in maize presents advantages for mutational analysis. Foremost among these is the ability to chemically select ADH-negative and ADH-low gametophytes owing to their resistance to allyl alcohol vapour. Immature tassels were irradiated with either 220 kV X-rays or 400 MeV/amu accelerated neon-ions; spontaneous mutants were also selected and recovered. RBE for neon-20 was about 5. A total of 70 presumptive mutants were placed into one of four classes on the basis of allozyme profiles following electrophoresis and ADH staining: (A) dysfunction, (B) underproducer, (C) overproducer, and (D) up-Adh2 gene. Mutants have been recovered and confirmed in the first three classes. These include two male-transmissible deletion-type lesions induced by X-rays, five underproducer transpositions and one overproducer transposition induced by neon-20. Certain of the neoninduced alleles are unstable in their expression. All 70 mutants are chromosomal aberrations; no intragenic lesions were recovered although our experimental design would have preferentially recovered them if they had occurred.

The Discussion considers the mutagenic action of ionizing radiation, and especially the well-documented differences between maize and Drosophila data. In particular, the effect of these chromosome derangements on the ‘programmable’ component(s) of the Adhl cistron is discussed.

Genetic fine-structure of the GA-1 locus in the higher plant Arabidopsis thaliana (L.) Heynh

Non-germinating gibberellin (GA) responsive mutants are a powerful tool to study genetic fine structure in higher plants. Nine alleles (EMS-and fast neutron-induced) of the ga-1 locus of Arabidopsis thaliana were tested in a complete half-diallel. No wild type ‘recombinants’ were found in the selfed progeny of 9 homoallelic combinations (in total 3 × 105 plants); in the progenies from the 36 selfed hetero allelics the wild type frequency ranged from zero to 6·6 × 10−4. These frequencies allowed the construction of an internally consistent map for five different sites representing eight alleles. The ninth allele covered three sites and thus behaved like an intragenic deletion. The estimate of the total genetic length of the ga-1 locus was 0·07 cM. The order of the sites was also clearly reflected by the association with proximal outside markers. On the assumption that wild type gametes predominantly arise from reciprocal events, it was shown that a cross-over within the ga-1 locus leads to positive interference in the adjacent region.

The results are discussed with respect to the mutagen used, the frequencies found in other plant and Drosophila genes, and the possible occurrence of gene conversion.

LineUp: statistical detection of chromosomal homology with application to plant comparative genomics

The identification of homologous regions between chromosomes forms the basis for studies of genome organization, comparative genomics, and evolutionary genomics. Identification of these regions can be based on either synteny or colinearity, but there are few methods to test statistically for significant evidence of homology. In the present study, we improve a preexisting method that used colinearity as the basis for statistical tests. Improvements include computational efficiency and a relaxation of the colinearity assumption. Two algorithms perform the method: FullPermutation, which searches exhaustively for runs of markers, and FastRuns, which trades faster run times for exhaustive searches. The algorithms described here are available in the LineUp package (http://www.igb.uci.edu/ approximately baldig/lineup). We explore the performance of both algorithms on simulated data and also on genetic map data from maize (Zea mays ssp. mays). The method has reasonable power to detect a homologous region; for example, in >90% of simulations, both algorithms detect a homologous region of 10 markers buried in a random background, even when the homologous regions have diverged by numerous inversion events. The methods were applied to four maize molecular maps. All maps indicate that the maize genome contains extensive regions of genomic duplication and multiplication. Nonetheless, maps differ substantially in the location of homologous regions, probably reflecting the incomplete nature of genetic map data. The variation among maps has important implications for evolutionary inference from genetic map data.

Analysis of recombination sites within the maize waxy locus

Genetic fine structure analysis of the maize wx locus has determined that the ratio of genetic to physical distance within wx was one to two orders of magnitude higher than the average for the maize genome. Similar results have been found at other maize loci. In this study, we examined several mechanisms that could account for this pattern. First, crossovers in two other maize genes resolve preferentially at specific sites. By mapping exchanges between wx-B1 and wx-I relative to a polymorphic SstI site, we found no evidence for such a hotspot at wx. Second, deletion of promoter sequences from wx alleles had little effect on recombination frequencies, in contrast to results in yeast where promoter sequences are important for initiating recombination in some genes. Third, high levels of insertion polymorphism may suppress intergenic recombination. However, the presence of a 2-kb Ds element 470 bp upstream of the wx transcription start site did not further suppress recombination between Ds insertions in nearby wx sequences. Thus, none of these mechanisms is sufficient to explain the difference between intergenic and intragenic recombination rates at wx.

