玉米自交系幼胚培养能力与遗传关系研究(简报)

玉米幼胚培养获得的胚性愈伤组织具有较强的长期继代能力和再生植株能力,常被用于构建转基因受体系统。然而基因型是制约玉米组织培养植株再生的重要因素之一,不同玉米基因型的幼胚培养能力关系到其遗传转化研究的结果[1]。     

玉米籽粒性状的遗传效应分析

采用二倍体胚和三倍体胚乳种子遗传模型及其分析方法,以5个玉米自交系及其配制的F1,F2,BC1,BC2世代为材料,研究5个玉米种子性状的胚直接效应、胚乳直接效应、母体效应和细胞质效应。分析结果表明,除粒宽外,各性状的遗传同时由细胞质效应和胚、胚乳、母体基因效应所控制,百粒重主要受胚乳和母体效应的影响,粒长的遗传以母体效应为主,粒宽和粒厚以胚乳效应为主。各部位籽粒百粒重的胚乳直接加性效应与母体加性效应的协方差达到显著或极显著水平,其余性状的胚、胚乳直接效应与母体效应间的协方差均不显著,通过母体植株的遗传表现可以对这些性状进行有效的选择。S22 是改良百粒重的优良亲本。 Abstract：The embryo,endosperm and cytoplasm effects of seven seed traits were studied by genetic model for diploid embryo and triploid endosperm plant seeds using five inbreds and their F1, F2, BC1 and BC2 generations. The estimates of genetic variance components indicated that the inheritance of all other kernel traits was controlled by the four effects except kernel width. The 100?kernel weight was mainly controlled by endosperm and maternal effects , and kernel length was controlled by the maternal effects,while endosperm conrrolled kernel width and kernel thickness. Except the significant or highly significant covariances between the endosperm direct additive and maternal additive effects for 100－kernel weight,all other traits between the embryo or endosperm direct effect and the maternal were not significant. So,maize inbreds could be developed by direct selection based on maternal plants for these traits. S22 was the best inbred of the improvement for kernel weight in this study.     

Recovery of maize (Zea mays L.) inbreds and hybrids from chilling stress of various duration: photosynthesis and antioxidant enzymes

The differences between two maize (Zea mays L.) inbred lines and their F1 hybrids in their response to chilling periods of various duration (1, 2, 3 or 4 weeks) and subsequent return to optimum temperatures were analysed by the measurement of the photosystem (PS) 1 and 2 activity, the photosynthetic pigments' content and the activity of antioxidant enzymes. The PS2 activity and the chlorophyll content decreased in plants subjected to 3 or 4 weeks of chilling, but not in those subjected to 1 or 2 weeks of chilling. This decrease was more pronounced in inbreds compared to their hybrids. The activity of superoxide dismutase did not much change with the increasing length of chilling period in the inbreds but decreased in the hybrids, the glutathione reductase activity increased in both types of genotypes but more in the inbred lines, while for ascorbate peroxidase and catalase the changes in parents-hybrids relationship did not show any specific trend. The PS1 activity and the carotenoids' content was not much affected.     

普通玉米籽粒性状的遗传效应分析

采用二倍体种子遗传模型及其分析方法,研究了５个玉米籽粒性状的直接效应、母体效应和细胞质效应．分析结果表明,各性状的遗传同时受种子直接效应和母体效应的影响,细胞质基因对百粒重和粒宽具有极显著影响．除粒长、粒厚的直接显性效应与母体显性效应间的协方差外,直接效应与母体效应间的协方差均不显著．因此,通过母体植株的表现可对这些性状进行有效的选择．Ｓ_２２和 ８７－１是改良粒重的优良亲本．选择粒较宽的自交系作母本有利于提高后代选系及Ｆ_１的百粒重．     

ORGANOGENESIS FROM HELIANTHUS ANNUUS INBREDS AND HYBRIDS FROM THE COTYLEDONS OF ZYGOTIC EMBRYOS

Dormant mature and immature zygotic embryos of sunflower (Helianthus annuus L.) inbreds and hybrids were dissected and cultured on modified Murashige and Skoog's medium supplemented with 0.1 mg/l benzyl adenine (BA), 0.5 mg/l naphthalene acetic acid (NAA), adenine sulfate, and casamino acids. For certain inbreds and hybrids, adventitious shoot formation occurred from callused cotyledonary tissue, particularly along the cut edges. The developmental stage of the zygotic embryo was critical. Eighty percent of immature H. annuus ‘Mammoth Russian’ embryos produced adventitious shoots from cotyledons, while mature embryos did not. The organogenic potential of mature cotyledons diminished as soon as one day after germination was initiated. Different forms of regeneration were observed in two inbred lines, and when these were crossed, both forms of regeneration were observed at a lower frequency from the immature embryos of the hybrid. Rooted shoots were successfully grown to maturity in a greenhouse.     

