The density and utility of the molecular genetic linkage map of the widespread use of RFLP markers and maps in suncultivated sunflower (Helianthus annuus L.) has been greatly inflower has been restricted by a lack of public RFLP creased by the development and mapping of several hundred simple sequence repeat (SSR) markers. Of 1089 public SSR markers de-probes, consequent lack of a dense public RFLP map, scribed thus far, 408 have been mapped in a recombinant inbred line and low-throughput nature of RFLP markers. The diffi-(RIL) mapping population (RHA280 ϫ RHA801). The goal of the culties posed by the historic lack of public, single-copy present research was to increase the density of the sunflower map by DNA markers were only weakly offset by the emerconstructing a new RIL map (PHA ϫ PHB) based on SSRs, adding gence of facile, universal DNA markers, e.g., RAPDs loci for newly developed SSR markers to the RHA280 ϫ RHA801 RIL (Williams et al., 1990, 1993) and AFLPs (Vos et al., map, and integrating the restriction fragment length polymorphism 1995). RAPDs have primarily been used for tagging (RFLP) and SSR maps of sunflower. The latter was accomplished by phenotypic loci in sunflower, e.g., rust (Puccinia helianadding 120 SSR marker loci to a backbone of 80 RFLP marker loci thi Schw.) and Orobanche cumana Wallr. resistance on the HA370 ϫ HA372 F 2 map. The map spanned 1275.4 centimorgans (cM) and had a mean density of 6.3 cM per locus. The genes (Lawson et al., 1998; Lu et al., 2000). While RAPD PHA ϫ PHB SSR map was constructed from 264 SSR marker loci, and AFLP markers have a multitude of uses, both are spanned 1199.4 cM, and had a mean density of 4.5 cM per locus. The dominant, multicopy, and often nonspecific in nature RHA280 ϫ RHA801 map was constructed by adding 118 new SSR and, as a whole, unsatisfactory for establishing a geand insertion-deletion (INDEL) marker loci to 459 previously nome-wide framework of DNA markers for anchoring mapped SSR marker loci. The 577-locus map spanned 1423.0 cM and cross referencing genetic linkage maps. Single-copy, and had a mean density of 2.5 cM per locus. The three maps were codominant DNA markers, e.g., SSRs, are preferred for constructed from 1044 DNA marker loci (701 unique SSR and 89 such purposes and, until recently, have been lacking unique RFLP or INDEL marker loci) and supply a dense genomein sunflower. wide framework of sequence-based DNA markers for molecular breeding and genomics research in sunflower.

Resistance to the soybean cyst nematode (SCN) (Heterodera glycines Ichinohe) is difficult to evaluate in soybean [Glycine max (L.) Merr.] breeding. PI 437.654 has resistance to more SCN race isolates than any other known soybean. We screened 298 F6∶7 recombinant-inbred lines from a cross between PI 437.654 and 'BSR101' for SCN race-3 resistance, genetically mapped 355 RFLP markers and the I locus, and tested these markers for association with resistance loci. The Rhg 4 resistance locus was within 1 cM of the I locus on linkage group A. Two additional QTLs associated with SCN resistance were located within 3cM of markers on groups G and M. These two loci were not independent because 91 of 96 lines that had a resistant-parent marker type on group G also had a resistant-parent marker type on group M. Rhg 4 and the QTL on G showed a significant interaction by together providing complete resistance to SCN race-3. Individually, the QTL on G had greater effect on resistance than did Rhg 4, but neither locus alone provided a degree of resistance much different from the susceptible parent. The nearest markers to the mapped QTLs on groups A and G had allele frequencies from the resistant parent indicating 52 resistant lines in this population, a number not significantly different from the 55 resistant lines found. Therefore, no QTLs from PI 437.654 other than those mapped here are expected to be required for resistance to SCN race-3. All 50 lines that had the PI 437.654 marker type at the nearest marker to each of the QTLs on groups A and G were resistant to SCN race-3. We believe markers near to these QTLs can be used effectively to select for SCN race-3 resistance, thereby improving the ability to breed SCN-resistant soybean varieties.

Genetic marker systems have improved dramatically in the past 10 yr. Each new system needs to be evaluated for its distribution of markers on genetic linkage maps to validate its use for genetic analysis. The resulting maps are also useful for establishing the genetic positions of genes affecting important phenotypes. We have constructed a high density map in soybean [Glycine max (L.) Merr.] using a 300 RIL (recombinant inbred line) population from BSR‐101 × PI437.654 by first constructing an RFLP (restriction fragment length polymorphism) “scaffold” map based on the entire population. The RFLP anchored map was then further populated with AFLP (amplified fragment length polymorphism) markers based on only a 42 RIL subset. We report here an 840 marker map consisting of 165 RFLP, 25 RAPD (random amplified polymorphic DNA), and 650 AFLP markers spread over 28 linkage groups representing 3441 cM distance. Although clustering of AFLP markers occurred, markers were mapped to every linkage group and were well distributed relative to other marker systems. The AFLP marker system appears to be a useful approach for generating high density genetic maps in soybean.

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