TRANSMISSION ANALYSIS OF MITOTICALLY UNSTABLE B-CHROMOSOMES IN LOCUSTA-MIGRATORIA

Seventeen controlled crosses in which the mitotically unstable B chrom osome of Locusta migratoria was carried by one parent only have provid ed evidence that B chromosomes are significantly eliminated during sex ual transmission in males, at a mean rate that almost counteracts the premeiotic accumulation derived from mitotic instability during germ l ine development. On the other hand, B chromosomes are significantly ac cumulated in females, presumably by their preferential migration to th e secondary oocyte during the first meiotic division. These results su bstantially change the current knowledge about this B chromosome syste m, because the main B accumulation occurs in females and not in males, as was hitherto thought. Furthermore, this case shows that the mainte nance of a single B system in natural populations may be the result of many different forces and mechanisms acting for and against B chromos omes.