Integrating maize and sorghum RFLP maps
--C. Spike and M. Lee

We are using a recently completed sorghum RFLP map and established maize maps to investigate the extent and nature of genetic linkage between these organisms and to integrate our sorghum and maize maps. Sorghum and maize are diploid members of the tribe Andropogoneae with 2n=20. Previous reports have documented a high degree of homology and conserved linkage between maize and sorghum as determined by low-copy maize probes (Hulbert et al., PNAS 87:4251-4255, 1990; Whitkus et al., Genetics 132:1119-1130, 1992).

The information in this note focuses on linkage group F of our sorghum map. This map was created from a population of 78 F2 plants from a cross between CK60 and PI229828. The map consists of 201 loci placed into 10 linkage groups. Most (134) of the loci were defined by previously unmapped maize cDNA clones. The maize mapping population consisted of 150 F2 individuals from a cross between inbreds Mo17 and H99. The sorghum and maize maps were created from restriction fragment length polymorphisms detected by probes of several sources, Brookhaven National Laboratory (bnl), University of Missouri-Columbia (umc), Pioneer Hi-Bred Intl., Inc. (pio), and Native Plants, Inc. (npi). Other probes for the maize map were provided by C. Hannah, LC, Univ. of Florida (agp2) and M. Scanlon and M. James, ISU (dek*-1047, dek*-326). Additionally a library of maize cDNA clones (jc) was developed at Iowa State University and used as a source of probes for the sorghum map. The jc clones are the primary focus of this investigation because they identify most of the loci on the sorghum map and their map positions were unknown in maize.

jc clones which detect polymorphisms for the parents used to create the maize map were chosen, and those located on sorghum linkage group F were mapped in the maize population. The RFLP segregation data were analyzed and integrated into the established maize map by MAPMAKER (Figure 1). For assigning linkage, a LOD threshold of 3.0 and recombination frequency of 0.4 were used.

The map clearly indicates that jc clones from sorghum linkage group F mapped to maize chromosomes 2 and/or 7. This is consistent with previous observations that sorghum linkage groups often contain loci which map to two maize chromosomes. The order of the loci is substantially conserved, with only three obvious rearrangements. Two putative inversions are present, one spanning three loci on maize chromosome 2 (jc0767a, jc1410, jc0233), and the other involving two closely linked loci on maize chromosome 7 (jc0943, jc0878). Another rearrangement detected by jc0678 is also evident on the short arm of maize chromosome 7. Estimates of genetic linkage distances (Haldane) were compared for regions characterized by high colinearity. Excluding the putative inversions, map distances were very similar with the exception of the interval defined by pio20.728 and bnl14.07.

Among low copy sequences, there appear to be fewer duplicate loci in sorghum. Most probes detected one locus in this sorghum population although several probes (e.g. jc1362, jc0943 and jc0767) also hybridized to two or more monomorphic bands which could represent additional loci. When the same probes are hybridized to DNA of maize inbreds, additional monomorphic bands are detected; usually one or two more than detected in hybridizations with sorghum DNA. Two loci which map to sorghum linkage group F have duplicate loci in either sorghum or maize. jc0676 maps to maize chromosomes 2 and 7, but only to linkage group F in sorghum. jc1362 maps to maize chromosome 7, but detects a duplicate locus in sorghum on linkage group I. This locus on linkage group I is flanked by loci which map to chromosomes 3 and 8 in maize.

Explanations for these types of rearrangements (Whitkus et al., Genetics 132:1119-1130, 1992) often include either (1) differential duplication of chromosome segments in maize or (2) polyploidy in the ancestry of maize and sorghum. Segmental duplication predicts that duplicated segments should be found only in maize. This is consistent with our sorghum map which shows no significant regions of duplication. However, polyploidy could account for the observed relationships if there has been a differential loss of duplicated segments in sorghum, and/or if duplicated regions in sorghum exist but have not been discovered. Our observations currently do not strongly favor either theory.

Figure 1. A comparison of RFLP linkage maps between sorghum linkage group F and maize chromosomes 2 and 7. Numbers directly adjacent to the vertical lines identify genetic distances in cM. Numbers in parentheses next to loci on the sorghum linkage group indicate the maize chromosome containing that locus. Capital letters at the end of locus designators indicate duplicate loci.

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