Structure, expression, chromosomal location and product of the gene encoding Adh2 in Petunia

In most higher plants the genes encoding alcohol dehydrogenase comprise a small gene family, usually with two members. The Adh1 gene of Petunia has been cloned and analyzed, but a second identifiable gene was not recovered from any of three genomic libraries. We have therefore employed the polymerase chain reaction to obtain the major portion of a second Adh gene. From sequence, mapping and northern data we conclude this gene encodes ADH2, the major anaerobically inducible Adh gene of Petunia. The availability of both Adh1 and Adh2 from Petunia has permitted us to compare their structures and patterns of expression to those of the well-studied Adh genes of maize, of which one is highly expressed developmentally, while both are induced in response to hypoxia. Despite their evolutionary distance, evidenced by deduced amino acid sequence as well as taxonomic classification, the pairs of genes are regulated in strikingly similar ways in maize and Petunia. Our findings suggest a significant biological basis for the regulatory strategy employed by these distant species for differential expression of multiple Adh genes.

Genetic and molecular characterization of an Adh-1 null mutant in tomato

Treatment of tomato seeds with ethyl methanesulphonate (EMS) followed by allyl alcohol selection of M2 seeds has led to the identification of one plant (B15-1) heterozygous for an alcohol dehydrogenase (Adh) null mutation. Genetic analysis and expression studies indicated that the mutation corresponded to the structural gene of the Adh-1 locus on chromosome 4. Homozygous Adh-1 null mutants lacked ADH-1 activity in both pollen and seeds. Using an antiserum directed against ADH from Arabidopsis thaliana, which cross-reacts with ADH-1 and ADH-2 proteins from tomato, no ADH-1 protein was detected in seeds of the null mutant. Northern blot analysis showed that Adh-1 mRNA was synthesized at wild-type levels in immature seeds of the null mutant, but dropped to 25% in mature seeds. Expression of the Adh-2 gene on chromosome 6 was unaffected. The potential use of the Adh-1 null mutant in selecting rare transposon insertion mutations in a cross with "mutable" Adh-1+ tomato lines is discussed.

Ethanol-resistant mutants of Nicotiana plumbaginifolia are deficient in the expression of pollen and seed alcohol dehydrogenase activity

Six independent mutant lines of Nicotiana plumbaginifolia resistant to ethanol, designated E3, E8, E101, E112, E144 and E251, were isolated as germinating seedlings on selective medium. In all cases, resistance to ethanol was conferred by a single recessive nuclear mutation at the same locus. Mutant seeds and pollen lacked detectable ADH activity, with the exception of E251 where a residual activity was detected. An antiserum directed against Arabidopsis thaliana ADH detected an ADH-related polypeptide of 44 kDa present in wild-type seeds and, to a lesser extent, in the seeds of the leaky mutant E251. No ADH-related polypeptide could be detected in seeds of the other mutants. However, all of them had a nearly normal level of ADH mRNA except one which did not synthesize any mRNA. These results suggest that these ethanol-resistant mutants are impaired in one of the structural genes coding for alcohol dehydrogenase. The corresponding locus has been designated Adh1.

Characterization of a gene family abundantly expressed in Oenothera organensis pollen that shows sequence similarity to polygalacturonase

We have isolated and characterized cDNA clones of a gene family (P2) expressed in Oenothera organensis pollen. This family contains approximately six to eight family members and is expressed at high levels only in pollen. The predicted protein sequence from a near full-length cDNA clone shows that the protein products of these genes are at least 38,000 daltons. We identified the protein encoded by one of the cDNAs in this family by using antibodies to beta-galactosidase/pollen cDNA fusion proteins. Immunoblot analysis using these antibodies identifies a family of proteins of approximately 40 kilodaltons that is present in mature pollen, indicating that these mRNAs are not stored solely for translation after pollen germination. These proteins accumulate late in pollen development and are not detectable in other parts of the plant. Although not present in unpollinated or self-pollinated styles, the 40-kilodalton to 45-kilodalton antigens are detectable in extracts from cross-pollinated styles, suggesting that the proteins are present in pollen tubes growing through the style during pollination. The proteins are also present in pollen tubes growing in vitro. Both nucleotide and amino acid sequences are similar to the published sequences for cDNAs encoding the enzyme polygalacturonase, which suggests that the P2 gene family may function in depolymerizing pectin during pollen development, germination, and tube growth. Cross-hybridizing RNAs and immunoreactive proteins were detected in pollen from a wide variety of plant species, which indicates that the P2 family of polygalacturonase-like genes are conserved and may be expressed in the pollen from many angiosperms.

