ZOOLOGY: H. H. PLOUGH 
553 
THE EFFECT OF TEMPERATURE ON LINKAGE IN THE SECOND 
CHROMOSOME OF DROSOPHILA 
By Harold H. Plough 
ZOOLOGICAL LABORATORY. COLUMBIA UNIVERSITY 
Communicated by T. H. Morgan, July 5, 1917 
Some recent data have shown that certain influences affect the per- 
centage of crossing over. Bridges^ (1915) noted that the age of the 
mother altered the result in the second chromosome, and Sturtevant^ 
(1917) has recently discovered no less than three definite Mendelian 
factors which influence the strength of linkage in parts of the second 
and of the third chromosomes. For two years I have been studying 
the effect of changes in the environment on the percentage of crossing 
over in Drosophila melanogaster (ampelophila) . It has been found that 
very striking effects are produced by differences in temperature for the 
second chromosome. A full report of this work will appear in a forth- 
coming number of the Journal of Experimental Zoology. 
Tests of the effect of temperature on the percentage of crossing over 
were made in the following way. Virgin females from the stock collectea 
originally at Falmouth, Mass., were mated to males homozygous for 
the second chromosome factors for black body color, purple eyes and 
curved wings. The pairs were placed successively in two or more sets 
of bottles for three or four days each. The bottles of the first set were 
kept at room temperature — about 22°C — and each of the other sets at 
one of the temperatures to be tested. The normal females heterozygous 
for black purple curved which hatched from each set were then back 
crossed to males of the original mutant stock and the offspring allowed 
to hatch at room temperature. The offspring (F2) of the back cross 
show the genetic constitution of the (Fi) eggs, and enable one to calculate 
the percentage of crossing over between black and purple, and purple 
and curved. The percentages for the shorter region — black to purple — 
from a number of experiments involving more than 35,000 flies are shown 
in the curve given in figure 1. The value at 22°C is the average of the 
controls for all of the different sets. At lower and higher temperatures — 
5°C. and 35°C. — no fertile offspring hatched. 
The curve is of considerable interest since it shows that both high and 
low temperatures produce an increase in the percentage of crossing over, 
i.e., a reduction in the strength of hnkage. It is plain that the process 
does not follow van't Hoff's law, as do most physiological processes.^ 
The phenomenon therefore involves apparently some change in the phys- 
ical state of the colloidal substratum of the cells. 
