Limits...
All paired up with no place to go: pairing, synapsis, and DSB formation in a balancer heterozygote.

Gong WJ, McKim KS, Hawley RS - PLoS Genet. (2005)

Bottom Line: We have utilized the LacI-GFP: lacO system to visualize the effects of FM7 on meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes.However, the frequencies of failed pairing and synapsis were still 1.5- to 2-fold higher than were observed for corresponding regions in oocytes with two normal sequence X chromosomes, and this effect was greatest near a breakpoint.We propose that heterozygosity for breakpoints creates a local alteration in synaptonemal complex structure that is propagated across long regions of the bivalent in a fashion analogous to chiasma interference, which also acts to suppress crossing over.

View Article: PubMed Central - PubMed

Affiliation: Stowers Institute for Medical Research, Kansas City, Missouri, United States of America.

ABSTRACT
The multiply inverted X chromosome balancer FM7 strongly suppresses, or eliminates, the occurrence of crossing over when heterozygous with a normal sequence homolog. We have utilized the LacI-GFP: lacO system to visualize the effects of FM7 on meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes. Surprisingly, the analysis of meiotic pairing and synapsis for three lacO reporter couplets in FM7/X heterozygotes revealed they are paired and synapsed during zygotene/pachytene in 70%-80% of oocytes. Moreover, the regions defined by these lacO couplets undergo double-strand break formation at normal frequency. Thus, even complex aberration heterozygotes usually allow high frequencies of meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes. However, the frequencies of failed pairing and synapsis were still 1.5- to 2-fold higher than were observed for corresponding regions in oocytes with two normal sequence X chromosomes, and this effect was greatest near a breakpoint. We propose that heterozygosity for breakpoints creates a local alteration in synaptonemal complex structure that is propagated across long regions of the bivalent in a fashion analogous to chiasma interference, which also acts to suppress crossing over.

Show MeSH

Related in: MedlinePlus

Unpaired lacO Sites in c(3)G Mutant Oocytes for Both X/X and FM7/X OocytesCytoplasmic protein marker ORB was used to identify the 16-cell cysts. It is present from region 2a, where it is evenly distributed in the 16 cells. At region 2b and region 3, ORB concentrates in the pro-oocytes and the oocyte. The GFP foci (green) in merge images in conjunction with DNA (blue, DAPI staining) and ORB (red). Two optical sections are shown in wildtype, while six to seven optical sections are shown in c(3)G mutants. Bars = 1 μm.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC1285065&req=5

pgen-0010067-g005: Unpaired lacO Sites in c(3)G Mutant Oocytes for Both X/X and FM7/X OocytesCytoplasmic protein marker ORB was used to identify the 16-cell cysts. It is present from region 2a, where it is evenly distributed in the 16 cells. At region 2b and region 3, ORB concentrates in the pro-oocytes and the oocyte. The GFP foci (green) in merge images in conjunction with DNA (blue, DAPI staining) and ORB (red). Two optical sections are shown in wildtype, while six to seven optical sections are shown in c(3)G mutants. Bars = 1 μm.

Mentions: We were concerned that the associations of homologous chromosomes into regional domains within the nucleus might constrain both X chromosomes into a small enough nuclear region that we might fail to see unpaired lacO couplets even if they did occur. To confirm that high frequencies of failed pairing could be observed, if they indeed occurred, lacO pairing was analyzed in oocytes homozygous for the c(3)G mutation, which disrupts the pairing of euchromatic regions during zygotene/pachytene [1] (Figure 5). These experiments differ from those presented above only in that we used Orb staining to identify meiotic nuclei [23] rather than C(3)G itself. Examining oocytes homozygous for both a lacO insertion at 11A and for c(3)G revealed 15/33 nuclei with two unpaired foci. There were only three nuclei in which two distinct GFP foci were paired (for examples of unpaired foci in this genotype, see Figure 5). The distribution of distances between foci in oocytes with two GFP foci is shown in Figure 3C. By multiplying the frequency of unpaired foci by two or by computing the fraction of two foci nuclei that were unpaired (15/18), we can estimate that the lacO arrays were unpaired in 83%–91% of the oocytes examined. Thus, we can easily observe a failure in homolog pairing in X/X females that are homozygous for c(3)G.


