Apomictic Grass Research Articles

Callus Production and Plant Regeneration from Mature Embryos of Poa pratensis L.

AbstractPlant regeneration from callus cultures may provide a source ot somaclonal variation for the improvement ot the apomictic grass Poa pratensis L. It is first necessary to be able to induce callus and regenerate plants in this species at a high frequency. Variation was observed between 50 cutivars of Poa pratensis for callus induction and plant regeneration. Using the cultivars ‘Merion’ and ‘Victa’, three basal media were tested along with various media additives. Murashige and Skoog's basal medium with 0.2 mg 1−1 2,4‐dichlorophenoxyacetic acid, 0.1 mg 1−1 6‐benzylamanopurine, 100 mg 1−1 casein hydrolysate and 25 g 1−1 sucrose is considered to be a good medium for callus growth and plant regeneration. Embryo‐like structures were observed in the callus of some cultivars but plant regeneration appeared to be predominantly from shoot meristems on the callus surface. The majority of regenerated shoots were green, but chlorophyll deficient shoots were obtained from media containing coconut milk. Green plantlets could be transferred to soil without difficulty.

Patterns of Genetic Variation Among Populations of Poa pratensis L. and Agrostis capillaris L. from Britain and Iceland

SUMMARY (1) Populations of Agrostis capillaris L., an outbreeding grass species, and Poa pratensis L., an apomictic grass species, were simultaneously collected from contrasting climatic areas and soil conditions. The climatic areas were: south-east England (Rothamsted), southern Iceland (Samsstadir), northern Iceland (Akureyri), and two higher altitude sites in Iceland (Gullfoss and SkAlafell). Plots at the first three sites had received either no known fertilizer application, or applications of fertilizer for at least 30 years. The populations were grown in spaced-plant trials at Reading (England) and Korpa (Iceland). (2) Significant differences were found between populations for most of the attributes measured. Univariate and multivariate analyses showed that the largest differences existed between climatic areas, particularly between British and Icelandic populations. Smaller differences were found between populations from soils receiving different fertilizer applications. The pattern of differences was generally similar for the two species, although differences tended to be clearer in Poa pratensis than in Agrostis capillaris. (3) Genetic variation within populations differed significantly between sites, and sometimes between soils, but there was no consistent difference between the two species, in spite of their contrasting breeding systems.

Degrees of sexuality in sexual plants of guineagrass by the simplified embryo sac analysis.

Degrees of sexuality in lowly and highly sexual plants of guineagratss which were isolated by progeny tests in the previous paper of the authors, were investigated by using the simplified embryo sac analysis, and compared with degrees of sexuality by frequency of off-type plants in progenies. In embryo sac analysis, ovules were classified into sexual or apomictic reproduction to detect the type of embryo sacs. Sexual embryo sac was 8-nucleate, and aposporous one was 4-nucleate. As ovules examined kept their original forms intact in pistils, reproductive behavior in polyembryonic ovules was easily identifled by determining the dominant embryo sac in multiple ones. Highly sexual diploid plants were revealed to be completely sexual based upon the embryo sac type. All ovules had single sexual embryo sacs excluding several sterile ones. Highly sexual tetraploid plant, N68/96-8, was also identified to be completely sexual, and its offsprings after open-pollination were segregated in degrees of sexuality. Five out of 10 offsprings were completely sexual, one was obligate apomictic, and the other four were facultative apomictic. Two lowly sexual plants, N68/84-1 and 73/61-9, and their offsprings after self-and open-pollination, were shown to have low frequencies of sexual embryo sacs except N68/84-1 which had rather higher rate of sexual embryo sacs. No plants with high degrees of sexuality were segregated in their offsprings. Both degrees of sexuaiity by embryo sac analysis and progeny test, agreed well excluding degrees in a few facultative apomicts. This analysis was appeared to provide more accurate estimation of degree of sexuallty. Complete sexuality observed in this study was recognized to be useful for producing controlled hybrid plants in sexual X apomictic cross in the first procedure of hybridization breeding in this apomictic grass.

