Intergradation among Latin American maize based on an analysis of chromosome knob frequencies.

Published information on chromosome knobs found at 21 knob-forming positions and on abnormal 10 and B chromosomes in maize, Zea mays L., was used to place maize populations within a multidimensional space based on frequencies. From this space, similarities among populations were determined using a measure of gentic diversity based on a modified Cartesian distance. Populations were portrayed in 2 (or 3) dimensions based on these distances. The objective was to investigate patterns of "migration" that had occurred among indigenous populations of maize from Latin America. Widely dispersed collections classified as Tuxpeño had similar knob constitutions. Collections from Guatemala reflected continuous migration among adjacent areas with increased isolation (or association of knob types) with increased altitude of collection. Maize from southeastern Guatemala and their southeastern neighbours were similar. The high elevation collections from Guatemala and Mexico were surprisingly similar. The data reflected three distinct phenomena: long-term intergradation of maize germplasm among adjacent areas (as would result from pollen drift between closely cultivated areas or from seed exchange among neighbors), major, relatively recent shifts in gene flow (as had occurred with Tuxpeño's widespread distribution in Mexico), and precolonial dispersions (as between maize populations from the high elevations in Guatemala and Mexico).

Evaluation of an exotic maize population adapted to a locality.

An exotic Zea mays L. population ('Tuxpeno') was adapted to North Carolina conditions by first introducing genes for adaptability from two North Carolina varieties ([('Jarvis' X 'Indian Chief')'Tuxpeno']'Tuxpeno') including four generations of intermating, and then selecting for adaptability using maturity as the primary measure. The study evaluated selection for adaptability and the diversity available between adapted 'Tuxpeno' and the local varieties, 'Jarvis' and 'Indian Chief'. Analytical procedures were developed to quantify the diversity between populations and the complementation of local varieties by introduced germ plasms. The analyses utilized the specific effects available from the diallel mating design.Three replicate selections responded similarly under simple recurrent mass selection (1/10) for the earliest disease-free plants initially and additionally for plant types (primarily height) in the final generation. The 1/4 local germ plasm permitted rapid adaptation of 'Tuxpeno' gene pool to local conditions. The adapted 'Tuxpeno' populations yielded similarly to the local populations with an average heterosis for grain yield of 28% when crossed to the local populations used as source of genes for adaptability. The diversity found between adapted 'Tuxpeno' lines and these local varieties based on genes affecting grain yield was 1.5 to 2.5 times that measured between the local varieties ('Jarvis' and 'Indian Chief'). Diversity lost through intergradation with local material was a reasonable investment. Yield genes introduced from 'Tuxpeno' complemented local gene pools through nonadditive, primarily dominance-associated, gene effects. Reassortment of major gene blocks apparently occurred leading to significant divergence among replicate selections involving both additive-associated and dominance-associated gene effects.

Rates of electron transfer and of non-cyclic photophosphorylation for chloroplasts isolated from maize populations selected for differences in juvenile productivity and in leaf widths.

In vitro studies with chloroplasts isolated from maize populations differing in juvenile productivity by the sixth leaf stage of growth demonstrated differences in rates of ferricyanide reduction and of non-cyclic photophosphorylation. Similar results were obtained for two independent sets of replicate selections for high and for low productivity. Chloroplasts from low productive selections had about 115 percent of the reduction rate and 129 percent of the phosphorylation rate found for the high productive selections. Differences in ferricyanide reduction held for a fourfold range of experimentally induced rates. The contrasting rates of electron flow did not result from differential extraction environments following cell disruption nor from the failure to protect chloroplasts during extraction. Light-dependent phosphate uptake gave evidence, though inconclusive, of being so modified. No evidence was found for reciprocal effects between low-high and high-low hybrids. No difference in activities was found between chloroplasts isolated from two populations selected for narrow and for wide leaf widths. These two selections differed markedly in leaf areas. While in vitro measures of chloroplast efficiency reflected differences which were under genetic control, the results did not follow those expected from photosynthetic studies. The differences in rates appeared to reflect modification of processes other than those in the photosynthetic pathway.

Experimental Evaluation of Relationships among Populations Resulting from Intergradation among Cultivars of ZEA MAYS L.

Populations were developed as intergradations among three maize cultivars, two adapted to North Carolina and one to Mexico, with the objective of testing whether diallel information could be used to identify the relationships among populations. The data substantiated theoretical observations that the technique would identify relationships among populations resulting from recent intergradations.

Genotypic stability.

A description for genotypic stability was developed based on the concept of a genotypic stability space. Environments were considered the measurement criteria. The coordinate position for a genotype was determined by the deviations of expected yield of the genotype from its yield if stable. An expression for the response of a stable genotype was developed. The relative genotypic stability measure was defined as the distance of a genotype from the center of the arrangement and presented as a measure of homeostasis. The comparative genotypic stability measure was defined as the distance between the positions of two genotypes and designed to measure the similarity of stability responses. The development permitted the testing of hypotheses with respect to stability concepts. A set of regional soybean test data was evaluated. The results for relative stability and the regression approach were comparable. Limitations for the regression approach were noted. Genotypes analyzed had different homeostatic properties. The genotypic responses to environmental stimuli appeared to be unique to genotypes and difficult to predict.