Comparative recombination distances for maize inbreds, wide crosses and interspecific hybrids
--Claire G. Williams, C.W. Stuber and Major M. Goodman

Variability in recombination rates for maize was first reported by Bregger (Amer. Nat. 52:57-61, 1918). More recently, this has been confirmed using molecular markers (Tulsieram et al., TAG 84:65-72,1992; Fatmi et al., TAG 86:859-866, 1993). Several chromosomal and genic mechanisms for recombination rates are known in maize: supernumerary chromosomes, chromosomal rearrangements such as inversions or translocations, transposable elements, and modifier genes. A fifth mechanism, cryptic structural differentiation (CSD), inhibits recombination between chromosomes from genetically distant parents. Reduced recombination or "linkage drag" due to CSD is considered a deterrent to using exotic maize germplasm in U.S. maize breeding programs (Lonnquist, Ann. Corn Sorg. Ind. Res. Conf. Proc. 29:102-117, 1974).

The objective of this study is to survey "linkage drag" for a wide range of maize and teosinte races. Testcross pedigrees were constructed for a wide sample of maize and teosinte races using chromosome 1L (Chr1L) rare-allele stocks to ensure balanced polymorphism at all loci. The Chr1L stock is an admixture of U.S. and exotic maize germplasm. Data from eight linked isozymes on Chr1L were used to compare genetic variability in recombination rate among maize and teosinte races.

All exotic x Chr1L maize testcrosses showed enhanced recombination relative to domestic x Chr1L testcrosses. No linkage drag was apparent. Variability in recombination rate was also unrelated to genetic distance between parents; relic indigenous maize isolates did not have more or less recombination than more widespread exotic races. Segregation ratios, when distorted, showed a random pattern with respect to origin. Distorted ratios biased recombination distance only with respect to slight shifts in allele frequencies. Segregation distortion did not account for recombination shrinkage.

The four teosinte x maize testcrosses (Central Plateau, Guatemala teosinte, Balsas teostinte and Zea diploperennis) showed an opposing trend. Recombinant genotypes were largely absent, resulting in a severe deficiency of heterozygotes. Selective elimination of teosinte recombinants appears to have occurred due to a small inversion common to all three teosinte species. These inversions resulted in severe genetic map "shrinkage" and segregation distortion at all loci tested on Chr1L. 

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