Identification of essentially derived varieties with molecular markers: an approach based on statistical test theory and computer simulations

Genetic similarities (GS) based on molecular markers have been proposed as a tool for identification of essentially derived varieties (EDVs). Nevertheless, scientifically reliable criteria for discrimination of EDVs and independently derived varieties with GS estimates are scanty, and implementation into practical breeding has not yet taken place. Our objectives were to (1) assess the influence of chromosome number and length, marker density, and distribution, as well as the degree of polymorphism between the parental inbreds on the distribution of GS between parental inbreds and their progenies [GS(P1,O)] derived from F2 and different backcross populations and (2) evaluate these factors with regard to the power for distinguishing F2- versus BC1- and BC1- versus BC2-derived lines with molecular markers. We developed an approach based on statistical test theory for the identification of EDVs with molecular markers. Standard deviations and overlaps of distributions of GS(P1,O) of F2-, BC1-, and BC2-derived lines were smaller with (1) increasing chromosome number and length, (2) increasing marker density, and (3) uniformly instead of randomly distributed markers, approaching a lower boundary determined by the genetic parameters. The degree of polymorphism between the parental inbreds influenced the power only if the remaining number of polymorphic markers was low. Furthermore, suggestions are made for (1) determining the number of markers required to ascertain a given power and (2) EDV identification procedures.     

RFLP analyses of early-maturing European maize germ plasm

Summary Thirty inbred lines representing a wide range of early-maturing European elite germ plasm of maize (Zea mays L.) were assayed for RFLPs using 203 clone-enzyme combinations (106 DNA clones with restriction enzymes EcoR1 and HindIII). The genetic materials comprised 14 flint, 12 dent, and 4 lines of miscellaneous origin. Objectives were to (1) characterize the genetic diversity for RFLPs in these materials, (2) compare the level of genetic diversity found within and between the flint and the dent heterotic groups, and (3) examine the usefulness of RFLPs for assigning inbreds to heterotic groups. All but two DNA clones yielded polymorphism with at least one restriction enzyme. A total of 82 and 121 clone-enzyme combinations gave single-banded and multiple-banded RFLP patterns, respectively, with an average of 3.9 and 7.7 RFLP patterns per clone-enzyme combination across all 30 inbreds, respectively. Genetic similarity (GS) between lines, estimated from RFLP data as Dice's similarity coefficient, showed considerable variation (0.32 to 0.58) among unrelated inbreds. The mean GS for line combinations of type flint x dent (0.41) was significantly smaller than for unrelated flint lines (0.46) and dent lines (0.46), but there was considerable variation in GS estimates of individual line combinations within each group. Cluster and principal coordinate analyses based on GS values resulted in separate groupings of flint and dent lines in accordance with phylogenetic information. Positioning of lines of miscellaneous origin was generally consistent with expectations based on known breeding behavior and pedigrees. Results from this study corroborated that RFLP data can be used for assigning inbreds to heterotic groups and revealing pedigree relationships among inbreds.     

Influence of flower functionality and pollination system on the pollen size-pistil length relationship

Twenty-five biotically pollinated plants of the Chaco Serrano Forest (Córdoba, Argentina) were studied in order to analyze whether ‘flower functionality’ is related to the relationship between pollen size and pistil length. Because flower functionality may act on the respective mean values of pollen size and pistil length rather than on intraspecific variation in these traits, we expected (1) a high positive correlation between pollen size and pistil length in a set of sympatric species, independent of their degree of pollination specialization or generalization; and (2) no interspecific correlation between the coefficients of variation (CVs) of those traits. On the other hand, on the assumption that pollinators are influencing the variation in floral traits (e.g. in pistil length) we expected lower mean phenotypic variation of pollen size and pistil length in pollination-specialist plants than in pollination-generalist ones. A positive correlation between pollen size and pistil length was found for the set of species, but not between the CVs of these traits. This trend was maintained when pollination-specialist plants were analyzed separately, but no statistical significance was obtained for the correlation in pollination-generalist plants. Contrary to our expectations, pollination-specialist plants did not show less mean intraspecific variation in floral traits than pollination-generalist plants. Therefore, the relationship between pollen size and pistil length among species suggests that the pollination system may be of less importance as a selective force than flower functionality.     

