3

8
injection so that the ethylations measured in the sperm of the epididymides
ani vas through 16 days posttreatment represent ethylations that had
occurred within hours after treatment.
The ethylation pattern for the vas sperm, over the 16-day period
studied, closely parallels the pattern of dominant lethals induced in
postmeiotic germ cells of treated males given similar doses of EMS.3,9,ll
When the dominant lethal frequency is low (1-3 days and 13-16 days after
treatment) the sperm ethylations are relatively low and when the dominant
lethal frequency is high (7-10 days after treatment) the sperm ethylations
are also high. Thus there is a strong positive correlation between dominant
lethal frequency and the number of ethylations per sperm cell.
It is noteworthy that by 16 days after treatment with the [3H]EMS
almost no ethylation can be detected in the sperm cells recovered from the
vas. This is in spite of the fact that all germ cell stages in the testes
are apparently ethylated.8 One possible explanation for the decrease in
sperm ethylation in the vas by 16 days posttreatment is that histones which
may be ethylated in the earlier germ cell stages are removed and replaced
by protamine as the germ cells mature.22 Another possibility is that ethyla-
ted DNA contained in the earlier germ cell stages at the time of treatment
may undergo excision repair before these germ cell stages have matured into
spermatozoa in the vas. More will be said below about the repair of
alkylated germ cell DNA.
b. In vivo alkylation of sperm DNA
After determining the number of ethylations per sperm in the vas as
a function of time after injection with the [3H]EMS, the DNA was extracted
from the sperm heads recovered from 4 hours through 10 days after treat-
ment.36,40 The extraction procedures that were used assured the removal