Repeatability and heritability of divergent recombination frequencies in the Iowa Stiff Stalk Synthetic (Zea mays L.).

Variability in recombination frequency was reported in the Iowa Stiff Stalk Synthetic. The objectives of the present research were to verify the differences in recombination frequency among individuals in the Iowa Stiff Stalk Synthetic maize population and to determine if the recombination frequency differences persisted among the S1 progeny. Testcrosses to measure male recombination frequency on three chromosomes (4, su1-c2; 5, a2-bt1-pr1; 9, sh1-bz1-wx1) were repeated for eight S0 individuals. Recombination frequencies were repeatably divergent among those individuals which were selected based on high or low recombination frequencies on specific chromosomes. Individuals which had been selected for long and short total map distances across the three chromosome regions produced repeatably divergent recombination frequencies only at the su1-c2 region. The recombination frequencies of the S1 lines, derived from the S0 individuals which had the most divergent recombination frequencies on a single chromosome, were significantly different. The broadsense heritability estimates derived from the regression of six S1 lines on six S0 individuals ranged from 0.69 to 0.20 for the five chromosome regions. We conclude that genetic differences for recombination frequency exist in this population and that modification by selection should be possible.

Recombination rates of soybean varieties from different periods of introduction and release.

Theory predicts that selection for adaptability during the short term also favors selection for a reduced recombination rate in the population. The objective of this study was to test whether the cyclic short-term selection which has taken place in soybean breeding programs in the USA since the introduction of the crop has measurably reduced recombination frequencies. Thirteen soybean varieties separated into four different release periods (prior to 1940, 1940-1954, 1955-1969, after 1970) were evaluated for their recombination frequencies within three locus pairs. Recombination frequencies among the individual varieties ranged from 7.6 to 24.1 % at thep 1 r locus pair, from 20.9 to 30.1 % at thelnp 2 locus pair, and from 28.7 to 41.6% at thedt 1 l 1 locus pair. Recombination frequencies were significantly different among varieties within a release period for thep 1 r andlnp 2 locus pairs, but recombination frequencies did not differ among release periods for any locus pair. Thus, apparently, plant breeders have developed soybean varieties with improved adaptation without influencing recombination rates.

Variability of recombination frequencies in the Iowa Stiff Stalk Synthetic (Zea mays L.).

Variability in recombination frequency has been reported in several plant populations. The objectives of the present research were to establish the range in variability in recombination among genotypes in the important corn population Iowa Stiff Stalk Synthetic and to identify individual genotypes which produced increased or decreased recombination frequencies. Approximately 150 individual S0 plants were testcrossed to measure male recombination frequency on three chromosomes: 4, sul-c2; 5, a2-btl-pr1; and 9, sh1-bz1-wx1. Although the variance component for individuals accounted for only 20-33% of the total variation, highly significant variability among individuals was present at all chromosome regions. Thus the environmental effects did not prevent measurement of differences between S0 individuals. At each chromosome region, individual genotypes with recombination frequencies at least two standard deviations above or below the population mean were isolated. Reports in the literature suggest that the variability reported here for the BSSS population should be representative of that present in other corn breeding populations. Recombination frequencies were positively correlated between adjacent regions of chromosome 9 and also between adjacent regions of chromosome 5. Recombination frequencies were positively correlated between both regions on chromosome 5 with the su1-c2 region of chromosome 4. Negative correlations were observed between chromosome 9 recombination and recombination in each region of chromosomes 4 and 5. Thus rankings of S0 individual recombination frequencies were not consistent for all three chromosomes.

Apparent digestibility of amino acids in raw and heated conventional and low-trypsin-inhibitor soybeans for pigs.

An experiment was conducted with ileally cannulated pigs to determine the apparent digestibility of amino acids and N in raw or heated conventional or low-trypsin-inhibitor soybeans. Six littermate barrows initially averaging 24 kg were fed cornstarch-based diets (10.5% CP, .68% lysine, 3,558 kcal of ME/kg) supplemented with raw (unheated), conventional soybeans; raw, low-trypsin-inhibitor soybeans; heated, conventional soybeans; heated, low-trypsin-inhibitor soybeans; or solvent-extracted soybean meal. Heating was achieved by autoclaving soybeans (after grinding) for 20 min at 110 degrees C. The urease activities of the two raw soybean sources were similar, but the trypsin inhibitor activity of the raw, low-trypsin-inhibitor soybeans was about one-half that of the raw, conventional soybeans. The lower trypsin inhibitor activity of the raw, low-trypsin-inhibitor soybeans was associated with an improvement in the apparent digestibility of amino acids and N compared with the raw, conventional soybeans (P less than .05). Heating reduced the urease and trypsin inhibitor activities and improved (P less than .05) the apparent digestibility of amino acids and N in both types of soybeans. However, the heated, conventional soybeans contained more trypsin inhibitor activity than the heated, low-trypsin-inhibitor soybeans and the digestibilities of the amino acids were lower (P less than .05). Although the urease and trypsin inhibitor activities of the two heated soybean sources were similar to or less than those of soybean meal, the digestibilities of amino acids were not as great (P less than .05) in the heated soybeans as in soybean meal.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparisons of alfalfa somaclonal and sexual derivatives from the same genetic source.

Tetraploid alfalfa (Medicago sativa L.) clones were derived from the same diploid genetic background by four different methods. A phenotypically superior clone was selected from each method and compared for herbage yield and fertility. The four methods and their best clones were: a) In vitro somatic chromosome doubling of one diploid hybrid (HG2-4x); b) selection within a two allele tetraploid synthetic population derived from HG2-4x (HAG); c) somaclonal variant selection from cell suspension culture of the diploid hybrid (NS1); and d) sexual polyploidization of a sibling hybrid (HXG). Clones HG2-4x, HAG, and NS1 were likely diallelic or monoallelic at all loci. Clone HXG was probably tetrallelic or triallelic at most loci. Experiments measured fertility, clonal herbage yield, and herbage yield of test cross progeny for each selected clone. Fertility rankings were HXG = HAG > NS1 > HG2-4x. Clonal herbage yield rankings were HXG = HAG > NS1 > HG2-4x. Test cross progeny herbage yield rankings varied depending on the tester, but, in general, HXG ≧ HAG ≧ NS1 HG2-4x. Overall the best clones from the sexual methods exceeded the best somaclonal variant which, in turn, was better than the chromosome doubled clone.