Me. Dresser et al., NONHOMOLOGOUS SYNAPSIS AND REDUCED CROSSING-OVER IN A HETEROZYGOUS PARACENTRIC INVERSION IN SACCHAROMYCES-CEREVISIAE, Genetics, 138(3), 1994, pp. 633-647
Homologous chromosome synapsis (''homosynapsis'') and crossing over ar
e well-conserved aspects of meiotic chromosome behavior. The long-stan
ding assumption that these two processes are causally related has been
challenged recently by observations in Saccharomyces cerevisiae of si
gnificant levels of crossing over (1) between small sequences at nonho
mologous locations and (2) in mutants where synapsis is abnormal or ab
sent. In order to avoid problems of local sequence effects and of muta
tion pleiotropy, we have perturbed synapsis by making a set of isogeni
c strains that are heterozygous and homozygous for a large chromosomal
paracentric inversion covering a well marked genetic interval and the
n measured recombination. We find that reciprocal recombination in the
marked interval in heterozygotes is reduced variably across the inter
val, on average to similar to 55% of that in the homozygotes, and that
positive interference still modulates crossing over. Cytologically, s
table synapsis across the interval is apparently heterologous rather t
han homologous, consistent with the interpretation that stable homosyn
apsis is required to initiate or consummate a large fraction of the cr
ossing over observed in wild-type strains. When crossing over does occ
ur in heterozygotes, dicentric and acentric chromosomes are formed and
can be visualized and quantitated on blots though not demonstrated in
viable spores. We find that there is no loss of dicentric chromosomes
during the two meiotic divisions and that the acentric chromosome is
recovered at only 1/3 to 1/2 of the expected level.