Tagging of a maize gene involved in kernel development by an activated Uq transposable element

A quiescent Uq transposable element has been activated in a maize plant treated with 5-aza-2'-deoxycytidine. This activated Uq cosegregates with a heritable dominant miniature (Mn) kernel phenotype, indicating its physical association with a maize miniature locus (Mn::Uq). The Mn::Uq mutant is dominant in producing a miniature seed phenotype of variable size and in reducing seedling vigor in the early growth stage. Genetic experiments indicate that the Mn::Uq mutant also affects the activity of the male gametophyte, whereby pollen germination is inhibited, thus lacking pollen tube growth resulting in the male nontransmissibility of this mutant. Proof for the Uq element in this mutant is derived by its ability to transactivate the standard a-ruq reporter allele to yield spotted aleurone tissue. However, the Mn::Uq mutant does not transactivate a normally Uq-responsive c-ruq allele, suggesting a structural difference between the two ruq receptors at the A1 and C1 loci. It is anticipated that cloning of the Uq transposable element would facilitate the molecular cloning and characterization of the maize miniature gene.

Stability of deletion, insertion and point mutations at the bronze locus in maize

Phenotypic revertants from several kinds of mutations, including deletions, have been detected by pollen analysis at the wx and Adh loci in maize. Mutations in these genes give phenotypic revertants with median frequencies of 0.7 and 0.5×10(-5), respectively. However, the nature of such revertants can only be analyzed following their recovery from conventional matings. In the current study large seed populations derived from crosses involving several bz (bronze) locus mutations in maize were examined for reversion to a Bz (purple) expression. Deletion, insertion and point mutations were included in the study. Principally, over 2 million gametes of the bz-R mutation, which is shown here to be associated with a 340 base pair deletion within the transcribed region of the gene, have been screened for reversion. No revertants from it or any of the other bz mutations have been recovered, even though a total of almost 5 million gametes from homoallelic crosses have been examined to date. Results from seed analysis are discussed in reference to those from pollen analysis in maize.

The extent of gametophytic-sporophytic gene expression in maize

To determine the extent of gametophytic gene expression and the type of transcription, haploid or haplo-diploid, of the genes, isozymes were used as genetic markers. Fifteen enzymatic systems, including thirty-four isozymes, were studied. The determination of the type of expression of genes coding for multimeric enzymes was based on the comparison of electrophoretic patterns of pollen and of sporophytic tissues from plants heterozygous for electrophoretic mobility: if gene expression in pollen is of a gametophytic (haploid) origin, pollen, unlike the sporophyte, would reveal only the parental homomultimeric bands. The enzymes analyzed can be grouped in three categories according to type of gene expression: i) enzymes present in both pollen and sporophyte, coded by the same gene with haplo-diploid expression; ii) enzymes controlling analogous functions in pollen and sporophyte, coded by different genes, expressed in only one of the two phases; iii) enzymes present in two or more forms in the sporophyte and only in one form in the gametophyte. The data allow the proportion of haplo-diploid gene expression in the loci examined to be estimated at 0.72; 0.22 and 0.06 being the proportions attributable to the sporophytic and gametophytic domains, respectively.

Onset of Alcohol Dehydrogenase Synthesis during Microsporogenesis in Maize

During male gametophyte development the synthesis of several proteins occurs from transcripts of the haploid genome. Alcohol dehydrogenase (ADH1), a developmentally regulated protein, was chosen for study to determine the stage at which its synthesis was initiated and the pattern of its synthesis during microsporogenesis. The ability of ADH to reduce p-nitro blue tetrazolium chloride in situ was used as an indicator of enzyme activity. Maize strains heterozygous for adh1 were utilized to provide an internal control, 50% of the grains being adh1(+) and 50% being adh1(-). No ADH activity was detectable when tetrads were first formed after meiosis. Activity was initially detected soon after the tetrads began to break apart but before the microspores in the tetrads had completely separated. The transcription of the adh1 gene from the haploid genome must thus occur very soon after meiosis is completed. ADH activity increases at a constant rate thereafter until microspore mitosis when an increase in the rate takes place which lasts until generative cell division. Thereafter, there is a marked decrease in the rate of accumulation of ADH activity.