All paired up with no place to go: pairing, synapsis, and DSB formation in a balancer heterozygote.

Gong WJ, McKim KS, Hawley RS - PLoS Genet. (2005)

Unpaired lacO Sites in c(3)G Mutant Oocytes for Both X/X and FM7/X OocytesCytoplasmic protein marker ORB was used to identify the 16-cell cysts. It is present from region 2a, where it is evenly distributed in the 16 cells. At region 2b and region 3, ORB concentrates in the pro-oocytes and the oocyte. The GFP foci (green) in merge images in conjunction with DNA (blue, DAPI staining) and ORB (red). Two optical sections are shown in wildtype, while six to seven optical sections are shown in c(3)G mutants. Bars = 1 μm.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC1285065&req=5

pgen-0010067-g005: Unpaired lacO Sites in c(3)G Mutant Oocytes for Both X/X and FM7/X OocytesCytoplasmic protein marker ORB was used to identify the 16-cell cysts. It is present from region 2a, where it is evenly distributed in the 16 cells. At region 2b and region 3, ORB concentrates in the pro-oocytes and the oocyte. The GFP foci (green) in merge images in conjunction with DNA (blue, DAPI staining) and ORB (red). Two optical sections are shown in wildtype, while six to seven optical sections are shown in c(3)G mutants. Bars = 1 μm.
Mentions: We were concerned that the associations of homologous chromosomes into regional domains within the nucleus might constrain both X chromosomes into a small enough nuclear region that we might fail to see unpaired lacO couplets even if they did occur. To confirm that high frequencies of failed pairing could be observed, if they indeed occurred, lacO pairing was analyzed in oocytes homozygous for the c(3)G mutation, which disrupts the pairing of euchromatic regions during zygotene/pachytene [1] (Figure 5). These experiments differ from those presented above only in that we used Orb staining to identify meiotic nuclei [23] rather than C(3)G itself. Examining oocytes homozygous for both a lacO insertion at 11A and for c(3)G revealed 15/33 nuclei with two unpaired foci. There were only three nuclei in which two distinct GFP foci were paired (for examples of unpaired foci in this genotype, see Figure 5). The distribution of distances between foci in oocytes with two GFP foci is shown in Figure 3C. By multiplying the frequency of unpaired foci by two or by computing the fraction of two foci nuclei that were unpaired (15/18), we can estimate that the lacO arrays were unpaired in 83%–91% of the oocytes examined. Thus, we can easily observe a failure in homolog pairing in X/X females that are homozygous for c(3)G.

Bottom Line: We have utilized the LacI-GFP: lacO system to visualize the effects of FM7 on meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes.However, the frequencies of failed pairing and synapsis were still 1.5- to 2-fold higher than were observed for corresponding regions in oocytes with two normal sequence X chromosomes, and this effect was greatest near a breakpoint.We propose that heterozygosity for breakpoints creates a local alteration in synaptonemal complex structure that is propagated across long regions of the bivalent in a fashion analogous to chiasma interference, which also acts to suppress crossing over.

View Article: PubMed Central - PubMed

Affiliation: Stowers Institute for Medical Research, Kansas City, Missouri, United States of America.

ABSTRACT
The multiply inverted X chromosome balancer FM7 strongly suppresses, or eliminates, the occurrence of crossing over when heterozygous with a normal sequence homolog. We have utilized the LacI-GFP: lacO system to visualize the effects of FM7 on meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes. Surprisingly, the analysis of meiotic pairing and synapsis for three lacO reporter couplets in FM7/X heterozygotes revealed they are paired and synapsed during zygotene/pachytene in 70%-80% of oocytes. Moreover, the regions defined by these lacO couplets undergo double-strand break formation at normal frequency. Thus, even complex aberration heterozygotes usually allow high frequencies of meiotic pairing, synapsis, and double-strand break formation in Drosophila oocytes. However, the frequencies of failed pairing and synapsis were still 1.5- to 2-fold higher than were observed for corresponding regions in oocytes with two normal sequence X chromosomes, and this effect was greatest near a breakpoint. We propose that heterozygosity for breakpoints creates a local alteration in synaptonemal complex structure that is propagated across long regions of the bivalent in a fashion analogous to chiasma interference, which also acts to suppress crossing over.

Show MeSH
Related in: MedlinePlus