Supernumerary segments in a bivalent of Leptysma argentina (Orthoptera): intra- and interchromosome chiasma effects

SUMMARY

One of the most frequently reported cases of chromosome polymorphism amongst grasshoppers is the presence of supernumerary heterochromatin, involving either B chromosomes or supernumerary segments. Moreover, it has been long evident that these changes may exert intra- and/or interchromosome effects on chiasma frequency and/or position. In the present paper a cytogenetical analysis of 447 males of Leptysma argentina (Acrididae: Orthoptera) revealed that two supernumerary segments (both of them sited in the smallest member of the karyotype) produce effects on chiasma conditions. The proximal segment (s_p) precludes chiasma formation in the proximal region of the bivalent contiguous to the segment, causing a more distal chiasma repatterning; this effect is accumulative, since in sp homozygotes this pattern is significantly more evident. The interstitial segment (s₁) causes a significant increase in chiasma frequency (due to enhanced proximal chiasma frequency) in some populations of L. argentina, but not in others. Finally, it was noticed that the presence of heterozygous s₁ in the normally monochiasmate bivalent 10 raises average chiasma frequency of this bivalent from 1.00 to 1.24. This fact suggests that the presence of a discontinuity in pairing caused by the formation of a loop in the region of the segment—perphas due to a discontinuity in the synaptonemal complex—produces a relaxation of interference. However, other facts that reveal that interference, although relaxed, is still operative are mentioned and discussed. These facts suggest that the continuity of the synaptonemal complex may play a central role in the operation of interference.