Fax +41 61 306 12 34 E-Mail karger@karger.ch www.karger.com Special Chromosomes Cytogenet Genome Res 120:265–280 (2008) DOI: 10.1159/000121076 Chromosomes with a life of their own R.N. Jones a M. González-Sánchez b M. González-García b J.M. Vega b M.J. Puertas b a Aberystwyth University, Institute of Biological Sciences, Aberystwyth (UK) b Departamento de Genética, Facultad de Biología, Universidad Complutense, Madrid (Spain) (1924) and Kuwada (1925) correctly classified them in rye, and later when Longley (1927) found them in maize. Long- ley first called them supernumeraries, and Randolph (1928), also working with maize, later used the term B chromo- somes, to distinguish them from the chromosomes of the basic complement called the A chromosomes (As). The term supernumerary B chromosomes is now simplified to Bs. The essential features of Bs are: (i) they are dispensable; (ii) they pair only among themselves at meiosis (in species where they do pair) and do not recombine with the As; (iii) their inheritance is irregular, due to their polysomic nature and to the occurrence and elimination of univalents, all of which compromises their transmission through meiosis; (iv) meiotic elimination in some species is counter-balanced by processes of drive at mitosis, mainly in the gametophytes, and less frequently at meiosis, leading to equilibrium fre- quencies in populations; (v) they have adverse and quantita- tive effects on the phenotype when present in high numbers, especially on fertility which also contributes to their loss; (vi) they lack any known major gene loci, but rDNA se- Abstract. B chromosomes (Bs) can be described as ‘pas- sengers in the genome’, a term that has been used for the repetitive DNA which comprises the bulk of the genome in large genome species, except that Bs have a life of their own as independent chromosomes. As with retrotransposons they can accumulate in number, but in this case by various processes of mitotic or meiotic drive, based on their own autonomous ways of using spindles, especially in the game- tophyte phase of the life cycle of flowering plants. This self- ish property of drive ensures their survival and spread in natural populations, even against a gradient of harmful ef- fects on the host plant phenotype. Bs are inhabitants of the nucleus and they are subject to control by ‘genes’ in the A Request reprints from M. Puertas Departamento de Genética, Facultad de Biología Universidad Complutense, ES–28040 Madrid (Spain) telephone: +34 91 3945044; fax: +34 91 3944844 e-mail: majetas@bio.ucm.es © 2008 S. Karger AG, Basel 1424–8581/08/1204–0265$24.50/0 Accessible online at: www.karger.com/cgr chromosome (As) complement. This interaction with the As, together with the balance between drive and harmful effects makes a dynamic system in the life of a B chromo- some, notwithstanding the fact that we are only now begin- ning to unravel the story in a few favoured species. In this review we concentrate mainly on recent developments in the Bs of rye and maize, two of the species currently receiv- ing most attention. We focus on their population dynamics and on the molecular basis of their structural organisation and mechanisms of drive, as well as on their mode of origin and potential applications in plant biotechnology. Copyright © 2008 S. Karger AG, Basel B chromosomes (Bs) are a unique class of supernumer- ary chromosomes that are optional extras in the genomes of numerous plant and animal species. They are dispensable, being found in some individuals in populations that carry them, and absent from others, which raises significant bio- logical questions in terms of genome organisation, popula- tion cytogenetics and evolution. Bs were first discovered 100 years ago in an insect, in species of the leaf-footed plant bug Metapodius, by Edmund Wilson (Wilson, 1907a, b). They were first recognised in plants in the 1920s, when Gotoh This work was supported by the grants BFU 2006-10921 and AGL2005-05104 from the DGICYT-MEC of Spain, and PR27/05-13949-BSCH from the Ban- co de Santander-UCM. Accepted in revised form for publication by M. Schmid, 05 December 2007.