It is concluded from a review of the literature that plant cell culture itself generates genetic variability (somaclonal variation). Extensive examples are discussed of such variation in culture subclones and in regenerated plants (somaclones). A number of possible mechanisms for the origin of this phenomenon are considered. It is argued that this variation already is proving to be of significance for plant improvement. In particular the phenomenon may be employed to enhance the exchange required in sexual hybrids for the introgression of desirable alien genes into a crop species. It may also be used to generate variants of a commercial cultivar in high frequency without hybridizing to other genotypes.

Somaclonal variation — a novel source of variability from cell cultures for plant improvement

Apart from their agronomic importance in hybrid seed production, mutations that encode cytoplasmic male sterility (CMS) provide a means to probe the role of the mitochondrion in reproductive development. Fertility restorers are examples of nuclear genes that affect cytoplasmic gene expression, and

Interactions of Mitochondrial and Nuclear Genes That Affect Male Gametophyte Development

Preface. Part I: Molecules and genomes in plant systematics. Chloroplast DNA and the study of plant phylogeny: present status and future prospects - M T Clegg and G Zurawski Use of chloroplast DNA rearrangements in reconstructing plant phylogeny - S R Downie and J D Palmer Mitochondrial DNA in plant systematics: applications and limitations - J D Palmer Ribosomal RNA as a phylogenetic tool in plant systematics - R K Hamby and E A Zimmer Evolution of the NOR and 5S DNA loci in the Triticeae - R Appels and B Baum Part II: Molecular approaches to plant evolution Intraspecific chloroplast DNA variation: systematic and phylogenetic implications - D E Soltis, P S Soltis and B G Milligan Molecular data and polyploid evolution in plants - P S Soltis, J J Doyle and D E Soltis Molecular systematics and crop evolution - J Deobley Part III: Model studies of phylogenetic relationships Contributions of molecular data to polyploid evolution in plants - P S Soltis, J J Doyle and D E Soltis Molecular systematics and crop evolution - J Deobley Contributions of molecular data to papilionoid legume systematics - J J Doyle, M Levin and A Bruneau Chloroplast DNA variation in the asteraceae: phylogenetic and evolutionary implications - R K Jansen, H J Michaels, R S Wallace, K-J Kim, S C Keeley, L E Watson and J D Palmer Chloroplast DNA restriction site variation and the evolution of the annual habit in North American Coreopsis (Asteraceae) - D J Crawford, J D Palmer and M Kobayashi Molecular systematics of onagraceae: examples from Clarkia and Fuschia - K J Systema and J E Smith Floral morphology and chromosome number in the subtribe oncidiinae (Orchidaceae): evolutionary insights from a phylogenetic analysis of the chloroplast DNA restriction site variation - M W Chase and J D Palmer Part IV: Theoretical perspectives The suitability of molecular and morphological evidence in reconstructing plant phylogeny -M J Donaghue and M J Sanderson Character-site weighting for restriction site data in phylogenetic reconstruction, with an example from chloroplast DNA - V A Albert, B D Mishler and M W Chase Polymorphism, hybridization and variable evolutionary rate in molecular phylogenies - K Ritland and J E Eckenwalder Index.

