Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

B-chromosomes, also denoted as supernumerary or accessory chromosomes, have been described for a number of species of organisms. It is generally assumed that they were formed from normal chromosomes of the original species as a consequence of nondisjunction during nuclear division and were subsequently fully or only partly inactivated through “heterochromatization”. However, some observations have shown that they can also be formed as a consequence of hybridization amongst related species (Dobson & Tanouye 1998; Perfectti & Werren 2001). In some cases, their presence does not in any way affect the phenotype of the individual; however, especially in larger numbers, they can reduce its biological fitness. At the present time, they are mostly considered to be selfish (parasitic) genetic elements, i.e. elements that do not bear any biologically active (transcribed) genes that are useful for their bearers. Their only function is an ability to remain in the gene pool of the population for a long time. Meiotic drive is one of the mechanisms how the B-chromosomes achieve this goal (Crow 1979; King 1993; Jones 1991).

            In females of grasshoppers of the species Melanoplus femur-rubrum and Myrmeleotettix maculatus, it has been observed that, during meiosis, the B-chromosome has a 90% probability of ending up in the primary oocyte (and not in the polar body). Similar behaviour of B-chromosomes has also been observed in plants. In some cases, the B-chromosome affects the probability of its transfer to the sex cells already during the phase of mitotic division, which precedes meiosis. In this case, the precursor of the sex cell gains both copies of the B-chromosome during mitosis. On the other hand, in other cases, meiotic drive occurs only after completion of meiosis in subsequent mitoses. An example can be found in maize, in which uneven transfer of the two copies of the B-chromosome occurs during pollen formation in postmeiotic mitosis, where the cell containing the B-chromosome participates preferentially in fertilization of the oocyte (King 1993).

            In populations where a certain B-chromosome is present, gene modifiers gradually proliferate (through selection), which suppresses the ability of the B-chromosome to be transferred with elevated probability. However, experiments with forty various B-chromosomes have shown that the ability of B chromosomes to enter the nucleus of the oocyte is unchanged and is manifested   in foreign “naive” populations.