2004 West Coast Worm Meeting abstract 49
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| 1 | Department of Developmental Biology, Stanford University School of Medicine |
| 2 | Department of Genetics, Harvard Medical School |
In preparation for the meiotic divisions, chromosomes must identify and align with their appropriate homologous pairing partners, stabilize this arrangement through assembly of the synaptonemal complex (SC), and undergo crossover recombination events between their DNA molecules. Crossovers at the DNA level collaborate with sister chromatid cohesion to form temporary connections (chiasmata) that hold homologs together after loss of side-by-side alignment and allow them to orient toward and attach to opposite poles of the meiosis I spindle. Moreover, the connections afforded by chiasmata must be coupled with a two-step loss of cohesion, such that partial loss of cohesion occurs at meiosis I to permit dissolution of chiasmata and homolog separation while maintaining connections between sister chromatids that are required to permit chromosome congression and bipolar attachment at meiosis II. We are investigating how the large-scale organization of chromosomes contributes to these essential meiotic functions.
We have used immunofluorescence and high resolution imaging to visualize remodeling of chromosome structures during meiotic prophase progression. This analysis has begun to shed light on the reorganization of chromosomes during SC disassembly and the emergence of chiasmata. First, we found that SC disassembly at the pachytene-diplotene transition is accompanied by a major transition in organization of the chromosome axis, from an extended linear conformation to a highly coiled conformation. Further, while immunofluorescence signals for SC central region proteins SYP-1 and SYP-2 are greatly diminished over most of the chromosome lengths during this transition, they remain concentrated preferentially on the regions distal to the single emerging chiasma on each bivalent. Diplotene organization gives way to the highly compact diakinesis organization, with axis proteins detected as a "cross" localized at the interface between sister chromatids and centered at the chiasma; SYP-1/2 are concentrated specifically on the short axis of the bivalent during early/mid diakinesis, and are no longer detected on chromosomes by the end of diakinesis (the "-1 oocyte"). The region of SYP-1/2 localization in early/mid diakinesis corresponds to the region where cohesion will be lost at the meiosis I division; it was previously shown that Aurora-like kinase AIR-2, which is required for sister chromatid separation at meiosis I, becomes localized to this same region in the -1 oocyte.
Second, we found that asymmetric departure of SYP-1/2 at the pachytene/diplotene transition depends on recombination. We examined SYP-1/2 dynamics in spo-11 and msh-5 mutants, which are successful at assembling SC between fully aligned homologs but cannot form crossovers and consequently lack chiasmata. These mutants are severely impaired in achieving asymmetric departure of SYP-1/2 from chromosomes, tending to retain these proteins rather evenly along the entire lengths of desynapsing homologs. Third, because late prophase SYP-1/2 localization during wild-type meiosis seemed to presage where AIR-2 would become localized in the -1 oocyte and where cohesion would be released at meiosis I, we investigated the relationship between SYP-1 and AIR-2 localization. Although several publications report that AIR-2 first becomes localized to meiotic prophase chromosomes in the -1 or -2 oocyte, we detected much earlier chromosomal localization of AIR-2 during the pachytene and diplotene stages. Moreover, dynamic changes in AIR-2 localization during the pachytene/diplotene transition closely paralleled those of SYP-1, and AIR-2 localization tracked with SYP-1 in the spo-11 mutant and in other mutants where SYP-1 localization is altered. Furthermore, localization of AIR-2 to diakinesis chromosomes is lost in syp-1 and syp-2 mutants. Together our observations strongly support a model in which crossing over triggers asymmetric departure of SYP-1 at the pachytene/diplotene transition; this in turn directs localization of AIR-2 to the region distal to the chiasma, which in turn dictates where cohesion will be released at the meiosis I division.