B chromosomes in hybrids of temperate cereals and grasses

B chromosomes are considered to be genetically inert, yet often have pronounced and surprising effects upon the A chromosome behaviour at meiosis in inter-generic and inter-specific hybrids. We review here our current knowledge of these effects in a number of different hybrids of the temperate cereals and grasses. Through hybridisation, many effects comparable to the pairing control system of wheat are uncovered, together with complex interactions of B chromosomes with hybrid host genotypes. We discuss the genetic and physical basis of the effects and try to make sense of them in terms of what we know about the origin and evolution of B chromosomes in plants.