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.
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.