From micrometers to centiMorgans: building genetic maps by direct meiotic analysis

PIGOZZI M.I.

Atibaia, San Pablo

Mesa redonda; 4ta Reunion Brasilera de Citogenética; 2015

Precise approach to crossovers between closely linked genes can be provided by genetic linkage studies, but this method requires the analysis of large data sets, well-controlled crosses or well-characterized pedigree records. An alternate approach for crossover mapping has been developed on the basis of the cytological localization of recombination sites along the pachytene chromosomes using antibodies to MLH1 (mutL homolog 1), a mismatch repair protein of mature or late recombination nodules. MLH1 protein appears as discrete foci detectable by immunofluorescence along the linear synaptonemal complexes (SCs). In birds, antibodies to the mammalian MLH1 protein recognize foci along SCs during pachytene. MLH1-focus mapping on oocytes gave an excellent prediction of the total map length in the chicken, later confirmed after extensive linkage mapping in this species. Because each MLH1 focus corresponds to one crossover, the frequency of foci on defined chromosomal segments can be converted to centimorgan (cM) values. This reasoning has been the basis for the construction of crossover maps based on direct (cytogenetic) meiotic analysis where the cM values are shown along pachytene chromosomes, similarly to the information depicted by linkage maps based on pedigree analysis. This information combined with FISH is a source of recombination data for each linkage group to compare with classic or molecular linkage maps.