Triplo-X hermaphrodite of Caenorhabditis elegans: pachytene karyotype analysis, synaptonemal complexes, and pairing mechanisms

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Abstract
Pairing of the 3 X chromosomes in the triplo-X strain of C. elegans occurs at pachytene in a 2-by-2 fashion such that 1 bivalent and 1 univalent are formed. The XX bivalent pairs synchronously with the autosomes and the univalent X remains in a similar chromatic state as the rest of the chromosomal complement. Normal tripartite synaptonemal complexes (SC) are formed between all bivalents. The univalent X lacks a SC and an axial core is not observed. The condensation of the univalent X in the triplo-X is different than in the male where the univalent X is heterochromatic. This real difference in condensation states of the chromatin may explain the fact that the univalent X is maintained in the male line yet it is easily lost in the triplo-X strain.