A test for obligate apomixis in grain sorghum R473

Summary Segregation patterns in progeny arrays of selfed plants, heterozygous for the Mdh 1 isozyme marker locus, were used in an attempt to confirm the presence of apomixis in the grain sorghum line R473. No evidence for obligate apomictic reproduction was obtained. However, our studies did not rule out the possibility of a low level of facultative apomixis in R473.     

Chemical induction of apomictic seed formation in maize

Summary Silks of 18 maize (Zea may L.) F₁ hybrids were treated with different combinations of 9 growth regulators, colchicine, and dimethyl sulfoxide (DMSO) for the purpose to induce apomixis (agamospermy) in 1988 and 1989. Hybrid K301 × K303 gave the highest (0.36%) average frequency of seed induction among the hybrids. The most effective treatments were DMSO, gibberellic acid plus 6-benzyl aminopurine (6-BA), and DMSO plus methanesulfonic acid. Individually, the highest frequency of seed induction was 1.4% for hybrid K731×K306 when treated with -naphthalene acetic acid (NAA)-zeatin mixture. The frequency of seed induction seemed to depend partially on the interaction between chemicals and hybrids. Cytological observation of root-tip cells indicated that the majority of the seeds obtained were diploid, some were mixoploid, and a few were haploid. Diploid plants from induced seeds from the same parent were morphologically uniform and resembled the parent. Variations in plant and ear heights were comparable to those of the hybrid parent. Cytological and morphological investigations suggested that the chemically induced seeds originated mainly from somatic tissue but occasionally came from reduced cells in the embryo sac, leading to haploids. The results showed that chemical induction of adventitious embryony in maize hybrids is possible, but the more effective chemicals, their concentrations, and ways of application for increasing the frequency of seed induction need to be explored for practical use.     

Evolution and adaptedness in a facultatively apomictic grass, Poa pratensis L.

Summary Facultative apomixis confers great adaptedness and evolutive potential on biotypes belonging to the Poa pratensis L. agamic complex. Pioneering and recent research have dealt with and sometimes elucidated different aspects of apomictic expression, such as cytological mechanisms, distribution patterns, genetic control and environmental effects, in this species. We carried out an investigation on the mode of reproduction expressed in Italian natural populations, cultivars and selected genotypes together with an extensive time (56 years) and space (six countries) review in order to obtain a comprehensive picture of apomixis expression in P. pratensis. Histograms of the estimations were prepared. They showed that, due to breeder and natural selection, both cultivars and natural populations very frequently expressed a high degree of apomixis. Variations in the degree of apomixis across generations, different pollen donors, environments and estimation levels were generally low. It would, therefore, seem that the tenet which holds that apomixis in P. pratensis is strongly influenced by external factors should be reconsidered. Despite the fact that little is known of the genetic, physiological and molecular control underlying apomixis, the overall picture that emerges from our study helps to explain how the balance between sexual and asexual reproduction confers extreme versatility, adaptedness and evolutive capacity on this remarkable grass.     

The pattern of megasporogenesis and megagametogenesis in diploid Solanum species hybrids; its relevance to the origin of 2n-eggs and the induction of apomixis

Summary Megasporogenesis and megagametogenesis in diploid tuber-bearing Solanum species hybrids were studied and illustrated in detail. Megagametogenesis followed the normal or Polygonum type of embryo sac development. In megasporogenesis a few regularly occurring deviations from what is usually considered to be the normal course of megasporogenesis were detected.Firstly, the development of a surplus of adjacent or non-adjacent megaspore mother cells in a single ovule was frequently observed. As they eventually may give rise to normally reduced, sexual embryo sacs, it is concluded that the archesporium in potato cannot be delimited to a single cell or even a group of adjacent cells in the ovule. Secondly, the micropylar daughter cell which is formed after completion of the first division was often found to degenerate before the onset or completion of the second division giving rise to a triad instead of a tetrad of megaspores. The spatial arrangement of megaspores within the ovule was found to vary according to the variation in the relative orientation of second division spindles.It is concluded that these deviations should be considered random legitimate variations of megasporogenesis rather than systematic abnormal events.The implications of the overall pattern of megasporogenesis for the formation of 2n-eggs and the attempts to induce diplosporic or aposporic apomixis in potato are discussed.     

Chromosome doubling in Tripsacum: the production of artificial, sexual tetraploid plants

A collection of embryogenic diploid calli of Tripsacum was established and treated with colchicine to induce chromosome doubling. Sections containing duplicated cells in calli were identified using flow cytometry and ploidy level was determined in the regenerated plantlets. Tetraploid plants from several origins were obtained. In contrast to wild polyploid plants, which show apomictic development, the regenerated tetraploid plants reproduced sexually. By hybridizing these plants with wild tetraploid apomicts, various populations were established; these will allow a study of the inheritance of apomixis in Tripsacum.     

