Worm Breeder's Gazette 15(1): 45 (October 1, 1997)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

The Egl-c Index

Tom Barnes, Siegfried Hekimi

Dept Biol, McGill University, Montreal PQ Canada

        The egg-laying constitutive phenotype may be defined in a
number of ways. One definition is the laying of eggs more immature than
those laid by N2, which is a relative measure of uterine residency
time. Another definition is an elevated rate of egg-laying relative to
N2 in the absence of food, which can be conveniently quantitated by
counting the number of eggs laid by worms over a short time (1-3 h) in
microtitre wells containing M9. This measure, however, depends on both
the egg production rate and the uterine residency time, and will be set
by whichever is limiting. For strains with the classic Egl-c appearance
in the presence of food (i.e. retaining fewer eggs than normal in the
uterus), egg production rate will be limiting, since by definition
these worms! eggs have briefer uterine residency. (Egg production rates
on food will probably be limiting for most strains.) Off food, however,
N2s respond first by increasing uterine residency dramatically
(retaining eggs), and then by stopping egg production after a few
hours. Over the course of a microtitre well test, therefore, elevated
egg laying will correspond largely to a failure to retain eggs in
response to the lack of food. (Beyond ~3 h in M9, egg laying slows
dramatically in all strains we have looked at, as egg production
        If an elevated egg-laying rate in M9 in Egl-c strains
corresponds to a failure to retain eggs, then in these strains uterine
residency will be brief and may not be rate-limiting. Rather, the rate
measured is likely to be limited by the egg production rate, just as it
would be on food. Thus for two different strains with comparably brief
uterine residencies and which lay excess eggs in M9, the observed
egg-laying rate may be strongly influenced by the egg production rate.
Direct comparison of egg-laying rates in M9 (as is typically done) thus
assumes that all genotypes have the same egg production rate, which has
never been systematically examined. We have found that this assumption
does not always hold.
        Since the egg production rate can be measured as the on-food
rate of egg laying (see above), we propose that egg-laying rates be
normalized by dividing the off-food (M9) rates by the on-food rates,
which we measure on plates by counting the number of eggs laid in a
timed (~24 h) span in the middle of the brooding period. This gives a
value between 0 and 100% of the on-food rates; we call this the Egl-c
Index. As we reported in the last Worm meeting, unc-24 mutants are
Egl-c, but we thought less so than unc-2(e55), the strong Egl-c
positive control (see Fig). This might suggest that the failure to
retain eggs in M9 is more profound in unc-2 than unc-24 mutants. We
found however that egg laying rates of unc-24 mutants are about half
that of N2 (curious in its own right), while that of unc-2 mutants is a
little more than N2 (see Fig). Strikingly, we find that the Egl-c
Indices of unc-24 and unc-2(e55) are almost exactly the same (see Fig).
We therefore conclude that contrary to first impressions, the degree of
failure to retain eggs off food is identical in unc-2 and unc-24
mutants (i.e. they are equally Egl-c).