No abstract

U.S. soybean [Glycine max (L.) Merr.] germplasm has a narrow genetic base. The objective of this study was to identify G. soja Sieb. and Zucc. alleles for yield and yield component quantitative trait loci (QTL). Two populations of BC2F4 lines were generated from a mating between recurrent parent G. max ‘7499’ and donor parent G. soja PI 245331 with one line in each population tracing back to the same BC2 plant. Population A was used for the QTL identification analysis and Population B was used for the QTL verification test. The Population A lines were genotyped at 120 simple sequence repeat marker loci and one phenotypic marker locus. There were 11 putative QTL significantly associated with yield and yield component traits across three environments. One QTL for seed yield was found using the combined data. At this locus, the G. soja allele at Satt511 on LG A1 was associated with increased seed yield with an additive yield effect of 191 to 235 kg ha−1 depending on the QTL analysis method. Across environments in the validation population, lines that were homozygous for the G. soja allele at Satt511 demonstrated a 6.3% yield increase over lines that were homozygous for the G. max allele. One seed filling period QTL was identified on LG F with an additive effect of +1 d. This QTL also provided a +1 d additive effect on maturity. These results demonstrate the potential of identifying positive alleles in the exotic germplasm of soybean.

Seed growth and development is an important part of the yield production process in grain crops. Experiments were conducted in the field for 2 yr to determine the seed growth characteristics of soybean [Glycine max (L.) Merr.] genotypes that varied widely the duration of seed fill. Fifty‐nine genotypes were grown in 1982 on a Donerail silt loam soil (fine, mixed, mesic Typic Argiudolls) and measurements of seed growth rate (SGR), final seed size (FSS), and effective filling period (EFP) were made. The 59 genotypes exhibited a wide range in SGR, FSS, and EFP. Although SGR and FSS were significantly correlated (r = 0.76**, significant at the 0.01 probability level), subsets of genotypes could be identified that exhibited a wide range in FSS with no variation in SGR. In 1983, seven genotypes (six plant introductions and one cultivar) were selected, on the basis of the 1982 data, to have a relatively similar SGR but a wide range in FSS and EFP. These genotypes were grown on a Maury silt loam soil (fine, mixed, mesic Typic Palendalfs). Fruit samples were taken at weekly intervals to characterize seed development. Cotyledon cell numbers were measured on selected cultivars in 1982 and 1983. For genotypes that differed in EFP, the patterns of dry matter accumulation and seed moisture changes with time were similar to those previously reported. The number of cotyledon cells per unit mass of seed decreased as FSS increased indicating that differences in cell size were associated with the differences in seed size in these genotypes. These data indicate that variation in FSS among genotypes can be associated with either variation in SGR and/or filling period duration.

The cytological mechanism of 2n egg formation was studied in several diploid (2n = 2x = 16) and tetraploid (2n = 4x = 32) clones of cultivated alfalfa (Medicago sativa L.). The comparison of normal megasporogenesis with megasporogenesis that produced 2n eggs was made using an ovule clearing technique with methyl salicylate. Developmental sequences in the formation of n and 2n eggs were the same through anaphase II. Following anaphase II in 2n egg formation cytokinesis occurred only in the micropylar diad, not in the chalazal diad. Micropylar megaspores disintegrated leaving a functional unreduced megaspore of the second division restitution (SDR) type at the chalazal end. The two nuclei in the megaspore can fuse prior to the mitotic divisions or during the first two mitotic divisions. The SDR mechanism of 2n egg formation was confirmed in selected diploid clones by comparing half-tetrad analysis of 2n eggs with half-tetrad analysis of a known first division restitution (FDR) 2n pollen producer. During the study of 2n egg formation in tetraploid alfalfa, several diploid clones were found which produced octoploid (2n = 8x = 64) progeny from 4x-2x crosses. Pollen development in the diploids was normal through telophase II. Cytokinesis was absent, however, and the four telophase nuclei fused to produce a 4n pollen grain. The fusion of a 2n (2n = 4x = 32) egg and a 4n (4n = 4x = 32) pollen grain produced the octoploid progeny.