Initiation of Postmeiotic beta-Galactosidase Synthesis during Microsporogenesis in Oilseed Rape

The synthesis of beta-galactosidase during Brassica campestris pollen development results from the transcription of the haploid genome. A quantitative cytochemical method has been developed in which 5-bromo-4-chloro-3-indoxyl-beta-d-galactoside is used as substrate giving a blue-green final reaction product. We have recently detected oilseed rape plants which are heterozygous for the beta-galactosidase locus, in which 50% of the pollen grains produced are Gal (having enzyme activity), while the other 50% are gal (enzyme deficient). The gal pollen grains served as a built-in control during microspectrophotometric determinations of enzyme activity. The present study has identified the developmental phase at which synthesis of the enzyme commenced. Activity is absent in microsporocytes, tetrads, and at microspore release. Enzyme activity is first detected in the young microspores and, by early vacuolate period, there is an increase in the rate of enzyme activity. A second period of increased enzyme synthesis occurred prior to generative cell division, although the rate is reduced in mature pollen.

Polymorphism of alcohol dehydrogenase in Arabidopsis thaliana (L.) Heynh.: genetical and biochemical characterization

Zymograms of Arabidopsis alcohol dehydrogenase (ADH; EC 1.1.1.1) show a unique anodal migrating band. Three electrophoretic variants were identified among geographical races and designated slow (S), fast (F), and superfast (A), according to their mobility on Tris-citrate starch gels. In plants ADH activity is confined mainly to pollen, seeds, and grains and rapidly declines during the germination process. In callus and suspension cultures, growing on media containing 2,4-D, ADH appeared as one of the major polypeptides. Genetical analysis indicated that the three types of ADH isozymes are under the control of one gene with three alles (Adh1S, Adh1F, Adh1A), showing codominant expression. Crosses between the electrophoretic types and dissociation-reassociation experiments showed that the Arabidopsis enzyme behaves as a dimer, like ADH from most other species. The molecular weight of the enzyme has been estimated by gel filtration and by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis to be 87,000. The pH optimum for the oxidation of ethanol is 9.0 and two optima for reduction of acetaldehyde have been obtained, 6.0 and 8.5, respectively. The enzyme exhibits a wide substrate specificity for alcohols and is relatively heat resistant.

Extracellular conditions affecting the induction of yeast alcohol dehydrogenase II

The alcohol dehydrogenases in yeast form one of the best-understood eukaryotic regulatory systems at the genetic level, but very little is known about their regulation at the biochemical level. We report on a simple whole-cell assay system for the induction of the inducible isozyme, alcohol dehydrogenase II, which has been used to show that no single compound added to the medium is responsible for the induction. The compounds which show the greatest inducing activity--malate, glutamate and fumarate--are all directly or indirectly involved in mitochondrial transport systems. No single compound can be purified from extracts of yeast cells to give inducing activity at low concentrations, suggesting that the inducing activity is an endogenous function of the cell. Tentative models for regulation of this isozyme involving the mitochondrion are proposed, and suggestions are made for testing these models further.

Haplodiploid gene expression in maize and its detection

A method for the demonstration of the gametophytic origin of genetic variability in maize is described. For genes coding dimeric or multimeric enzymes, haploid expression can be demonstrated by means of translocations between A and B chromosomes (TB-A), which make it possible to obtain hyperploid pollen grains, partially diploid and heterozygous for electrophoretic mobility. Comparison of the electrophoretic pattern of this pollen type (three bands) and that of normal grains produced by a heterozygous F/S plant (two bands only) reveals haploid transcription of the monomeric forms. The procedure was tested on ADH-1 and used to demonstrate haploid expression for GOT-1. The data obtained suggest, moreover, that the reduction in male gamete transmission of duplications may be due to differences in pollen competitive ability rather than to processes affecting microspore maturation.

Embryo-lethal mutants of Arabidopsis thaliana: Evidence for gametophytic expression of the mutant genes

Normal and aborted seeds from two recessive embryo-lethal mutants (79A and 124D) of Arabidopsis thaliana were shown to be distributed nonrandomly along the length of heterozygous siliques. Significantly more than half of the aborted seeds in these two mutants were located in the top half of the silique, in the region closest to the stigma surface. Segregation ratios (percent aborted seeds) were unusually low at the base of the silique, and slightly higher than expected at the tip. In contrast, aborted seeds from four other embryo-lethal mutants (87A, 123B, 50B, and 71E) were distributed randomly along the length of the silique. These results suggest that the mutant genes in 79A and 124D are expressed during both the gametophytic (n) and sporophytic (2n) phases of development. These two mutants provide further evidence for the hypothesis that many genes expressed prior to fertilization also perform a critical function during growth and development of the sporophyte. Embryo-lethal mutants of Arabidopsis may therefore be useful in future studies of gametophytic gene expression and the regulation of pollen-tube growth in higher plants.

