Abstract
The hermaphroditic flatworm Dendrocoelum lacteum shows a very pronounced sex difference in chiasma frequency; oocytes have 66 per cent more chiasmata than spermatocytes. The ultrastructural basis of this difference was investigated by three-dimensional reconstruction of synaptonemal complexes from ten oocytes and ten spermatocytes at pachytene and smaller numbers of zygotene nuclei. Oocyte nuclei at pachytene are larger and have longer synaptonemal complexes than the equivalent spermatocyte stage. The difference in synaptonemal complex length (60 per cent) corresponds very closely to the difference in chiasma frequency shown by oocytes and spermatocytes and suggests that synaptonemal complex (SC) length may contribute to the regulation of chiasma frequency. Recombination nodules were observed in both spermatocytes and oocytes but their quantitation was unreliable because they are small and differentiation from the unusually dense and prominent central region is difficult. Analysis of the few zygotene nuclei available shows that pairing is predominantly proterminal in both spermatocytes and oocytes, but oocytes may have several additional interstitial initiations. These extra initiations are likely to be necessary for efficient pairing of the longer oocyte chromosomes.
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