The western corn rootworm, Diabrotica virgifera virgifera LeConte, is an established insect pest of maize (Zea mays L.) in North America.

It has been claimed that plant breeding reduces genetic diversity in elite germplasm which could seriously jeopardize the continued ability to improve crops. The main objective of this study was to examine the loss of genetic diversity in spring bread wheat during (1) its domestication, (2) the change from traditional landrace cultivars (LCs) to modern breeding varieties, and (3) 50 years of international breeding. We studied 253 CIMMYT or CIMMYT-related modern wheat cultivars, LCs, and Triticum tauschii accessions, the D-genome donor of wheat, with 90 simple sequence repeat (SSR) markers dispersed across the wheat genome. A loss of genetic diversity was observed from T. tauschii to the LCs, and from the LCs to the elite breeding germplasm. Wheat's genetic diversity was narrowed from 1950 to 1989, but was enhanced from 1990 to 1997. Our results indicate that breeders averted the narrowing of the wheat germplasm base and subsequently increased the genetic diversity through the introgression of novel materials. The LCs and T. tauschii contain numerous unique alleles that were absent in modern spring bread wheat cultivars. Consequently, both the LCs and T. tauschii represent useful sources for broadening the genetic base of elite wheat breeding germplasm.

The efficiency of breeding programs could be increased by predicting the prospects of crosses for line development before producing and testing lines derived from them. In this study, we (i) assessed the level of genetic diversity among German and Austrian winter wheat (Triticum aestivum L.) cultivars using 117 restriction fragment length polymorphism (RFLP) probes, 16 amplified fragment length polymorphism (AFLP) primer combinations, and 21 simple sequence repeat (SSR) primer pairs, (ii) investigated the correlation between coancestry (f) and genetic similarity (GS) estimated from molecular markers, and (iii) evaluated the use of f and GS for predicting the genetic variance (σ2g) within sets of 22 F4n (n = 7 or 8) lines derived from 30 crosses between these winter wheat cultivars. The average polymorphic information content (PIC) for polymorphic bands was not significantly different between the three marker systems (0.30 ≤ PIC ≤ 0.33), whereas the marker index was low for RFLPs and SSRs but high for AFLPs. Estimates of f between all 55 cultivar combinations ranged from 0.01 to 0.53. ‐values varied between 0.52 and 0.89 for RFLPs, between 0.40 and 0.83 for AFLPs, and between 0.16 and 0.91 for SSRs. The Mantel Z test statistic revealed no common pattern between the four dendrograms obtained by cluster analyses. Significant (P < 0.05) correlations among f̂, ‐RFLP, ‐AFLP, and ‐SSR were detected only for related parent combinations (f̂≥ 0.10). For all seven traits analyzed, estimates of σ2g were not significantly associated with any measure of GS between parents. On the basis of these results, we recommend AFLPs for fingerprinting wheat cultivars. However, predicting the progeny variance (σ2g) remains an unsolved problem.

linkage (Stuber et al., 1992; Crow, 1999). Epistasis, particularly between linked loci, may also be an explanation Heterotic groups and patterns are of fundamental importance in for heterosis in maize (Cockerham and Zeng, 1996). No hybrid breeding of maize (Zea mays L.). The major goal of this study data exclude the possibility of all three mechanisms was to investigate the relationship between heterosis and genetic distance determined with simple sequence repeat (SSR) markers. The contributing to heterosis, albeit in different proportions. objectives of our research were to (i) compare the genetic diversity Lamkey and Edwards (1999) coined the term panmicwithin and between seven tropical maize populations, (ii) test alterna-Breeding, Seed Sci.,

for manipulating QTL in foreground selection. Further, they investigated the combination of foreground and Marker-assisted selection can accelerate recovery of the recurrent background selection in QTL introgression. Openshaw parent genome (RPG) in backcross breeding. In this study, we used et al. (1994) determined the population size and marker computer simulations to compare selection strategies with regard to (i) the proportion of the RPG recovered and (ii) the number of marker density required in background selection. They recomdata points (MDP) required in a backcross program designed for mended the use of four markers per chromosome (of introgression of one target allele from a donor line into a recipient 200-cM length) and a selection strategy for proximal line. Simulations were performed with a published maize (Zea mays recombinants of the target allele. L.) genetic map consisting of 80 markers. Selection for the target Although efficient PCR-based DNA markers such as allele was based on phenotypic evaluation. In comparison to a constant simple sequence repeats and amplified fragment length population size across all generations, increasing population sizes from polymorphisms are available (Ribaut et al., 1997), their generation BC 1 to BC 3 reduced the number of required MDP by as use in background selection is restricted by the large much as 50% without affecting the proportion of the RPG. A fournumber of required MDP. In this study, we investigate stage selection approach, emphasizing in the first generations selection strategies for reducing the total number of MDP needed for recombinants on the carrier chromosome of the target allele, reduced the required number of MDP by as much as 75% in compari-in background selection. Our research objectives were son to a selection index taking into account all markers across the to (i) determine the number of MDP required in backgenome. Adopting the above principles for the design of markerground selection, (ii) investigate the effects of varying assisted backcross programs resulted in substantial savings in the population sizes from early to late backcross generations number of MDP required.on the level of RPG and the MDP required, and (iii) compare a two-stage selection procedure, consisting of one foreground and one background selection step, with