Genetic and non-genetic factors affecting anther culture of Brussels sprouts (Brassica oleracea var. gemmifera)

Summary Eight inbred lines of Brussels sprouts and ten F₁ hybrids derived from them were tested for their response to anther culture. From 5–19 plants per genotype were tested, and each plant was tested on 3–6 separate occasions. Results from the inbred lines were broadly similar to those from the F₁ hybrids, despite the inbreds producing fewer buds and having a higher frequency of anther deformities. The maximum embryo yield from an inbred line was 215 embryos per 100 anthers, and from a hybrid was 275. From estimation of the variance components it was calculated that, for both inbreds and hybrids, about half the total variation was genetic whereas variation due to plants within genotypes and to occasions within plants were each about 13% of the total. The narrow sense heritability of responsiveness to anther culture (estimated by the proportion of variation between inbred lines which was genetic) was 0.48, and there was partial dominance for this character. In three cases the hybrid outyielded the better inbred, and this heterosis may well be due to dispersed dominant genes.     

RFLP studies of genetic relationships among inbred lines of the cultivated sunflower,Helianthus annuus L.: evidence for distinct restorer and maintainer germplasm pools

One-hundred-and-eighty-one nuclear DNA probes were used to examine restriction-fragment length polymorphism in inbred lines of the cultivated sunflower (Helianthus annuus L.). The probes were from six libraries: two genomic libraries — one made with PstI and the other with HindIII, and four cDNA libraries — from etiolated plantlets, green leaves, ovaries, petals and anthers. Total DNA from 17 inbred lines representing an overview of the genetic stocks of sunflower, including restorer and maintainer lines of the classical cytoplasmic male sterility, was digested with four different restriction enzymes and probed in 331 probe-enzyme combinations. Of 181 clones analysed, 73 probes were found to be polymorphic. Genetic distances between inbreds were calculated from the resultant proportion of shared bands and submitted to principal component analysis and the UPGMA tree-making method. The RFLP analysis allowed a clear differentiation between restorer and maintainer lines of the cytoplasmic male sterility, together with a grouping of some of the genotypes from the same origin. The analysis of the accuracy of distance estimation as a function of the number of probe-enzyme combinations used, indicates that 40–50 combinations ensure a confidence level of near 95%. Considering the inbreds as representatives of the range of cultivated inbreds, estimates of gene diversity, as well as estimates of average gene diversity between and within the sets of restorer and maintainer lines, were calculated. Estimation of gene diversity showed that the available genetic variability in cultivated sunflower, based on allelic frequencies, is lower than that of other plants (H=0.20). Moreover, we show that the proportion of genetic variability due to the difference between maintainer and restorer lines (D_m) is about 2%.     

Variation of DNA fingerprints among accessions within maize inbred lines and implications for identification of essentially derived varieties: II. Genetic and technical sources of variation in AFLP data and comparison with SSR data

Accuracy and reproducibility of genetic distances (GDs) based on molecular markers are crucial issues for identification of essentially derived varieties (EDVs). Our objectives were to investigate (1) the amount of variation for amplified fragment length polymorphism (AFLP) markers found among different accessions within maize inbreds and doubled haploid (DH) lines, (2) the proportion attributable to genetic and technical components and marker system specific sources, (3) its effect on GDs between maize lines and implications for identification of EDVs, and (4) the comparison to published SSR data from the same plant materials. Two to five accessions from nine inbred lines and five DH lines were taken from different sources of maintenance breeding or drawn as independent samples from the same seed lot. Each of the 41 accessions was genotyped with 20 AFLP primer combinations revealing 988 AFLP markers. Map positions were available for 605 AFLPs covering all maize chromosomes. On average, six (0.6%) AFLP bands were polymorphic between different accessions of the same line. GDs between two accessions of the same line averaged 0.013 for inbreds and 0.006 for DH lines. The correlation of GDs based on AFLPs and SSRs was tight (r = 0.97**) across all 946 pairs of accessions but decreased (r = 0.55**) for 43 pairs of accessions originating from the same line. On the basis of our results, we recommend specific EDV thresholds for marker systems with different degree of polymorphism. In addition, precautions should be taken to warrant a high level of homogeneity for DNA markers within maize lines before applying for plant variety protection.     

