Fort Collins, Colorado
Colorado State University The relationship between synapsis, recombination nodules, and crossing over in maize --Anderson, LK, Stack, SM We examined spreads of synaptonemal complexes (SCs) from inbred KYS maize using electron microscopy to determine the pattern of synapsis in relationship to the frequency and distribution of recombination nodules (RNs). In maize, synapsis generally initiates distally (near ends) and proceeds proximally, although there are also a few interstitial SC initiation sites. Segments near centromeres are ordinarily the last to synapse.

Early RNs (ENs), like late RNs (LNs), are proteinaceous ellipsoids found on the central element of SCs, but ENs are found during zygotene and occur six times more frequently than maize LNs, which occur later in pachytene (see below). ENs are thought to have roles in synapsis and/or early recombination events. Evidence from yeast, mammals, and plants indicates that a subset of ENs gives rise to LNs. Maize ENs do not appear to show interference with respect to one another, although the distribution of ENs along the length of bivalents varies. For example, distal SC segments that synapse early in zygotene have nearly two-fold more ENs per unit length of SC than more proximal segments that synapse later. It is interesting that as zygotene progresses, the frequency of ENs does not increase in distal segments, indicating that ENs do not continue to assemble on already formed SC. This, coupled with the observation that the highest frequency of ENs occurs at synaptic forks, suggests that ENs are assembled on SCs at synaptic forks where synapsis is temporarily delayed.

ENs disappear early in pachytene to reveal LNs. In contrast to ENs, LNs show interference just like crossover events, and, indeed, LNs appear to reside at sites of crossing over. The frequency of LNs per m of SC is six-fold higher in distal compared to proximal segments of SCs. Thus, distal chromosomal segments that synapse first have more ENs in zygotene and more LNs in pachytene. These results suggest that (1) synaptic patterns are important in determining crossover patterns in maize and (2) the first ENs to associate with SC are more likely to give rise to LNs (and crossover events) than ENs that associate with the SC later in zygotene.
 
 

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