MOLECULAR-MARKER ANALYSIS OF SEED WEIGHT - GENOMIC LOCATIONS, GENE-ACTION, AND EVIDENCE FOR ORTHOLOGOUS EVOLUTION AMONG 3 LEGUME SPECIES

The objectives of this study were to use molecular markers to: (1) ide ntify quantitative trait loci (QTL) controlling seed-weight in soybean , (2) characterize the genetic basis of seed-weight: expression, and ( 3) determine whether soybean shares orthologous seed-weight genes with cowpea and/or mung bean, An F-2 population was developed between a la rge-seeded Glycine max breeding line and a small-seeded G. soja plant introduction. DNA samples from 150 F-2 individuals were analyzed with 91 polymorphic genetic markers, including RFLPs, RAPDs and SSRs. Seed- weight was analyzed by randomly sampling 100 seeds from each of 150 gr eenhouse-grown F-2 individuals, and their 150 F-2:3 lines, from a repl icated field trial. Markers associated with seed-weight were identifie d using the computer program MapMaker-QTL and a one-way analysis of va riance. Three and five markers were significantly associated with seed -weight variation (P < 0.01) in the F-2 and F-2:3 generations, respect ively. Tests for digenic epistasis revealed three significant interact ions in both generations. In a combined analysis, these markers and in teractions explained 50 and 60% of the phenotypic variation for seed-w eight in the F-3 and F-2:3 generations, respectively. Comparison of ou r results in soybean (Glycine) with those previously reported in cowpe a and mung bean (Vigna) indicated that soybean and cowpea share an ort hologous seed-weight gene. In both species, a genomic region significa ntly associated with seed-weight spanned the same RFLP markers in the same linkage order. A significant digenic interaction involving this g enomic region was conserved in all three species, These results sugges t that the exploitation of ''comparative QTL mapping'' is an invaluabl e tool for quantitative geneticists working with poorly characterized plant systems.