In the plant-parasite nematode Heterodera glycines, 2 forms, in addition to the diploid (9 bivalents), have been isolated and analysed: a tetraploid (18 bivalents) and an aneuploid (14 bivalents, hybrid between the diploid and the tetraploid). Observations on the formation of their karyotypes indicates normal and non-homologous pairing. Eighteen normal synaptonemal complexes (SC) are present in pachytene nuclei of the tetraploid. Two of the SCs are enclosed in a large heterochromatin mass that is displaced to one side of the nucleus. Such a mass has not been observed in the diploid or the aneuploid. Another 2 normal SCs of the tetraploid have each a ‘modified SC region’ (MSC) within which the SC appears disorganized. The aneuploid has 14 SCs that are unattached at either end. Only 25% of the karyotype length is normal in the appearance of the SCs. The rest can be traced by the presence of disorganized SC material and condensed chromatin. Four MSCs are present in the hybrid nuclei. The possible role of the MSCs in the sex determination system is discussed.

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