Maize pollen test systems to detect nondisjunction

Three pollen test systems to detect the induction of nondisjunction in maize are actively being explored. (1) Each member of a tetrad of haploid microspores produced by meiosis contains a single chromosome 6 which carries the only nucleolar organizing region in the maize genome. Thus, each member of a normal tetrad contains one nucleolus. If nondisjunction took place at the first or second meiotic division, 2:2:0:0 or 2:1:1:0 tetrads would be produced respectively. (2) If a male parent carrying a dominant endosperm marker is crossed by a female carrying a recessive allele of this gene, all normal kernels would be heterozygous and would express the dominant phenotype in their endosperm. If nondisjunction of the chromosome carrying this gene took place at the second microspore division and the nullisomic and disomic sperm fertilized the polar nuclei and egg of a given embryo sac respectively, an exceptional kernel is produced which would express the recessive endosperm phenotype and contain a trisomic embryo. (3) Complementing null mutations of genes expressed in individual pollen grains can be utilized to detect nondisjunction. Normal haploid pollen grains from plants heterozygous for two complementing null alleles of a locus would each express the recessive phenotype. If nondisjunction took place during either meiotic division, disomic pollen grains containing both alleles could be produced expressing the dominant phenotype due to complementation. We are exploring this test system utilizing appropriate alcohol dehydrogenase mutants.

Pollen genetic markers for detection of mutagens in the environment

To utilize and exploit pollen for in situ mutagen monitoring, screening and toxicology, the range of genetic traits in pollen must be identified and analyzed. Traits that can be considered include ornamentation, shape and form, male sterility viability, intraspecific incompatibility, proteins and starch deposition. To be useful for the development of mutagen detection systems proteins should be: (1) activity stainable or immunologically identifiable in the pollen, (2) the products of one to three loci, and (3) gametophytic and nuclear in origin. Several proteins including alcohol dehydrogenase in maize, which meet those criteria will be discussed. The waxy locus in barley and maize which controls starch deposition has been characterized genetically and methods have been developed for pollen screening and mutant detection. At Washington State University a waxy pollen system is being developed in barley for in situ mutagen monitoring. The basis is an improved method for staining and scoring waxy pollen mutants. Specific base substitution, frameshift, and deletion mutant lines are being developed to provide information about the nature of the mutations induced by environmental mutagens. Thirty waxy mutant lines, induced by sodium azide and gamma-rays have been selected and are being characterized for spontaneous and induced reversion frequencies, allelism, karyotype, amylose content, and UDP glucose glucosyltransferase (waxy gene product) activity. Twelve mutant alleles are being mapped by recombinant frequencies.

Mutational study of the alcohol dehydrogenase-1 FCm duplication in maize

The alcohol dehydrogenase-1 FCm (Adh-FCm) duplication in maize was subjected to ethyl methanesulfonate (EMS) mutagenesis. Of the mutants recovered, eight produced ADH polypeptides with altered electrophoretic mobility. Four produced new mobilities of the progenitor F with no change of the Cm molecule; the remainder altered only the Cm enzyme. No cases were found in which the electrophoretic mobilities of the two types of subunits were simultaneously altered, and no complete nulls lacking both F and Cm were recovered. These observations confirm the duplicate nature of the FCm complex.

Spontaneous forward mutation versus reversion frequencies for maize Adh1 in pollen

Reliable quantitative data on spontaneous, specific gene mutation frequencies in higher plants and animals are few. Detergents include low natural frequencies and difficulty in obtaining in excess of 10(6) scorable organisms or gametophytes. The alcohol dehydrogenase-1 gene (Adh1 gene; ADH enzyme EC 1.1.1.1.), as expressed in pollen grains, is among the exceptionably suitable; much is known about the maize ADHs, Adh1 function is totally dispensible in an aerobic environment and maize pollen is a trinucleate gametophyte which expresses much of its haploid genome, including Adh1. In particular, there have been two recent methodological advances. First, I am able to cytochemically stain pollen, before or after in vitro germination, for the presence of above 5% normal ADH activity. And second, ADH1- pollen grains survive allyl alcohol (C=C-C-OH) vapour concentrations which kill ADH1+ grains; this selection scheme was developed for yeast by Megnet. My genetic resolution is approximately one mutant (Adh1+-->ADH-) per 10(7) chemically selected, viable gametophytes, and one (phenotypic) revertant (Adh1--->ADH+) per 10(8) unselected gatetophytes. In this note, I compare spontaneous forward mutant frequency with previously published revertant frequencies for one naturally occurring and six ethyl methanesulphonate-induced Adhl-deficient (Adh1-) alleles.