Genetic properties of linkage group XIX in Chlamydomonas reinhardtii.

A unique linkage group has been identified in Chlamydomonas. To date, all mutations that have been mapped to linkage group XIX affect flagellar and basal body functions. Linkage group XIX shows several other striking genetic properties. First, the genetic map of this linkage group is circular. Genetic circularity can be achieved because the chromosome is a physically circular molecule or because of constraints on the types of recombination events that occur. A linear molecule that shows complete chromatid interference cannot be distinguished from a circular molecule. Complete chromatid interference is defined as the property that every chromatid is always involved in an even number of recombination events. If interference is not complete, three factor crosses will distinguish between a circular chromosome and a linear chromosome. Experiments of this type are underway (S.K. Dutcher, work in progress). Second, recombination levels on linkage group XIX are affected by temperature; recombination on 12 other linkage groups in Chlamydomonas is not affected by changes in temperature during any part of the meiotic life cycle (S.K. Dutcher, ms. in prep.). Patterns of interference and recombination on linkage group XIX are also different from other linkage groups. Basal bodies/centrioles are cellular organelles that are precisely replicated and partitioned in cell division. This fidelity distinguishes basal bodies/centrioles from all other cellular organelles, with the exception of the nucleus and the chromosomes. Because of the odd genetics of linkage group XIX and the strict replication and segregation of basal bodies, it is intriguing to speculate on the location of linkage group XIX. There are numerous reports in the literature of nucleic acid being associated with basal bodies. Both RNA and DNA have been reported to be localized to these structures. To date no unique species has been identified. Lwoff has suggested that basal bodies are genetically autonomous, and Sagan has suggested that they could have a symbiotic origin. Could linkage group XIX be located in the basal body and not in the nucleus? No definitive answer is available to this question. The number of chromosomes in the nucleus of Chlamydomonas has not been determined reliably. Linkage group XIX segregates as expected for a nuclear chromosome and appears to contain a region that behaves genetically as a centromere. However, any genetic information that is partitioned at meiosis in a regular manner and is present in a limited number of copies could resemble a nuclear chromosome in its segregational properties.(ABSTRACT TRUNCATED AT 400 WORDS)