Chiasma localization, heterochromatin and synaptonemal complexes in the grasshopper Pyrgomorpha conica.

Surface-spread synaptonemal complexes and chiasma distributions in spermatocytes with different C-banding patterns and chiasma distributions in oocytes were analyzed in the grasshopper Pyrgomorpha conica. Male meiosis was characterized by a proximal/distal chiasma localization and complete pairing of homologous chromosomes at pachytene. However, there were indications of a relationship between the frequency and location of pairing initiation sites and chiasma distribution. The presence of a proximal supernumerary segment in a medium-sized chromosome does not increase the mean cell chiasma frequency of carrier individuals compared with those lacking it but may modify chiasma distribution in at least some carrier bivalents. This effect could be related to heterosynapsis in the region near the segment. Mean cell chiasma frequency was significantly lower in females than in males. Females also showed altered chiasma distributions compared with males, with fewer proximal chiasmata and more interstitial and distal chiasmata.