Heterosis and Inbreeding Depression in Two Soybean Single Crosses

Heterosis is considered to be of little importance in soybean (Glycine max L. Merr.) because the crop is produced as “pure-line” cultivars or blends of inbred lines. The F1 generations Holladay/ Hutcheson (Cross 1) and Brim/Boggs (Cross 2) were generated by hand pollinations. Inbred generations were generated by bulk selfing. The F1, F2, F3, F4, and F5 generations were yield-tested in replicated bordered single row plots in multiple years and locations. The average yield of Cross 1 F1 was 16% greater than that of the highest-yielding parent and the average yield of the Cross 2 F1 was 5% greater than the highest-yielding parent. Cross 1 showed significant inbreeding depression when regressed on percentage inbreeding which is clear evidence of dominance for yield. Possible genetic bases for heterosis in soybean include gene complementation or interaction of duplicate favorable loci in repulsion, linked dominant alleles that are inherited as a unit, a greater number of dominant alleles in the F1 than either parent separately, multiple dosage-dependant regulatory loci, and/or overdominance. The existence of heterosis should be evidence that superior gene combinations are possible. The magnitude of yield heterosis may be a useful criterion for selection among biparental crosses. HETEROSIS generally is considered to be of little importance in soybean (Glycine max [L.]). A selfpollinated species, soybeans are produced as “pure-line” cultivars or blends of inbred lines. An efficient system for hybrid seed production on a commercial scale has not yet been realized. When yield heterosis has been measured in bordered row plots in more than one environment (of different studies), average high-parent heterosis has been reported as high as 20% (Palmer et al., 2001). Thus, significant yield increases appear to be possible with some F1 hybrid combinations. The source of this heterosis is often thought to be effects of additive 3 additive epistasis that have been found in soybean breeding populations (Burton, 1987). Inbreeding depression (evidence for dominance effects) like heterosis is thought to be rare or nonexistent in soybean breeding populations (Pioneer Hi-Bred International, 1991). Because most quantitative genetic studies have shown genetic variation for yield to be primarily additive (Burton, 1987), soybean breeders practice mostly pedigree selection, single seed descent (Brim, 1966), or some modification of those methods. Typically, a single cross between two good cultivars is followed by rapid inbreeding, derivation of F4, F5, or F6 lines, line evaluation, and selection. In this process, the F1 generation, which may be only three or four plants, is never compared with succeeding generations. Thus, there rarely has been opportunity to observe or measure inbreeding depression where it would be most obvious, in F2 performance compared with F1 performance. In two studies where this was done, Brim and Cockerham (1961) and more recently Lewers et al. (1998), significant inbreeding depression was found. Brim and Cockerham worked with the parents and F1 through F5 generations derived from two single crosses, N48–4860 3 ‘Lee’ and ‘Roanoke’ 3 Lee. In one case, the F2 generation was 10% lower-yielding than the F1 and in the other the F2 was 11% lower than the F1. Lewers et al. (1998) tested F1 and F2 generations and parents of six test crosses. The cultivar ‘Harosoy’ was the pollen parent for all six. Four of the six test crosses showed significant mid-parent heterosis in the F1 generation. Two with significant midparent heterosis in the F1 showed 11 and 16% yield declines in the F2 generation. The lines that Brim and Cockerham (1961) used to develop populations for their study were relatively unimproved and low yielding when compared with modern soybean cultivars, and Harosoy, used by Lewers et al. (1998), is in a similar category released in 1955. It may be that as yield improves with breeding in a selfpollinated species like soybean, and favorable genes accumulate, heterosis will decline. The objectives of the research reported here were (1) to estimate heterosis and inbreeding depression in populations derived from crosses between modern cultivars that would be typical of a standard pedigree selection program, and (2) to compare the results with those obtained by Brim and Cockerham (1961) and Lewers et al. (1998). MATERIALS AND METHODS The cultivars in this study were ‘Holladay’ (Burton et al., 1996), ‘Hutcheson’ (Buss et al., 1988), ‘Brim’ (Burton et al., 1994), and ‘Boggs’ (Boerma et al., 2000). Holladay (?) and Hutcheson (/) were cross-pollinated to generate one F1 hybrid, hereafter designated Cross 1 and Brim (/) and Boggs (?) were crossed to generate the other, Cross 2. Holladay and Hutcheson were both of maturity group Vand Brim and Boggs were both of maturity group VI. Thus, segregation for maturity among plants within each inbred generation was minimized. Generating Hybrid and Inbred Generation Seeds In 1998 and again in 2000, F1 hybrid seed were generated by multiple hand pollinations over a period of 6 wk in the soybean nursery at Central Crops Research Station, Clayton, NC. The F2 generation seeds were generated by self-pollination of ten F1 plants in the 1998–99 USDA winter soybean nursery at Isabella, PR. In 1999, the F1, F2, and parents of the two crosses were field-tested in replicated trials. In 2000, the F2 and F3 generations and parents were tested using bulk-harvested seeds from 1999 F1 and F2 plots as the source of F2 and F3 generation seeds. In 2001, the F1, F2, F3, and F4 generations were field-tested along with parents. Bulk-harvested seeds J.W. Burton, USDA-ARS, 3127 Ligon St., Raleigh, NC 27607; C. Brownie, Statistics, North Carolina State Univ., Raleigh, NC 27695. Received 9 Mar. 2006. *Corresponding author ([email protected]). Published in Crop Sci. 46:2643–2648 (2006). Crop Breeding & Genetics doi:10.2135/cropsci2006.03.0156 a Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA R e p ro d u c e d fr o m C ro p S c ie n c e . P u b lis h e d b y C ro p S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 2643 Published online November 21, 2006