The synaptonemal complexes of the meiotic parthenogenetic Antarctic nematode Panagrolaimus davidi: karyotype analysis and three-dimensional reconstruction of pachytene nuclei.

This study represents the first complete reconstruction of synaptonemal complexes (SCs) and analysis of the pachytene stage of meiosis prophase I in the genus Panagrolaimus. In the free-living meiotic parthenogenetic Antarctic nematode P. davidi, pachytene nuclei had an average nuclear volume of 40 microns 3. Seven synaptonemal complexes were present (n = 7) and the average total length was 65.1 microns. The SCs were attached to the NE at only one end. The nucleolar organizer region was restricted to a central position on the SC. The unstriated central element consisted of two linear components and lacked transverse filaments. Such a modification of the SC has been seen in only two other organisms; the amphimictic nematode Heterodera glycines and the meiotic parthenogenetic nematode Heterodera betulae. Recombination nodules were not evident in the pachytene nuclei. There was no apparent interlocking of bivalents. Morphologically distinct sex chromosomes were not observed.