Palestis et al. 2004

Palestis BG, Burt A, Jones RN, Trivers R. 2004. B chromosomes are more frequent in mammals with acrocentric karyotypes: support for the theory of centromeric drive. Proceedings of the Royal Society of London B (Supplement) 271: S22-S24.

These authors tested the predictions of the theory of centromeric drive (apparently proposed by Pardo-Manuel de Villena and Sapienza, 2001) using a dataset of cytological and taxonomic information for mammals.

The theory of centromeric drive explains the distribution of B chromosomes among species by asymmetries during female meiosis. Female meiosis involves the production of only one gamete from a diploid progenitor cell, compared with the four gametes from one diploid progenitor in males. This unbalance in female meiosis offers an opportunity for selfish genetic elements like B chromosomes to achieve meiotic drive by exploiting differences in spindle capabilities between eggs and polar bodies.

A “strong” spindle in the egg compared with the polar body will capture more centromeres during meiotic division when there are odd numbers, as in the case of a centromeric fission or fusion translocation event that leads to meiotic pairing of one metacentric with two acrocentric chromosomes. This stronger spindle will also tend to capture the centromeres of any B chromosomes, as well. Conversely, if the polar body has the stronger spindle, B chromosomes will be lost from the population more often than chance.

This study found support for the theory of centromeric drive, in that most mammals with Bs had predominantly acrocentric A chromosomes, while acrocentrics were relatively rare among species without Bs. These data support both the proliferation of Bs in predominantly-acrocentric genomes and the typical failure of Bs to establish in predominantly-metacentric genomes.

Two alternative hypotheses for these data are presented, but no other evidence consistent with either acrocentric chromosomes being prone to generating Bs nor with other selection pressures on centromeres are described, apparently because such evidence has not been found. Additionally, although mammal Bs are poorly studied, two cases of strong female meiotic drive in rat Bs are known, associated with weak or absent drive in males, and two other species in which nothing is known about female meiotic drive but mitotic instability in males has been found do not contradict the theory of centromeric drive.