MAMMALIAN MEIOTIC RECOMBINATION - A REEXAMINATION

Recombination nodules (RNs) are small electron-dense structures associ ated with the synaptonemal complex. Two types have been identified: ea rly RNs present during zygonema-early pachynema, which are thought to be involved in gene conversion and synaptic initiation, and late RNs p resent during mid-to-late pachynema, which are thought to be involved in reciprocal recombination leading to chiasma formation. In organisms as diverse as Sodaria, Drosophila, and plants there is indeed a close correlation between the observed number of late RNs and crossovers, o r their cyto,genetic manifestation, chiasmata. However, as this reexam ination of the human data shows, there is not a similar correlation in mammals. Instead, there is a severe deficiency in RNs in eutherian ma les and marsupial females near chromosome ends and other recombination al ''hot spots'' (defined genetically), or ''localized chiasmata'' (de fined cytogenetically). Many of these sites of hyper-recombination cor respond to sites of telomere or telomere-associated sequences. Togethe r these observations suggest the possibility of a second, mechanistica lly different, recombination pathway that does not involve RNs, but ma y directly involve telomere or telomere-associated sequences. This pat hway may be responsible for sex-specific hat-spots of recombination ob served at highly localized sites throughout the genome.