The pattern of sex chromosome kinetochore phosphorylation during nonrandom segregation in a flea beetle

Abstract

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.

Microtubule distribution during meiosis I in flea-beetle[Alagoasa(Oedionychus)] spermatocytes: evidence for direct connections between unpaired sex chromosomes

“…Our results show that the X and Y chromosomes are not always syntelic, as suggested by Virkki (Virkki, 1971;Virkki, 1972;Virkki, 1985), nor initially amphitelic, as suggested by others (Kupfer and Wise, 2000b;GreenMarroquin et al, 2001). Although there are bidirectional associations between the sex-chromosome kinetochores and microtubules in prometaphase, they appear in many cases to be lateral associations rather than truly amphitelic associations.…”

“…Since the sex chromosomes are mobile in prometaphase (Virkki, 1971;Virkki, 1972), it is likely that the kinetochores are sliding along the microtubules, as observed in other cells (e.g. Reider and Alexander, 1990) and as suggested previously (Kupfer and Wise, 2000b). During prometaphase, the sex chromosomes move towards opposite poles, with the poleward fibre becoming thicker, more developed and acetylated, whereas the fibre in the opposite direction remains weak or disappears.…”