Mitotic instability leading to an accumulation of B-chromosomes in grasshoppers

The study of mitotically unstable B-chromosomes (supernumeraries) of two grasshopper species confirmed a suggestion made earlier (Nur, 1963) that the instability should always be associated with a tendency of the B's to increase in frequency. Among 780 Camnula pellucida (Scudder) males from California, 105 had B's. In the testes of these males the number of B's varied from follicle to follicle and ranged between 0 and 4. Because of this variation, the number with which each male started to develop could not be determined. However, the relatively low frequency of males with B's and the regular meiotic behavior of the latter suggested that most of the 105 males started with a single B. Cytological analysis of the cells of the gastric caeca of 31 males whose testes contained B's confirmed this suggestion by showing that only one male had two B's in these cells; all the rest had one. In the testes of the 74 other males the mean number of B's ranged from 0.89–2.50, but only two males had means higher than 2.00. The observed ratio of one male with two B's to 30 with one, suggested that only the two males with the highest means started to develop with two B's and that the other 72 males all started with one. Since the mean for the 72 males was 1.37 B's per male, it was concluded that during the development of the testes of these males the mean increased by 37%. The males with B's had fewer follicles in their testes and apparently had also a lower frequency of normal sperm. — The analysis of the testes of Locusta migratoria L. males from Japan gave results which agreed with those from C. pellucida.Supported by grants GB 1585 and GB 6745 from the National Science Foundation, Washington, D.C.