A nucleolar-organizing B chromosome showing segregation–distortion in the grasshopper Dichroplus pratensis (Melanoplinae, Acrididae)

Abstract

A nucleolar-organizing B chromosome occurs at low frequency in some populations of the grasshopper Dichroplus pratensis. This B chromosome is telocentric, mitotically stable, and has a proximal secondary constriction. Haematoxylin and silver staining both demonstrate that the constriction organizes a nucleolus in addition to the standard nucleolar organizing region present in the S8 bivalent. In the single 2B male carrier studied, both B's organized nucleoli. The analysis of the meiotic behaviour of 1B males revealed that the B and the X chromosome associate preferentially during early prophase I. At leptotene the B and the X are associated in approximately 50% of the nuclei, but by zygotene – early pachytene the mean frequency of association is 86 reaching 100% in one male. This difference in association frequency between the two stages is highly significant. Persistent X–B associations lead to both chromosomes moving to the same pole in approximately 60% of the first anaphases. In the 2B male, both B's are paired in all early pachytene nuclei, while X–B associations seem to be random and much less frequent than in 1B males. Both B's remain associated in 60% of the metaphase I meiocytes, although true chiasmata do not seem to be involved. Nevertheless the B bivalent disjoins regularly. In 2B cells where both B's are univalents their segregation is random with respect to the X.Key words: B chromosome, nucleolus, nucleolar-organizing region, segregation–distortion, grasshopper.