No abstract

In males of the flea beetle, Alagoasa bicolor L., spermatocytes have two achiasmate sex chromosomes, X and Y, each of which is approximately five times larger than the ten pairs of chiasmate autosomes. At metaphase I, these univalent sex chromosomes are located on a spindle domain separated from the autosomal spindle domain by a sheath of mitochondria. A single centriole pair is located at each pole of the spindle. In prometaphase I, each sex chromosome appears to maintain an attachment to both spindle poles via kinetochore microtubules (i.e., amphitelic orientation). Before anaphase I, this orientation changes to the syntelic orientation (both sister kinetochores connected to the same pole), perhaps by the release of microtubule attachments from the more distant pole by each of the chromosomes. The syntelic orientation just prior to anaphase I leaves each sex chromosome attached to the nearest pole via kinetochore microtubules, ensuring nonrandom segregation. As the sex chromosomes reorient, the autosomes follow in a sequential manner, starting with the bivalent closest to the sex spindle domain. We report here data that shed new light on the mechanism of this exceptional meiotic chromosome behavior.

In the flea beetle species, Alagoasa bicolor, males have two sex chromosomes, X and Y, each of which is larger than the rest of the genome combined. These large sex chromosomes do not pair at meiosis I, and are therefore not joined at metaphase I. Nevertheless, they always segregate from each other at anaphase I. As prometaphase I progresses, the unpaired X and Y undergo reorientation from a parallel to a linear configuration. Using 3F3/2, an antibody that detects the level of phosphorylation of a kinetochore protein or proteins, we have determined that this reorientation is not accompanied by a change in the level of phosphorylation of the kinetochores of either X or Y. This implies that: i) either the reorientation does not involve the loss or gain of kinetochore microtubules, or ii) if such loss or gain occurs, it does not effect a change in the tension placed on the nonrandomly segregating kinetochores, or iii) the sex chromosomes, as in some other species, have lost the ability to sense kinetochore tension changes. Evolution of nonrandom segregation may necessitate the inability of the participating chromosomes to affect the metaphase checkpoint.

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.

In the flea beetle species, Alagoasa bicolor, males have two sex chromosomes, X and Y, each of which is larger than the rest of the genome combined. These large sex chromosomes do not pair at meiosis I, and are therefore not joined at metaphase I. Nevertheless, they always segregate from each other at anaphase I. As prometaphase I progresses, the unpaired X and Y undergo reorientation from a parallel to a linear configuration. Using 3F3/2, an antibody that detects the level of phosphorylation of a kinetochore protein or proteins, we have determined that this reorientation is not accompanied by a change in the level of phosphorylation of the kinetochores of either X or Y. This implies that: i) either the reorientation does not involve the loss or gain of kinetochore microtubules, or ii) if such loss or gain occurs, it does not effect a change in the tension placed on the nonrandomly segregating kinetochores, or iii) the sex chromosomes, as in some other species, have lost the ability to sense kinetochore tension changes. Evolution of nonrandom segregation may necessitate the inability of the participating chromosomes to affect the metaphase checkpoint.