Production of 27-, 28-, and 56-chromosome apomictic hybrid derivatives between pearl millet (2n=14) and Pennisetum squamulatum (2n=54)

Summary An interspecific hybridization program designed to transfer gene(s) controlling apomixis from Pennisetum squamulatum Fresen. (2n=6x=54) to induced tetraploid (2n=4x=28) cultivated pearl millet, Pennisetum americanum (L.) Leeke resulted in four offtype plants, two with 27 chromosomes and two with 28 chromosomes. These plants were found among 217 spaced plants established from open-pollinated seed of an apomictic 21-chromosome polyhaploid (2n=21) plant derived from an apomictic interspecific hybrid (2n=41) between tetraploid pearl millet and Pennisetum squamulatum. It appeared that a 21- (or 20-) chromosome unreduced egg from the apomictic polyhaploid united with a 7-chromosome pearl millet (2n=2x=14) gamete to produce a 28- (or 27-) chromosome offspring. Meiotic chromosome behavior was irregular averaging from 3.60 to 4.05 bivalents per microsporocyte in the 27- and 28-chromosome hybrids. The 27- or 28-chromosome hybrids, like the 21-chromosome female parent, shed no pollen, but set from 1.8 to 28 seed per panicle when allowed to outcross with pearl millet. Progeny of the 28-chromosome hybrids were uniform and identical to their respective female parents, indicating that apomixis had been effectively transferred through the egg. In addition, a 56-chromosome plant resulting from chromosome doubling of a 28-chromosome hybrid was identified. Pollen was 68 per cent stainable and the plant averaged 2.3 selfed seeds per panicle. Chromosomes of the 56-chromosome plant paired as bivalents (x=10.67) or associated in multivalents. Three to nine chromosomes remained unpaired at metaphase I. Multiple four-nucleate embryo sacs indicated the 56-chromosome hybrid was an obligate apomict. The production of 27-, 28-, and 56-chromosome hybrid derivatives were the results of interspecific hybridization, haploidization, fertilization of unreduced apomictic eggs, and spontaneous chromosome doubling. These mechanisms resulted in new unique genome combinations between x=7 and x=9 Pennisetum species.     

Genetic diversity revealed in the apomictic fruit species Garcinia mangostana L. (mangosteen)

The novel molecular marker technique Randomly Amplified DNA Fingerprinting (RAF)was used to survey genetic relationships between 37 accessions of the tropical fruit G. mangostana (mangosteen) and among 11 accessions from eight other Garcinia species. Although mangosteen is believed to reproduce exclusively through apomixis, our results show that considerable genetic diversity exists within G. mangostana and between other Garcinia species. Among the 37G. mangostana accessions examined, nine different genotypes were identified which clustered into three distinct groups based on correspondence analysis(reciprocal averaging). For 26 (70%) of the accessions no marker variation was detected over 530 loci screened. A further eight (22%) accessions exhibited very low levels of variation (0.2–1%) suggesting at least one well conserved mangosteen genotype. The remaining three accessions (8%) showed extensive variation (22–31%)compared with the majority of accessions. The three mangosteen groups were 63–70% dissimilar to the other Garciniaspecies investigated. The genetic diversity identified in this research will assist in the conservation of Garciniagermplasm and provides a valuable framework for the genetic improvement of mangosteen. This revised version was published online in August 2006 with corrections to the Cover Date.     

Powdery mildew resistance in Kentucky bluegrass genotypes regenerated from excised seed pieces

Summary Severity of powdery mildew was assessed on seven cultivars and lines of Kentucky bluegrass propagated by seed and tissue culture. Tissue culture plants were started from embryo axes cultured on Murashige and Skoog medium containing different concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) and 6-benzylaminopurine (BAP), and incubated (1 to 4 weeks) or not incubated in the dark prior to transfer to a lighted culture room. There were significant differences in disease severity (DS) among seed propagated and tissue culture regenerated plants. DS ranged from highly susceptible (100% of leaf covered by mildew) (DS=9) to resistant (DS=3.0). In some tissue culture regenerants the disease severity was significantly affected by the tissue culture process. Ten clones expressing resistance were selected, and plants propagated vegetatively. In six clones, disease resistance was sustainable in subsequent vegetatively propagated plants, while resistance was lost in four of the selected clones. Results are discussed with a view to using tissue culture to produce Kentucky bluegrass genotypes with resistance to powedery mildew.Abbreviations BAP benylaminopurine - 2,4-D 2,4-dichlorophenoxyacetic acid - DS disease severity - MS Murashige & Skoog - SDW sterile distilled water     

Seed of Ullucus tuberosus—An apomict or not

Summary The paper deals with variations in enzyme pattern in the seeds of ulluco, Ullucus tuberosus. Six seeds per clone, four clones and three enzyme systems (malate dehydrogenase, glutamate oxaloacetate transaminase and 6-fosfoglucose dehydrogenase) were examined. There was variation in enzyme pattern between the seeds of a single clone. It is concluded that ulluco is not an apomictic species but a truly sexual one. Consequences of these findings for breeding programmes of ulluco are pointed out.