ORIGINAL PAPER Meiotic segregation of a homeologous chromosome pair Received: 17 September 2002 / Accepted: 5 December 2002 / Published online: 12 February 2003 Ó Springer-Verlag 2003 Abstract During meiosis, the alignment of homologous chromosomes facilitates their subsequent migration away from one another to opposite spindle poles at anaphase I. Recombination is part of the mechanism by which chromosomes identify their homologous partners, and serves to link the homologs in a way that, in some organisms, has been shown to promote proper attach- ment to the meiotic spindle. We have built a diploid strain that contains a pair of homeologous chromo- somes V’: one is derived from Saccharomyces cerevisiae and one originates from S. carlsbergensis. Sequence analysis reveals that these chromosomes share 71% se- quence identity. The homeologs experience high levels of meiotic double-stranded breaks. Despite their related- ness and their competence to initiate recombination, the meiotic segregation behavior of the homeologous chro- mosomes suggests that, in most meioses, they are par- titioned by a meiotic segregation system that has been shown previously to partition non-exchange chromo- somes and pairs with no homology. Though the ho- meologous chromosomes show a degree of meiotic segregation fidelity similar to that of other non-exchange pairs, our data provide evidence that their limited se- quence homology may provide some bias in meiotic partner choice. Keywords Homeologous chromosomes Æ Meiosis Æ Distributive segregation Æ Saccharomyces carlsbergensis Æ Non-exchange Introduction Meiosis is the process by which the chromosomal con- tent of the cell is halved. In most organisms, re- combination is a critical component of the meiotic chromosome segregation process (reviewed in Bascom- Slack et al. 1997). Chiasmata, the physical manifesta- tions of genetic exchange, link chromosome pairs, thereby enabling them to be attached to the spindle in such a way that they will migrate to opposite poles at meiosis I (reviewed in Bascom-Slack et al. 1997). Con- sequently, in many organisms, homologous chromo- some pairs that fail to experience exchange show elevated levels of mis-segregation at meiosis I. In yeast, mutations that eliminate meiotic recombination result in extremely low levels of spore viability, resulting from non-disjunction events in meiosis I (reviewed in, Bascom-Slack et al. 1997). Some organisms possess systems capable of par- titioning pairs of homologs that do not experience exchange (Wolf 1994). Systems for partitioning non-ex- change homologs have been best characterized in Drosophila females (reviewed in Hawley and Theurkauf 1993). In female fruit flies, the fourth chromosome pair never recombines, yet the pair still segregates with high fidelity. Heterochromatic DNA sequences play an im- portant role in directing the segregation of non-exchange chromosomes in Drosophila, and chromosome size and shape may also contribute to this process. In the yeast Saccharomyces cerevisiae, meioses in which a homolog pair has failed to recombine are less frequent, but here too, a non-exchange segregation mechanism has been demonstrated. This system can partition artificial non-exchange chromosomes consist- ing of yeast or non-yeast DNA (Dawson et al. 1986; Mol Gen Genomics (2003) 268: 750–760 DOI 10.1007/s00438-002-0796-9 R. Maxfield Boumil Æ B. Kemp Æ M. Angelichio T. Nilsson-Tillgren Æ D.S. Dawson Communicated by E. Cerda´-Olmedo R. Maxfield Boumil Æ B. Kemp Æ M. Angelichio D.S. Dawson (&) Department of Molecular Biology and Microbiology, Tufts University, 136 Harrison Avenue, Boston, MA 02111, USA E-mail: dean.dawson@tufts.edu Fax: +1-617-6260337 T. Nilsson-Tillgren Department of Genetics, Institute of Molecular Biology, University of Copenhagen, 1353 K, Copenhagen, Denmark Present address: R. Maxfield Boumil Department of Molecular Biology, Massachusetts General Hospital, 55 Fruit Street, Boston, MA 02114, USA