Population Genetic Diversity in a Maize Reciprocal Recurrent Selection Program

to selection for yield has increased through Cycle 11, while genetic variance within populations showed a The genetic structures of the Iowa Corn Borer Synthetic #1 (CB) nonsignificant decrease. and Iowa Stiff Stalk Synthetic (SS) maize (Zea mays L.) populations are important because these populations serve as the model for develLabate et al. (1997) evaluated several measures of opment of modern commercial hybrids. In 1949, CB and SS were genetic diversity of the progenitors and 100 plants each used to start a reciprocal recurrent selection (RRS) breeding program from Cycle 0 and Cycle 12 in both populations using at Iowa State University. This study was conducted to analyze more 82 restriction fragment length polymorphism (RFLP) thoroughly the genetic diversity within this RRS program and illusmarkers. Gene diversity (expected heterozygosity) and trate how the RRS program has changed over time at the molecular average number of alleles per locus decreased in both level. The progress of this program was measured by analyzing the populations. Nei’s (1978) unbiased genetic distance bevariation at 86 SSR loci among 28 progenitor lines and 30 plants samtween populations increased from the progenitors to pled from each of seven cycles (Cycle 0, Cycle 1, Cycle 3, Cycle 6, Cycle 0 and from Cycle 0 to Cycle 12. The progenitors Cycle 9, Cycle 12, and Cycle 15) in each population. The progenitors of both populations were closely related (Nei’s genetic of these populations show a high amount of variation on the basis of expected heterozygosity (0.557). As the RRS program proceeded, this distance 0.07). Over time, the distance between the variation decreased (Cycle 15, 0.245). In total, a larger amount of gepopulations continued to increase (Cycle 0, Nei’s genetic netic variation was found among plants within cycles (66%) than among distance 0.21; Cycle 12, Nei’s genetic distance 0.66). cycles (13%) or between populations (21%). The repartitioning of variThe genetic distance between the progenitors and ation from within populations (96% in progenitors) to between popCycle 0 of CB was approximately zero (Nei’s genetic ulations (58% in Cycle 15) over time is consistent with theoretical distance 0.02), while the same comparison in SS was expectations of divergence between the populations. By sampling larger (Nei’s genetic distance 0.13). Most loci did meet intermediate time points, we gained a comprehensive genetic view the expectation of random mating or, more specifically, of CB and SS permitting evaluation of the molecular-level changes independence of occurrence of pairs of alleles under occurring as a result of reciprocal recurrent selection. Hardy-Weinberg equilibrium. Excess homozygosity was commonly a factor in deviations from random mating. The tendency to intermate plants with similar flowering I corn borer Synthetic #1 and Iowa Stiff Stalk Syntimes increases the chances of homozygosity (Labate thetic maize populations are important for research et al., 2000). because they characterize the basis of the modern hybrid The objective of this research was to answer questions corn industry (Senior et al., 1998). Both populations are remaining as a result of the work that has been reported involved in a reciprocal recurrent selection program and previously (Labate et al., 1997, 2000). Our approach was are thought to have complementary alleles fixed as a to obtain a more thorough molecular characterization of result (Keeratinijakal and Lamkey, 1993). Many phenothe RRS program between CB and SS by evaluating an typic evaluations of this program have been made since increased number of cycles of selection in each populathe first cross between the CB and SS populations was tion using simple sequence repeat (SSR) markers. We made in 1949 (Penny and Eberhart, 1971). The goal of wanted to see if a smaller sample size is adequate to charRRS is to improve the mean performance of the interacterize these populations, to observe the variations in population cross while maintaining the variability within allele frequency that occur in intermediate time points, populations (Comstock et al., 1949). Results from Keerato see if additional information could be gained from tinijakal and Lamkey (1993), Schnicker and Lamkey SSR versus RFLP analysis, and to more rapidly perform (1993), and Holthaus and Lamkey (1995) indicate that dithe laboratory procedures. The main questions we were rect response (i.e., the value of the interpopulation cross) interested in answering were (i) how has allelic diversity, measured with microsatellites, changed over time in the L.L. Hinze, USDA-ARS, Crop Germplasm Research Unit, Texas populations and (ii) has reciprocal recurrent selection A&M Univ., College Station, TX 77845; S. Kresovich, Institute for changed the population structure or the relationships Genomic Diversity, Cornell Univ., Ithaca, NY 14853; J.D. Nason, Dep. of Ecology, Evolution, & Organismal Biology, Iowa State Univ., between the populations and among the cycles? Ames, IA 50011; K.R. Lamkey, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011. This journal paper of the Iowa Agriculture and MATERIALS AND METHODS Home Economics Experiment Station, Ames, Iowa, Project No. 3755, was supported by Hatch Act and State of Iowa funds. Part of a Maize Populations dissertation submitted by L.L. Hinze in partial fulfillment of the requirements for the Ph.D. degree. Received 16 Nov. 2004. *CorreThe plants genotyped in this study were from two maize sponding author ([email protected]). populations, Iowa Corn Borer Synthetic #1 and Iowa Stiff Stalk Synthetic. These populations were formed from crosses of 12 Published in Crop Sci. 45:2435–2442 (2005). Crop Breeding, Genetics & Cytology doi:10.2135/cropsci2004.0662 Abbreviations: CB, Iowa Corn Borer Synthetic #1; RRS, reciprocal recurrent selection; SS, Iowa Stiff Stalk Synthetic; SSR, simple se© Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA quence repeat. 2435 Published online October 27, 2005