These species allowed a more profound understanding of B-chromosome meiotic behavior, including mechanisms of elimination and accumulation through meiotic drive
, B chromosome structure in terms of molecular composition, effects on carriers at the endo- and exophenotypic levels, distribution in natural populations, including cases of clinal variation, and the relationships of Bs with the A genome and their evolution through genomic conflict, especially in E.
Control of meiotic drive
of B chromosomes in the mealybug Pseudococcos affinis (obscurus).
The transition to new hierarchical levels implied conflicts between adjacent levels, examples of which persist today: some traits seem not to favor the multi-cellular organism, but give advantage to lower level units (e.g., meiotic drive
) or to higher ones (classic group selection overriding individual selection).
Because asymmetric distribution of centromeric activity in structural heterozygotes has the potential to select for chromosomal variants through meiotic drive
(Pardo-Manuel de Villena & Sapienza, 2001), increased rate of centromere evolution may lead to higher rates of fixation of chromosomal variants.
This is not surprising, because cycles of cell fusion and chromosome segregation provide very strong selective advantages to molecular parasites, such as transposable elements and meiotic drive
genes, which have clearly modified the sexual process to further their own interests.
The t haplotypes of the house mouse were first discovered in 1927 (Dobrovolskaia-Zavadskaia and Kobozieff 1927), and remain one of the best studied mammalian examples of meiotic drive
, or ultraselfish DNA (Wu and Hammer 1991).
4 Meiotic drive
: contribution to the fitness goal as an additional needed constraint
Whatever keeps psr from destroying the species, the chromosome apparently has an ambitious "meiotic drive
" of its own, says Nur.
Genotypes suppressing meiotic drive
of a B chromosome in the mealy bug Pseudococcus obscurus.
What about sex-ratio meiotic drive
? Or the rare-male mating advantage?
for B-chromosomes in the primary oocytes of Myrmeleotettix maculatus (Orthoptera: Acrididae).
Therefore, the most likely mechanism for B accumulation is meiotic drive