Molecular Systematics of Plants

Cell/Tissue Culture and in Vitro Manipulation

The Genetics of Corn

Plants resistant to Helminthosporium maydis race T were obtained following selection for H. maydis pathotoxin resistance in tissue cultures of susceptible, Texas male-sterile (T) cytoplasm maize. The selected lines transmitted H. maydis resistance to their sexual progeny as an extranuclear trait. Of 167 resistant, regenerated plants, 97 were male fertile and 70 were classified male sterile for reasons that included abnormal plant, tassel, anther or pollen development. No progeny were obtained from these male-sterile, resistant plants. Male fertility and resistance to the Phyllosticta maydis pathotoxin that specifically affects T cytoplasm maize were co-transmitted with H. maydis resistance to progeny of male-fertile, resistant plants. These three traits previously were associated only with the normal (N) male-fertile cytoplasm condition in maize. Three generations of progeny testing provided no indication that the cytoplasmic association of male sterility and toxin susceptibility had been broken by this selection and regeneration procedure. Restriction endonuclease analysis of mitochondrial DNA (mtDNA) revealed that three selected, resistant lines had distinct mtDNA organization that distinguished them from each other, from T and from N cytoplasm maize. Restriction patterns of the selected resistant lines were similar to those from T cytoplasm mtDNA; these patterns had not been observed in any previous analyses of various sources of T cytoplasm. The mtDNA analyses indicated that the male-fertile, toxin-resistant lines did not originate from selection of N mitochondrial genomes coexisting previously with T genomes in the T cytoplasm line used for selection.

Mitochondrial DNA variation in maize plants regenerated during tissue culture selection.

Spontaneous reversion to fertility in S male-sterile cytoplasm of maize is correlated with the disappearance of the mitochondrial plasmid-like DNA's, S-1 and S-2, and changes in the mitochondrial chromosomal DNA. Hybridization data indicate that one of the plasmid-like DNA's, S-2, is prominently involved in the mitochondrial DNA rearrangements.

Cytoplasmic Reversion of cms-S in Maize: Association with a Transpositional Event

Chloroplast and mitochondrial DNAs from six races of annual teosinte (Guatemala, Huehuetenango, Balsas, Central Plateau, Chalco, and Nobogame), perennial teosinte, and maize were compared and grouped by restriction endonuclease fragment analyses. Three groups of chloroplast DNAs were detected: (i) perennial teosinte and Guatemala; (ii) Balsas and Huehuetenango; and (iii) all other teosintes. Four groups of mitochondrial DNAs were separated: (i) perennial teosinte; (ii) Guatemala; (iii) Nobogame; and (iv) all other teosintes. Separation of the teosinte and maize organelle DNAs into five groups (Guatemala; perennial teosinte; Balsas and Huehuetenango; Central Plateau and Chalco; Nobogame and maize) approximated the biosystematic relationships of the taxa. It was suggested that the evolutions of the chloroplast and mitochondrial DNAs may be independent of each other, that variation of organelle DNA within a species complex of an organism may be the common condition, and that the DNAs of the organelle and nuclear systems evolve in reasonable harmony.

Organelle DNA variation and systematic relationships in the genus Zea: Teosinte.

Organelle Genome Diversity in Sorghum: Male-Sterile Cytoplasms 1

Plasmid-like, linear DNAs were detected in preparations of mitochondrial DNA from cytoplasmic male-sterile sorghum. Designated N-1 and N-2, the DNAs exhibited molecular sizes of ca. 5,700 and 5,300 bp, respectively. The DNAs occurred in only the IS1112C entry among 24 entries examined. Electron microscopy of the DNAs indicated that the molecules were linear as isolated. Nick translation of N-1 and N-2 followed by membrane hybridization indicated substantial homology between the two DNAs, and some homology to the S-1 and S-2 maize plasmid-like DNAs. At least four additional DNA species, ranging from ca. 1,000–4,000 bp if linear, were also detected in sorghum mitochondrial DNA. The detection of plasmid-like DNAs in sorghum, with homology to the plasmid-like DNAs of maize, suggests an etiological relationship of the molecules to the inheritance and expression of cytoplasmic male sterility in sorghum, perhaps in a manner analogous to the postulated role of these elements in maize.

M. F. Conde

Papers

Mitochondrial DNA variation in maize plants regenerated during tissue culture selection.

Cytoplasmic Reversion of cms-S in Maize: Association with a Transpositional Event

Organelle DNA variation and systematic relationships in the genus Zea: Teosinte.

Organelle Genome Diversity in Sorghum: Male-Sterile Cytoplasms 1

Plasmid-like DNAs associated with mitochondria of cytoplasmic male-sterile Sorghum