Development of the synaptonemal complex of two types of pachytene in oocytes and spermatocytes of Carausius morosus Br. (Phasmatodea)

During oogenesis of the parthenogenetic stick insect Carausius morosus (2n =61+XXX) pachytene is followed by a duplication of the desynapsed chromosomes, which results in a second type of pachytene (tetrapachytene) consisting of paired sister chromosomes (autobivalents). Electron microscopic studies on sections revealed that synaptonemal complexes (SCs) are formed during tetrapachytene only. This means that the parthenogenetically produced progeny have the genetic constitution of the mother. During spermatogenesis of rare fatherless males (2n=61 + XX) and intersexes (2n=61 +XXX) either an incomplete chromosome doubling (demonstrated by up to 10% additional DNA synthesis) or a complete chromosome doubling takes place during zygotene. EM studies on sections and spreads of germ cells of the first type of meiosis showed that unpaired lateral components (LCs), pieces of SCs and complete SCs are formed during pachytene only, the sex chromosomes being represented by unpaired thickened LCs. The incomplete SC formation reflects the complex heterozygosity of the chromosome complement. In the duplicated type SCs are found in tetrapachytene nuclei only; they are wider than the SCs in oocytes. The sex chromosome bivalents are represented by unpaired thickened LCs or partially paired LCs, in which localized chiasma formation was found. The idea is discussed that formation of SCs does not take place as long as a germ cell has been programmed either to replicate or to be able to replicate its chromosomes and that consequently SCs can be formed only once per meiosis.