Maintenance of a "Parasitic" B Chromosome in the Grasshopper MELANOPLUS FEMUR-RUBRUM.

About 10-15% of the males and females of the grasshopper Melanoplus femur-rubrum collected near Rochester, New York, possessed a supernumerary B chromosome. The frequency of the B chromosome remained fairly constant during the years 1971-1974. The B chromosome was shown previously to be transmitted at a rate of about 0.5 and 0.8 by 1B males and females, respectively. This study was designed to determine the forces preventing the B chromosome from increasing in frequency due to the high rate of transmission by the females. Eighty inseminated females collected in the wild were analyzed cytologically together with their embryos (10-20 per female). Ten of the 80 females had a B chromosome, and they transmitted it at a rate of about 0.75. Among the 983 embryos analyzed, 0.141 had one B, 0.007 had two, and the mean number of B chromosomes per embryo was 0.155. The frequency of the B chromosome in the sperm pool (0.061) was consistent with a 0.5 rate of transmission. Individuals with two B chromosomes apparently have low viability, because about six were expected, but none was found among 851 adult males and females examined. The data suggest that the viability of the 1B individuals was only about 0.86 that of the OB individuals. There was no evidence that the B chromosome increased the fecundity of either the 1B males or females. It was concluded, therefore, that the B chromosome reduced the fitness of all the individuals carrying it and was thus "parasitic," and that it was maintained in the population only because of its high transmission rate. The maintenance of other B chromosomes with high transmission rates is reviewed.