Use of Sister-Lines and the Performance of Modified Single-Cross Maize Hybrids E. A. Lee,* A. Singh, M. J. Ash, and B. Good ABSTRACT In hybrid maize (Zea mays L.) seed production, yield of the female parent is a major factor affecting production costs. As an alternative to using an inbred line as the female parent, sister-lines (SLs), the F 1 between two highly related inbred lines (A3A*), have been used in seed corn production. Hybrids produced using SLs are referred to as modified single-cross (MSC) hybrids. This study examined (i) yield changes associated with MSC hybrids compared with their respective single-cross (SC) hybrid counterparts and (ii) differences in grain yield of inbred lines and SLs. Three families of six inbred lines each were used to produce three diallel groups of 15 SLs. Simple sequence repeat (SSR) primer pairs were used to establish the degree of relatedness between inbred lines. The SLs and the inbred lines were mated to four unrelated inbred lines to form MSC and SC hybrids. The SC and MSC hybrids were evaluated for grain yield, grain moisture, test weight, and broken stalks in six environments. The six inbred lines and 15 SLs from each family were grown in five environments and evaluated for grain yield, grain moisture, test weight, and broken stalks. Most of the MSC hybrids (72–83%, depending on inbred line family) were not significantly different than the ‘‘best’’ SC counterpart. However, a low frequency of the MSC hybrids, 11 out of 180 (6.1%), had grain yields that were significantly lower than both SC hybrid counterparts. And surprisingly, three out of 180 (1.7%) of the MSC hybrids had grain yields that were significantly greater than both SC hybrid counterparts. The SLs used in this study exhibited an average grain yield that was two-fold greater and more stable or predictable than the inbred lines. These results suggest that there are definite advantages in utilizing SLs in hybrid seed production and that, in general, the performance of the resulting MSC hybrids is expected to be similar to the ‘‘best’’ SC hybrid counterpart. E SSENTIALLY all of the maize acreage grown in the USA and Canada today is planted to hybrid maize, with an increasing percentage of the acreage worldwide (|65%) moving from open-pollinated populations, im- proved synthetics, and variety crosses to hybrids (Duvick, 1999). Production of hybrid maize seed is an intricate process as seed quality, seed purity, and cost of produc- tion are all critically important factors (for review see Wych, 1988; Beck, 2004). One aspect that influences cost of production is grain yield of the female seed parent. Generally. grain yield of inbred lines is in the range of 3.8 to 5.4 Mg ha 21 , which is two- to three-fold lower than hybrid grain yields (Duvick, 1999). Histori- cally, poor inbred seed yield was the impetus behind using double-cross and three-way hybrids (Jones, 1918; 1922; Hallauer and Miranda, 1988; Crow, 1998). With the improvement in performance of inbred lines, SC hybrids began to replace double-cross and three-way hybrids in the marketplace (Wych, 1988; Hallauer and Miranda, 1988; Duvick, 1999). In the late 1980s, it was estimated that SC hybrids represented approximately 90% of the hybrid seed sold in the USA and Canada (Wych, 1988). In the 1960s and early 1970s, MSC hybrids and three-way hybrids were grown extensively in the USA and Canada and in the late 1980s, it was estimated that they still occupied about 10% of the hybrid market (Wych, 1988). Modified single-cross hybrids involve crossing two highly related inbred lines together (A and A*) and using the related-line F 1 (A3A*; i.e., SL) as the female parent in the seed corn production fields. Several factors, whether real or inferred, were behind the shift from double-cross to three-way to SC hybrids. The results of hybrid type comparison studies conclude that SC hybrids are higher yielding and more uniform in appearance (e.g., Eberhart and Russell, 1969; Jugen- heimer, 1976; Wych, 1988). When hybrid type compari- son studies are summarized, however, the differences in grain yield are not as great as predicted on the basis of quantitative genetic theory [100% for SC, 103.8% for three-way cross, 93.1% for double-cross (weighted averages) (c.f., Hallauer and Miranda, 1988). This ex- pectation of SC performance . MSC . three-way . double-cross may be due more to inferences based on quantitative genetic theory. Single-cross hybrid superi- ority over MSC, three-way, and double-cross hybrids is expected on the basis of a single-locus model of additive and non-additive genetic action (Hallauer and Miranda, 1988). Double-cross, three-way, and MSC hybrids repre- sent heterogeneous mixtures, with double-cross hybrids potentially being the most heterogeneous (four different parental genotypes) and MSC hybrids being the least heterogeneous (three parental genotypes, two which are .50% identical). Heterogeneous mixtures of genotypes may result in temporal variability in the field. Temporal variability due to emergence has been shown to reduce grain yield by 4 to 6%, depending on the extent of the variability (Liu et al., 2004). While the presence of temporal variability in double-cross, three-way, and MSC hybrids was not discussed in the hybrid compari- son studies, it could explain the yield advantage of SC hybrids over these other hybrid types. Regardless of the reason, it is quite common to find in hybrid selection recommendations that type of cross is an important con- sideration, as ‘‘single crosses have the maximum hybrid Dep. of Plant Agriculture, Crop Science Building, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1. Financial support, in part, from the Ontario Ministry of Agriculture and Food, Natural Science and Engineering Research Council, Canadian Foundation for Innovation, Ontario Innovative Trust, and Ontario Corn Producers’ Association. A. Singh is supported by a graduate scholarship from Pioneer Hi- Bred International. Received 1 Feb. 2005. *Corresponding author (lizlee@uoguelph.ca) Published in Crop Sci. 46:312–320 (2006). Seed Physiology, Production & Technology doi:10.2135/cropsci2005.0103 ª Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: I, inbred; IBD, identity-by-descent; MSC, modified single-cross; PCR, polymerase chain reaction; RCBD, randomized complete block design; SC, single-cross; SL, sister-line; SSR, simple sequence repeat. Reproduced from Crop Science. Published by Crop Science Society of America. All copyrights reserved. 312 Published online January 24, 2006 Published online January 24, 2006