Meiotic Pairing and Chiasma Localization in Scyllina Signatipennis (Gomphocerinae, Acrididae)

SUMMARY

Scyllina signatipennis is a Gomphocerine grasshopper whose male karyotype of 23 telocentric chromosomes includes 11 autosomal pairs (3 long: L₁-L₃; 5 medium-sized: M₄-M₈; short: 3 S₉-S₁₁) and an X-chromosome. Males show an extreme pattern of proximal chiasma localization during meiosis which is characterized by a preleptotenic contraction stage followed by an unravelling stage which precedes a typical leptotene. The nucleus then enters a prolonged diffuse stage that comprises most of zygotene and pachytene. Post-diffuse nuclei represent an intermediate stage between pachytene and diplotene and at this time some homologous pairing is evident. Usually, only one chiasma per bivalent is observed and chiasma frequency per cell ranges from 11.00 to 11.15. An analysis of the percentage of pairing in post-diffuse nuclei suggests that incomplete pairing cannot be the cause of chiasma localization in this species. A role for the diffuse stage in determining chiasma localization is suggested since other species with localization also show a prolonged diffuse stage which replaces conventional zygotene and pachytene. A further interesting feature of meiosis in Sc. signatipennis is the behaviour of the X univalent which is negatively heteropycnotic during most of the diffuse stage suggesting that it could be involved in transcriptional activities. It also showed a high frequency of neocentric activity in metaphase I in most of the individuals. This was correlated with stable equatorial orientation whereas in individuals in which neocentric activity was not apparent, the X usually migrated precociously to one of the spindle poles.