Chiasma function

Meiosis is an evolutionarily ancient process. Since it is nearly ubiquitous across modern eukaryotes and since its basic features are similar across the plant and animal kingdoms, the basic process probably evolved only once, and therefore information from a variety of organisms can be compared to address specific questions. However, evolution has had time to tailor particular parts of meiosis to meet the needs of the organism it is in - witness the wide range of levels of crossing over - so information from different species should be compared with caution; it is not necessarily safe to assume that a feature of meiosis in one organism is obliged to be present in all. However, the topic of this minireview - the relationship between crossing over and chromosome segregation - does appear to be remarkably consistent. In all organisms in which meiosis has retained the (presumably original) feature of crossing over, the resulting chiasmata are used to orient homologous chromosomes to opposite poles at metaphase I. There are organisms whose meioses lack crossing over; here, proper disjunction occurs by other means. Although interesting in their own right, such cases are ignored here. We are also going to ignore the very interesting questions of the DNA mechanics of crossing over itself and of how particular sites are chosen for the privilege in any given meiosis. We will start with the chiasmata themselves and follow their behavior.