Homolog groups preferentially achieve pairwise synapsis, but supernumerary chromosomes challenge this process.

<p><b>A.</b> Synapsis patterns in the indicated karyotypes illustrated by half-nuclear projections of nuclei from the mid-pachytene region. HTP-3 (red) marks chromosome axes and SYP-1 (green) marks SC central region; overlap (yellow) indicates synapsed segments with full synapsis illustrated in the diploid. The altered karyotypes display a mixture of nuclei containing unsynapsed regions (arrows) and nuclei exhibiting apparently complete synapsis (circles; assessments were made from full projections). Images are not deconvolved; bars = 4 µm. <b>B.</b> Major classes of X chromosome synapsis configurations in the mid-pachytene region illustrated by 3-D surface renderings of individual nuclei stained for SYP-1 (white) and HIM-8 (green) 24 h after S-phase labeling (red) of X chromosomes. Pairwise synapsis, characteristic of the diploid, also predominates in tetraploid and triplo-X nuclei; whereas in triploids, all three X chromosomes typically occupy a joint domain containing a single SC track. White dotted traces indicate SYP-1 tracks associated with X chromosome territories. iv: region where a single SC track is associated with four X chromosomes (purple arrow), and complex SC tracks associated with the remainder of the X territory (gray dotted traces); v: unsynapsed third X (red dotted trace); vi: short SC track associated with a self-synapsed third X (yellow arrow); vii and viii: single SC track within the domain occupied by all three X chromosomes (blue arrow). Frequency of each synapsis configuration is indicated below each image pair. Nuclei from 4–6 germ lines were analyzed. See <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1003963#pgen.1003963.s004" target="_blank">Figure S4</a> for full quantitation of classes in triplo-X and triploids; the remaining two tetraploid nuclei appeared to display partial pairwise homologous synapsis with partial asynapsis or partial heterologous synapsis.