Efficient mapping of plant height quantitative trait loci in a sorghum association population with introgressed dwarfing genes

Of the four major dwarfing genes described in sorghum, only Dw3 has been cloned. We used association mapping to characterize the phenotypic effects of the dw3 mutation and to fine map a second, epistatic dwarfing QTL on sorghum chromosome 9 (Sb-HT9.1). Our panel of 378 sorghum inbreds includes 230 sorghum conversion (SC) lines, which are exotic lines that have been introgressed with dwarfing quantitative trait loci (QTL) from a common parent. The causal mutation in dw3 associates with reduced lower internode length and an elongation of the apex, consistent with its role as an auxin efflux carrier. Lines carrying the dw3 mutation display high haplotype homozygosity over several megabases in the Dw3 region, but most markers linked to Dw3 do not associate significantly with plant height due to allele sharing between Dw3 and dw3 individuals. Using markers with a high mutation rate and the dw3 mutation as an interaction term, significant trait associations were detected across a 7-Mb region around Sb-HT9.1, largely due to higher detection power in the SC lines. Conversely, the likely QTL interval for Sb-HT9.1 was reduced to approximately 100 kb, demonstrating that the unique structure of this association panel provides both power and resolution for a genomewide scan.     

Relationship between maize stem structural characteristics and resistance to pink stem borer (Lepidoptera: Noctuidae) attack

The pink stem borer, Sesamia nonagrioides (Lefebvre), is one of the most important insect pests of maize (Zea mays L.) in northwestern Spain. The objectives of this work were to evaluate, at different times during the growth of maize, structural traits related to the entry point and tissues on which larvae feed and to determine the relationship between these structural traits and the stem borer resistance. Six inbred lines with different levels of stem resistance to S. nonagrioides were evaluated in several trials. Potential structural resistance factors included rind and pith puncture resistance (RPR and PPR), rind thickness, length of the meristematic area (LMA), and pith parenchyma interlumen thickness (PPIT). Surprisingly, the inbred lines that showed the strongest stalks, EP42 and EP47, were not stem resistant to pink stem borer attack, while the stem resistant inbreds A509, CM151, and PB130 were among the least resistant to rind puncture. There were no significant differences among resistant and susceptible inbreds for the rind thickness. However, the susceptible inbred EP42 had the softest internode pith, and the resistant inbred PB130 showed the hardest, as was expected. Susceptible inbred lines in general showed higher values for the LMA, while the PPIT was important for individual inbreds. The results suggest that the usefulness of these characters as estimators of pink stem borer resistance is limited to some genotypes. Besides, even among those genotypes, other mechanisms of resistance that do not involve stalk strength could be present. Among the traits considered, the LMA was the most promising as an indicator of resistance to pink stem borer, although further experimentation is necessary.     

Genetic diversity of maize inbred lines within and among heterotic groups revealed by RFLPs

Summary The objectives of this study were (1) to investigate genetic diversity for RFLPs in a set of important maize inbreds commonly used in Italian breeding programs, (2) to compare genetic similarities between unrelated lines from the same and different heterotic groups, and (3) to examine the potential of RFLPs for assigning maize inbreds to heterotic groups. Forty inbreds were analyzed for RFLPs with two restriction enzymes (EcoRI and HindIII) and 82 DNA clones uniformly distributed over the maize genome. Seventy clone-enzyme combinations gave single-banded RFLP patterns, and 79 gave multiple-banded RFLP patterns. The average number of RFLP patterns detected per clone-enzyme combination across all inbreds was 5.8. RFLP data revealed a wide range of genetic diversity within the two heterotic groups assayed, Iowa Stiff Stalk Synthetic (BSSS) and Lancaster Sure Crop (LSC). Genetic similarity (GS) between lines was estimated from binary RFLP data according to the method of Nei and Li (1979). The mean GS for line combinations of type BSSS × LSC (0.498) was substantially smaller than for unrelated line combinations or type BSSS × BSSS (0.584) but almost as great as for un-related line combinations of type LSC × LSC (0.506). Principal coordinate and cluster analyses based on GS values resulted in the separate groupings of lines, which is consistent with known pedigree information. A comparison between both methods for multivariate analyses of RFLP data is presented.     

Quantitative trait loci for callus initiation and totipotency in maize (Zea mays L.)

