Behavior of sex chromosomes, autosomes, and the spindle during nonrandom segregation in a flea beetle.

We have analyzed autosome, sex chromosome, and spindle behavior in spermatocytes of the flea beetle, Alagoasa bicolor. In this species, males have very large X and Y chromosomes, which, although they are never physically connected, always segregate to opposite spindle poles at anaphase I, thus preserving the sex ratio in the next generation. We find that the sex chromosomes are partitioned to a peripheral spindle domain early in prometaphase I and that their segregation can be accounted for mainly by their reorientation from the parallel to the linear configuration, and little by chromosome-to-pole movement. Further, the behavior of the autosomes and that of the sex chromosomes seem to have little to do with each other. Spindle elongation is minimal; barely segregating the large sex chromosomes into the daughter cells at telophase I.