B Chromosomes

The earliest record of B chromosomes in plants comes from Anne Lutz in 1916 who found them in Oenothera, and referred to them as ‘‘diminutive chromosomes.’’ Later, they also became known as accessories or extra fragment chromosomes. The term B chromosomes is credited to Randolph, who used this name in 1928 to describe certain additional chromosomes found in some plants of maize, and to distinguish them from those of the regular chromosome complement (2n = 2x = 20), which are the A chromosomes. B chromosomes is now the universally accepted term, and can be conveniently shortened to B or Bs.

The special feature of Bs, which makes them so fascinating and enigmatic, is that they are only found in some individuals of a species and are completely absent from others: In other words, they are dispensable. We may look upon them as optional extras, which is a puzzling idea. It would not be puzzling were it not for the fact that Bs are not rarities at all, but are known in more than one thousand plants and several hundred animals. Bs are part of the genome in those species that carry them, but not an obligatory part like the basic set of A chromosomes. The standard reference that covers all aspects of Bs through the 1980s is by Jones and Rees; two other recent reviews update the story.

In general terms we can profile them as follows:

• At mitosis, Bs are morphologically distinct from the As in size (usually smaller), centromere position, and status of chromatin (often more heavily heterochromatic), which is how we first recognize them and distinguish them from extra copies of the As.
• Their diagnostic feature is that they show no homology with any of the As and never pair or recombine with them—they follow their own evolutionary pathway.
• They display non-Mendelian modes of inheritance due to their presence in variable numbers in different individuals and their special property of ‘‘selfishness’’ in terms of their numerical increase over generations.
• In high numbers they reduce the vigor and fitness of plants.
• Their phenotypic effects are of a quantitative nature, and they lack genes with major effects.

OCCURRENCE

The latest estimate[1] gives the number of flowering plants with Bs as 1372, of which 12 are conifers and 1360 angiosperms. A few examples are known in ferns and fungi.

Among flowering plants they occur in 738 monocots and 622 dicots, and they are found in polyploids as well as diploids. Their presence among families varies enormously, but this variation cannot be interpreted to fit any special pattern. The only thing we can say with any certainty is that they are most often seen in species favored for chromosome studies, such as the Liliaceae and Gramineae.

INHERITANCE

The non-Mendelian mode of transmission of Bs occurs because their number is variable, and meiosis is irregular due to complexities in pairing. In addition, there are sys- tems of mitotic drive in some species (especially Gramineae) based on nondisjunction in gametophytes. In rye there is directed nondisjunction at the first pollen grain and first egg cell mitosis; this leads to an unreduced number of Bs being directed into the gametes. In maize the nondisjunction takes place at the second pollen mitosis, followed by preferential fertilization of the egg by the B- carrying sperm, and likewise constitutes a selfish drive mechanism that causes the Bs to spread in natural populations Equilibrium frequencies of Bs are reached when the forces of accumulation are balanced by those of meiotic loss and reduced fitness.