Induction of embryogenic callus in culture is an important step in plant transformation procedures, but response is genotype specific and the genetics of the trait are not well understood. Quantitative trait loci (QTL) were mapped in a set of 126 recombinant inbred lines (RILs) of inbred H99 (high Type I callus response) by inbred Mo17 (low Type I callus response) that were evaluated over two years for Type I callus response. QTL were observed in a total of eleven bins on eight chromosomes, including eight QTL with main effects and three epistatic interactions. Many of the QTL were mapped to the same or bordering chromosomal bins as candidate genes for abscisic acid metabolism, indicating a possible role for the hormone in the induction of embryogenic callus, as has previously been indicated in microspore embryo induction. Further examinations of allelic variability for known candidate genes located near the observed QTL could be useful for expanding the understanding of the genetic basis of induction embryogenic callus. The QTL observed herein could also be used in a marker assisted selection (MAS) program to improve the response of agronomically useful inbreds, but only if the resources required for MAS are lower than those required for phenotypic selection.     

Seedling growth, mitochondrial characteristics, and alternative respiratory capacity of corn genotypes differing in cold tolerance

Four maize (Zea mays L.) inbreds representing genetic differences in seedling cold tolerance were used to determine the effect of growth temperatures on dry weight accumulation and mitochondrial properties, especially the alternative oxidase capacity. Seedlings were grown in darkness at 30°C (constant), 14°C (constant), and 15°C for 16 hours and 8°C for 8 hours. Inbreds B73 and B49 were characterized as cold tolerant while G50 and G84 were cold sensitive. Shoot growth rate of cold-sensitive inbreds in the lower temperatures was slower relative to the tolerant inbreds. Mesocotyl tissue was particularly sensitive to low temperatures during growth after germination. There were no significant differences in relative rates of mitochondrial respiration in the cold-tolerant compared to cold-sensitive inbreds measured at 25°C. Mitochondria from all seedlings grown at all temperatures had the ability to phosphorylate as indicated by the observation of respiratory control. This result indicated that differences in low temperature growth were probably not related to mitochondrial function at low temperatures. Alternative oxidase capacity was higher in mitochondria from seedlings of all inbreds grown at 14°C compared to 30°C. Capacities in seedlings of 14°C-grown B73 and G50 were higher than in B49 and G84. Capacities in seedlings grown for 16 hours at 15°C and 8 hours at 8°C were similar to those from 14°C-grown except in G50 which was lower and similar to those grown at 30°C. Mesocotyl tissue was the most responsive tissue to low growth temperature. Coleoptile plus leaf tissue responded similarly but contained lower capacities. Antibody probing of western blots of mitochondrial proteins confirmed that differences in alternative oxidase capacities were due to differences in levels of the alternative oxidase protein. Male sterile lines of B73 were also grown under the three different temperature regimes. These lines grew equally as well as the normal B73 at all temperatures and the response of alternative oxidase capacity and protein to low growth temperature was similar to normal B73.     

Relationship of restriction fragment length polymorphisms to single-cross hybrid performance of maize

Summary Isozymes and restriction fragment length polymorphisms (RFLPs) have been proposed for use in varietal identification and selection for agronomic traits. Although the use of isozymes for these purposes has been well documented, evaluation of the efficacy of RFLP technology as applied to crop improvement is far from complete. This investigation was conducted to study the relationship between RFLP-derived genotypes and heterotic patterns of a group of maize (Zea mays L.) inbred lines. A total of 22 inbreds was crossed to four testers (B73, B76, Mo17, and Va26) in combinations that minimized crossing within heterotic groups. Forty-seven single-cross progeny were subsequently evaluated for several agronomic traits (including grain yield and moisture, ear height, and root lodging) over 2–4 consecutive years at two to four Iowa locations in a randomized complete-block design. The inbred lines were subjected to RFLP analysis, which involved 47 genomic clones and the restriction enzymes EcoRI and HindIII. Hybrid RFLP patterns were predicted from their inbred parents. Modified Roger's distances were computed to estimate genetic distance among the inbred lines. Principal component analysis facilitated ascertainment of relative dispersion of the inbreds based on the frequency of variants at specific RFLP loci. Evident associations of variants with genes affecting agronomic traits were identified by principal component regression analysis, in which adjusted hybrid means were regressed on the matrix of hybrid variants frequencies. The hybrid means were adjusted by removing environmental effects, using residuals as dependent variables in the regression analysis. Results from this study suggest that RFLP analysis may be of value in allocating maize inbreds to heterotic groups, but no relationship between RFLP-based genetic distance and hybrid performance was apparent. Principal component regression identified variants potentially linked to genes that control specific agronomic traits.Joint contribution: USDA-ARS and Journal Paper No. J-13590 of the Iowa Agriculture and Home Economics Experiment Station, Ames, IA 50011, USA. Projects No. 2818 and 2778     

