SSR Variation in Important U.S. Maize Inbred Lines

that some of the inbreds continued to contribute substantially to hybrids marketed in the USA. For example, Historically important public inbred lines continue to play an imB73 and Mo17 were used in about 28% of all seed portant role in maize (Zea mays L.) improvement in many different breeding programs. Their continued use means they have undergone planted in the USA in 1979. This fell to 12.8% of the numerous seed increases in diverse programs since their original retotal seed requirements in 1985 (Zuber and Darrah, lease. Our objective was to estimate the level of genetic diversity 1980; Darrah and Zuber, 1985). This may be attributed among and within inbred lines from different sources using SSR markto a shift in resource allocation from public to private ers. We sampled six inbred lines (B73, CM105, Mo17, Oh43, W153R, breeding efforts (Frey 1996). Mo17 was released from and Wf9) obtained from 14 sources (breeding programs). The data the University of Missouri in 1964 and B73 was released were analyzed by analysis of molecular variance (AMOVA), genetic from Iowa State University in 1972 (Troyer, 1999). diversity statistics, and genetic distance (Dice’s coefficient). Of the Other lines such as Wf9, released in 1936 at Purdue total variation observed in gene frequency, 87.8% was found among University, and Oh43, released from the Ohio Agriculinbred lines, 7.6% among sources within inbred lines, and 4.6% within sources. Genotypes of identically named inbred lines from eight differtural Research and Development Center at Wooster in ent sources differed slightly on the basis of 44 SSR loci. The mean 1949, have been used in maize improvement for over genetic similarity between sources of the same inbred was greater 50 yr. than 85%. It can be concluded that although more diversity exists The most widely recognized and utilized inbreds in among these six inbred lines than within them, a small but significant the USA fall into one of three heterotic groups. The amount of variation exists among seed sources within inbreds. This most commonly used heterotic group is Reid Yellow variation may have arisen through differences in seed maintenance, Dent, of which the most utilized source population is since we found no evidence to suggest high mutation rates or extensive Iowa Stiff Stalk Synthetic (BSSS). The Lancaster Sureoutcrossing. The small but statistically significant level of variation crop group consists of material that is more flinty raises concerns in germplasm conservation, mapping studies, marker development, and long-term recombinant inbred line development, (harder starch in the endosperm) in nature than Reid especially when high resolution is desired. Yellow Dent (Gerdes and Tracy; 1993; Baker, 1984; Mumm and Dudley, 1994). The other miscellaneous heterotic category comprises inbred lines developed from crosses between the two major heterotic groups, H maize production in the USA is based on development and crossing of inbred lines. Since between adapted and exotic germplasm, or derived from the 1920s, over 600 public inbred lines have been develdistantly related material such as Minnesota 13. oped, some of which are now extinct (Zuber and Darrah, Once released, inbred lines have been maintained for 1980; Gerdes et al., 1994). Most of the modern inbreds decades through periodic seed increases in breeding being used in public breeding programs are second or programs and at germplasm repositories. Effects of artithird cycle lines that were developed from other inbred ficial selection regimes, natural selection in maintenance lines or from synthetic populations derived from crossenvironments, drift, migration (contamination), and ing inbreds (Baker, 1984). Although the older generamutations could lead to genetic changes (Ajmone-Martion inbred lines have been retired from hybrid seed san et al., 1998; Senior et al., 1998; Mumm and Dudley, production in the USA, they are still widely used in 1994). Such genetic changes would be influenced by the inbred line development, genetic studies, and as testers frequency of regeneration, methods used for regenerain many breeding programs (Smith and Gracen, 1993; tion, unintentional outcrossing, and addition of newer Nedev et al., 2000; Marcon et al., 1999; Bing et al., 1992; versions of the same inbred from other sources. Hallauer et al., 2000). The availability of different inbred Variation can be investigated by means of phenotypic lines from different sources, both within and outside and genotypic measures. Quantitative character studies the USA, indicates their continued importance (Mauria of long-time inbred lines detected genetic changes larger et al., 2000; Livini et al., 1992). Surveys conducted in than those expected by breeders (Russell et al., 1963; the late 1970s and mid 1980s on inbred lines showed Fleming et al., 1964; Russell and Vega, 1973). These changes could affect yields of hybrid combinations after several cycles of regeneration. Genetic changes were J.G. Gethi and M.E. Smith, Dep. of Plant Breeding, 252 Emerson not constant across inbred lines and sources, and were Hall, Cornell Univ., Ithaca, NY 14853-1902; J.A. Labate, USDAARS, Plant Genetic Resources Unit, Cornell Univ., Geneva, NY thought to have resulted from residual heterozygosity, 14456-0462; K.R. Lamkey, USDA-ARS, Corn Insects and Crop Gemutation, or a combination of both (Fleming et al., netics Research Unit, Dep. of Agronomy, Iowa State Univ., Ames, 1964). Busch and Russell (1964) studied 31 sublines IA 50011-1010; S. Kresovich, Institute for Genomic Diversity, Biotechfrom two inbred lines (Os420 and M14) and reported nology Building, Cornell Univ., Ithaca, NY 14853-2703. KRL, JAL, and SK acknowledge funding from the USDA-NRI plant genome biological changes in more than one character in a subprogram (CSREES Award No. 99-35300-7757). Received 28 Feb. line. They attributed such changes either to pleiotropism 2001. *Corresponding author ([email protected]). or mutations at more than one locus. Russell and Vega (1973) reported that genetic changes occurring in inbred Published in Crop Sci. 42:951–957 (2002).