FRACTIOUS CHROMOSOMES - HYBRID DISRUPTION AND THE ORIGIN OF SELFISH GENETIC ELEMENTS

Supernumerary B chromosomes are dispensable elements of the genome whi ch can be retained in populations at high frequencies, despite being d eleterious, through the ability to undergo non-Mendelian inheritance. Their mode of origin is, however, obscure. Recent work on gynogenetic fish has demonstrated the incorporation of small, unstable, centromere -containing microchromosomes, probably of interspecific derivation, in to an asexual lineage((1)). That these resemble B chromosomes both in structure and behaviour is consistent with the proposal that hybridisa tion between closely related species may be a significant mode of orig in for such selfish genetic elements. Additional work on the B chromos ome of a parasitoid wasp and observations on patterns of chromosome br eakage from somatic cell hybrids also support this hypothesis.