Worm Breeder's Gazette 14(1): 98 (October 1, 1995)

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.

Sex is good for males but bad for hermaphrodites

David Gems and Don Riddle

Division of Biological Sciences, University of Missouri, Columbia, MO 65211.

According to the evolutionary theory of aging the life span characteristic
of a species may reflect optimal division of resources between
reproductive output and processes which enhance longevity (1). In some
organisms this division is flexible, e.g. in Drosophila, where there is an
inverse relationship between life span and reproductive output (2).
Understanding the biology of the trade-offs underlying mating costs offers
a possible means to identify determinants of longevity. To this end we
have studied the effect of mating on the life span of C. elegans
hermaphrodites and males at 20 C. Under conditions which guarantee 100%
outcrossing, hermaphrodite life span (50% survival) was shortened by
49-50% (sample sizes, 513 unmated, 354 mated). Maximum life span was
similarly reduced. In contrast, mating caused a slight increase in male
longevity: although 50% survival values were similar in populations of
virgin and mated males, the age-specific mortality rate of mated males was
reduced at advanced ages. This was most pronounced on day 11 where
mortality rates of virgin and mated males were 41.3% and 23.2%,
respectively (initial sample size, 574 virgin and 655 mated males). This
difference resulted, for example, in 20% survival values of 12.6 and 15.4
days for virgin and mated males, respectively. These results are quite
different to those reported by Van Voorhies (3) who observed a cost of
reproduction to males but not hermaphrodites. We repeated our mating cost
experiments using conditions similar to those described in (3) but in
three trials saw a similar decrease in hermaphrodite longevity and an
increase in male longevity.

In contrast to Drosophila and a number of other insect species, we found
no obvious inverse relationship between reproductive output and longevity
in C. elegans. When the brood sizes of individual, briefly-mated
hermaphrodites were compared to their life spans, little correlation was
seen between egg production and life span (N = 70, 48,350 progeny and
3,736 oocytes counted). Also, mating was found to have no effect on the
rate of egg laying per hour. However, a large decrease in hermaphrodite
life span was caused by mating with fer-6(hc6ts) males, which copulate,
but neither stimulate oocyte production nor transfer sperm. This result
indicates that copulation rather than increased egg production reduces
hermaphrodite longevity.

The corpses of mated hermaphrodites often contain larvae. To test the
possibility that internal hatching causes early hermaphrodite mortality we
used fog-2(q71) females. These produce no sperm, so mating with fer-6
males at 25 C produces very few or no fertilized eggs.  A cost of mating
was observed similar to that seen with N2 males, so internal hatching of
larvae is not the cause of the reduced hermaphrodite life span. In
Drosophila seminal fluid both aids in displacement of sperm and shortens
female life span, and it has been suggested that these effects are linked
(4). We found that mating N2 hermaphrodites with fer-6 males (25 C) did
not affect egg output, suggesting that the shortening of life span caused
by mating with fer-6 males is not a side effect of sperm displacement.
Thus, it appears that the physical act of copulation rather than increased
reproductive output reduces hermaphrodite life span.

It is a little hard to accept that the halving of hermaphrodite life span
by mating is unlinked to some reproductive advantage, but merely reflects
a gender-specific maladaptation to copulation. However, our results to
date support this latter interpretation.

(1) Kirkwood, Nature 270 301-304 1977.
(2) Partridge and Harvey, Science 241 1449-1455, 1988.
(3) Van Voorhies, Nature 360 456-458 1992.
(4) Chapman et al, Nature 373 241-244 1995.