CHEMICAL ALTERATION OF CROSSING-OVER FREQUENCY IN CHLAMYDOMONAS

Mazia and Ambrose and Gopal-Ayengar have recently suggested that chromo- somes are composed of particulate "macromolecular complexes of nucleic acid and proteins. .linked together by bridges of divalent ions (Ca, Mg, or both)"' or by hydrogen bonds.2This hypothesis was based on various lines of evidence: that chromosomes disperse when treated with chelating agents;1 the observations of Steffensen3 that deficiencies in either calcium or magnesium increase the frequency of chromosome breaks in Tradescantia; and Williamson and Gulick's4 finding that mammalian cell nuclei contain unusually large amounts of calcium and magnesium.In this connection it is of interest that Jungner5 has found relatively high concen- trations of magnesium in purified nucleic acids from several sources and has ob- served that the magnesium content varied within fairly narrow limits, apparently characteristic of the various nucleic acids examined.Genetic crossing over6 would be expected to be intimately related to chromosome structure.Accordingly, an investigation was undertaken of the effects of calcium and magnesium and of treatment with a chelating agent on crossing over in Chlamydomonas.This haploid uninucleate green alga is in some ways perhaps better suited experimentally7' 8 for such studies than is Drosophila, in which Levine9 has recently demonstrated modification of crossing-over frequency by changes in calcium level.Methods.ix growth-factor-requiring strains of Chlamydomonas reinhardi Stock 137C, representing three pairs of linked genes, were used for the cross-over frequency studies to be reported.The arg-l strain requires arginine, citrulline, or ornithine,7 and the arg-2 strain utilizes arginine only; the two genes responsible for the deficiencies are about 6 map units apart.The pab-1 strain will grow only if supplied p-aminobenzoic acid, and its requirement is controlled by a gene which is roughly 23 map units from the gene responsible for the nicotinamide requirement of the nic- 5 strain.The intact thiamine molecule is necessary for growth of the thi-1 mutant,