The costs and benefits of promiscuity in nematodes
Zusammenfassung der Projektergebnisse
Sexual selection is a part of evolution that can explain many traits and behavioral differences between the sexes. When it comes to mating and reproduction, males and females differ in their interests. Males are less limited in reproduction than females are and therefore compete for matings and paternity. This can lead to male harm due to male-male struggles and female manipulation. However, females can adapt to these manipulations, resulting in a complex dynamic process been males and females. In species, where both sexes mates repeatedly during one reproductive season, females can suffer from male harm as well as benefit from being promiscuous. Mating with more than one male can, for example, lead to a sufficient sperm supply, increase compatibility of gametes and offspring diversity. To test these theoretical predictions, the nematode Caenorhabditis remanei is a well-fitting model organism. Male and female mate repeatedly, while the exact number depends on the sex ratio. Males can potentially father around seven more offspring than females can produce. Female fitness is depending on the number of males around, what means on the number of matings she has. While too few and too many matings can lead to a reduced fitness, presumably due to sperm depletion and male harm, a balanced sex ratio and therefore a medium mating rate results in the female fitness maximum. Knowing the potential for male harm an experimental evolution gave the possibility to increase or relax selection pressure on males and females by using a polyandry versus a polygyny approach. After 20 generations, the effects of male harm did not increase, but female resistance to a male manipulation factor increased in the polyandry treatment, showing that this trait is more flexible and open to adaptation. This gives the hind that antagonistic coevolution between the sexes in present in C. remanei and can be manipulated artificially. Quantifying the effects of direct fitness reduction in females, one can ask the question, whether females can also benefit from mating repeatedly; being promiscuous would only pay off if there are advantages involved, too. One of the ways to manipulate females is to seal the vulva after sperm transfer with a copulatory plug. What if this copulatory plug, can lower the mating rate of a female in her benefit? Surprisingly, experiments showed that a plugged female C. remanei has the same mating rate and is as attractive for males as an unplugged female. Still, females with a plug had a higher fitness, what can be interpreted as an advantage of mating for females besides from receiving sperm (Press release by Frontiers in Zoology and the University of Tübingen). Investigating the benefits of promiscuity, one has to take into account the natural environment of the model species. C. remanei lives in fluctuating environments with different abiotic factors, food sources and micro parasites. One of them is Bacillus thuringiensis, which is naturally occurring with Caenorhabditis nematodes and decreases lifespan and fitness of their host. A females living in contact with these parasites can benefit from multiple mating with different partners, increasing the chance of receiving sperm from a more resistant male. Crossing females with males from different stains, which were diverse in surviving an infection with B. thuringiensis, resulted in a higher indirect fitness of promiscuous females measured as population growth. This experiment shows that parasites, which are recognized being responsible for the evolution sexual reproduction in general, can also lead to benefits of increased sexuality, namely promiscuity.
Projektbezogene Publikationen (Auswahl)
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(2010) The function of copulatory plugs in Caenorhabditis remanei: hints for female benefits. Frontiers in Zoology 7:74
Timmermeyer N, Gerlach T, Guempel C, Knoche J, Pfann JF, Schliessmann D, Michiels NK
