Early recombination nodules (ENs) are multiprotein complexes that are thought to be involved in synapsis and recombination,
but little is known about their components or how they may be involved in these events. In this study, we describe the cytological
behavior of a possible EN component, MRE11, a protein that is important for the repair of the numerous, programmed deoxyribonucleic
acid double-strand breaks (DSBs) that occur early in the meiotic prophase. By immunofluorescence, many MRE11 foci were associated
with chromosomal axes during early prophase I in both wild-type Arabidopsis and tomato primary microsporocytes. Similar patterns
of MRE11 foci were observed in two Arabidopsis mutants (
Atspo11-1 and
Atprd1) that are defective in DSB formation and synapsis. In tomato chromosomes, MRE11 foci were more common in distal euchromatin
than in proximal heterochromatin, consistent with known EN patterns. However, electron microscopic immunogold localization
demonstrated that only about 10% of ENs were labeled, and most MRE11 label was associated with synaptonemal complex components.
Thus, in plants, MRE11 foci are not dependent on DSB formation, and most MRE11 foci do not correspond to ENs. More generally,
our results show that the simple presence of large numbers of fluorescent foci associated with synapsing chromosomes is insufficient
evidence to equate these foci with ENs.
Communicated by E.A. Nigg.
Leslie D. Lohmiller and Arnaud De Muyt contributed equally.