ANALYSIS OF RFLP MAPPING INACCURACY IN BRASSICA-NAPUS L

We identified sources of mapping inaccuracy during the construction of RFLP linkage maps from one F-2 population and two F-1 microspore-deri ved populations from the same cross of oilseed Brassica napus. The gen etic maps were compared using a total of 145 RFLP marker loci includin g 82 loci common to all three populations. In the process, we identifi ed a series of mapping events that could lead to ambigous conclusions. Superimposed restriction fragments could be mistaken as a single domi nant restriction fragment in a F-2 population and, when analyzed as su ch, would yield inaccurate linkage information. Residual heterozygosit y in parental lines resulted in complicated allelic assignment and yie lded subsequent difficulties in linkage determination. Loose and spuri ous linkages occurred during mapping and were identified by comparing maps derived from different populations. LOD scores and chi(2) test of independence were compared for their capacity to detect loose linkage s or generate spurious ones. Extreme segregation distortions towards t he same parental allele also contributed to an additional source of sp urious linkage. Small but significant segregation distortions resulted in reduced estimates of the recombination fraction. The use of the sa me 'probe x enzyme' combinations in doubled haploid populations allowe d the identification of the correct allele assignment as well as loose and spurious linkages. A translocation between two homoeologous linka ge groups was observed. The consequences of such a chromosomal event a s a source of error in mapping applications are discussed.