Cleared-pistil and thick-sectioning techniques for detecting aposporous apomixis in grasses

Two techniques were evaluated for detecting sexual and aposporous embryo sacs in an apomictic grass. Pistils of Cenchrus ciliaris L. were fixed in formalin – acetic acid – alcohol (FAA), cleared in methyl salicylate, and examined in toto by interference contrast microscopy. The same pistils were then embedded in plastic, sectioned, and studied by conventional microscopy. The two methods yielded identical interpretations of embryo sac morphology. The cleared-pistil technique was simple and required about 1/10 the time of the sectioning method.

Methods for induction and utilization of variability in the improvement of an apomictic grass, Dichanthium annulatum complex

Many problems and difficulties are encountered in making genetic improvements in plants where both apomixis and polyploidy occur together. From biosystematic studies on an agamic species complex, Dichanthium annulatum, information is presented on: (A) Mechanisms which create variability in apomicts - (i) genome building and reduction, (ii) hybridization between ecotypes of facultative apomicts, (iii) fertilization of unreduced gametes, (iv) introgressive hybridization, (v) preferential pairing and genotypic control of bivalent formation and (vi) induced mutation; (B) Embryo-sac variations, vis-a-vis sexual/apomictic sacs - (i) production of sexual embryo-sac in apomicts, (ii) balance between apomixis and sexual process, (iii) effect of environment and experimental manipulation of the type of embryo-sac; and (C) Heterosis and fixation of apomixis.The utilization and exploitation of these mechanisms and phenomena for accelerating the genetic improvement of apomictic plants is discussed.Mating systems impose certain restrictions on the breeding methodology to be used in the genetic improvement of crop plants. Allogamous species have built-in mechanisms for self-improvement and, for them, the breeding techniques are well worked out. Little information is, however, available on the procedures to be followed for the genetic improvement of apomicts. Recently gathered information on the causal mechanisms of apomixis and its mode of inheritance, the genetic systems which regulate the balance between apomixis and sexuality, the physical and chemical agents for artificial induction of sexuality in apomicts, and the processes which promote variability and adaptive polymorphism in apomicts show a way for the creation, exploitation and fixation of superior genotypes. Such information, based on biosystematic studies on an agamic species complex, Dichanthium annulatum, at the Oklahoma State University, Stillwater, Oklahoma, U.S.A., is presented here.Breeding procedures commonly followed for the genetic improvement of apomicts are outlined below:1. Collection of varieties, strains or ecotypes from diverse sources; 2. Evaluation of the germ plasm for the presence of desirable characters; 3. Building up of selection indices and estimation of genetic parameters; 4. Determination of mode of reproduction and isolation of sexual types or clones; 5. Hybridization using the sexual types; 6. Progeny testing, comparisons, multiplication and release of superior types.Thus, the success of the breeding programme would depend on the range of variability already present in the germ plasm collections, the relative proportion of sexual/apomictic seed produced and the exploitation of variability from the crossbred progenies. Since large collections of plants with different genotypes are not often available, one would like to look for the mechanisms which can create variability in the apomicts. Such mechanisms are as follows.

A Modified Bradley's Squash Technique for Studying Embryo Sacs in Gramineae

This procedure is especially suited for studying the embroyology of sexual and apomictic grasses. Material is fixed in a 2:2:1 alcohol-chloroform-propionic acid mixture for a minimum period of 2 days, soaked in 4% iron alum at 75 C for 7 min, and 2 min each in 2 changes of distilled water, also at 75 C. After 2–3 min in cold water, it is macerated in 50% HCI for 10 min at about 22–25 C, washed and mordanted for 12–16 hr in 50% alcohol saturated with ferric acetate. Ovules are then dissected out and squashed in 1% carmine in 45% propionic acid. Squashing should be firm enough to separate and flatten the embryo sacs but not to burst them. The slides are set aside for 12–24 hr for intensification of the stain.