Genetic diversity for restriction fragment length polymorphisms and heterosis for two diallel sets of maize inbreds

Summary Changes that may have occurred over the past 50 years of hybrid breeding in maize (Zea maize L.) with respect to heterosis for yield and heterozygosity at the molecular level are of interest to both maize breeders and quantitative geneticists. The objectives of this study were twofold: The first, to compare two diallels produced from six older maize inbreds released in the 1950's and earlier and six newer inbreds released during the 1970's with respect to (a) genetic variation for restriction fragment length polymorphisms (RFLPs) and (b) the size of heterosis and epistatic effects, and the second, to evaluate the usefulness of RFLP-based genetic distance measures in predicting heterosis and performance of single-cross hybrids. Five generations (parents, F₁; F₂, and backcrosses) from the 15 crosses in each diallel were evaluated for grain yield and yield components in four Iowa environments. Genetic effects were estimated from generation means by ordinary diallel analyses and by the Eberhart-Gardner model. Newer lines showed significantly greater yield for inbred generations than did older lines but smaller heterosis estimates. In most cases, estimates of additive x additive epistatic effects for yield and yield components were significantly positive for both groups of lines. RFLP analyses of inbred lines included two restriction enzymes and 82 genomic DNA clones distributed over the maize genome. Eighty-one clones revealed polymorphisms with at least one enzyme. In each set, about three different RFLP variants were typically found per RFLP locus. Genetic distances between inbred lines were estimated from RFLP data as Rogers' distance (RD), which was subdivided into general (GRD) and specific (SRD) Rogers' distances within each diallel. The mean and range of RDs were similar for the older and newer lines, suggesting that the level of heterozygosity at the molecular level had not changed. GRD explained about 50% of the variation among RD values in both sets. Cluster analyses, based on modified Rogers' distances, revealed associations among lines that were generally consistent with expectations based on known pedigree and on previous research. Correlations of RD and SRD with f₁ performance, specific combining ability, and heterosis for yield and yield components, were generally positive, but too small to be of predictive value. In agreement with previous studies, our results suggest that RFLPs can be used to investigate relationships among maize inbreds, but that they are of limited usefulness for predicting the heterotic performance of single crosses between unrelated lines.Joint contribution from Cereal and Soybean Research Unit, USDA, Agricultural Research Service and Journal Paper no. J-13929 of the Iowa Agric and Home Economics Exp Stn, Ames, IA 50011. Projects no. 2818 and 2778A.E.M. is presently at the Iowa State University on leave from University of Hohenheim, D-7000 Stuttgart 70, Federal Republic of Germany     

Cytology of inbreds and f 1 hybrids of pearl millet

Summary The meiotic behaviour of chromosomes was studied in four inbred lines and their F ₁ hybrids of P. typhoides. The inbred lines showed a decrease in mean chiasma frequency when compared with the population plants, but differed from one another in their mean chiasma frequencies. Together with the decrease in mean chiasma frequency the inbreds showed variation in mean chiasma frequencies. The inbred lines showed a number of meiotic abnormalities such as extra chromosomes, extra fragments, desynapsis, persistent nucleoli and differential condensation of chromosomes. The F ₁ hybrids of these inbreds exhibited heterosis for chiasma frequency. All the F ₁'s had mean chiasma frequencies higher than the means of the respective participating parents. The F ₁'s, however, differed in the degree of heterosis exhibited. In the F ₁ hybrids, the variation in mean chiasma frequency, both between plants and between PMC's within plants, was significantly lower than that of the inbred lines. The effect of environment was studied in the inbred lines and their F ₁ hybrids. The mean chiasma frequencies of the inbred lines were significantly lower, and the variation in mean chiasma frequencies was greater, in the stress season. The mean chiasma frequencies of F ₁'s did not show any significant differences between the two seasons. The F ₁'s exhibited less variation in mean chiasma frequency than the inbred lines, showing that F ₁'s were developmentally more stable. The F ₁'s did not show any meiotic abnormalities in either season.     

Plant Nucleases: V. Survey of Corn Ribonuclease II Isoenzymes

Corn (Zea mays L.) ribonuclease II of the root microsomal fraction was isolated from 11 inbreds and seven hybrids. Polyacrylamide gel electrophoresis revealed a total of five bands of activity among corn lines tested. Most inbreds had only one isoenzyme, but three had two isoenzymes. The hybrids tested contained all of the isoenzymes found in the parental inbreds.