Evolutionary Biology
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Schärer Group
Evolutionary Biology
Zoological Institute
University of Basel
Vesalgasse 1
CH-4051 Basel
Switzerland

Sex allocation
Trade-off
A fundamental assumption of sex allocation models is a direct trade-off between investment into male or female reproductive function. However, despite its generality the assumption has rarely been explicitly tested in any animal species. There are a number of studies in plants, both on a phenotypic level and based on genetic correlation analyses, but support for a trade-off remains equivocal at best. Macrostomum lignano allows both of these approaches. In a recent project we have shown that the phenotypic trade-off is only visible under very specific conditions (Schärer et al. 2005). A study on genetic correlations is planned.
Mating Group Size
Sex allocation theory for simultaneous hermaphrodites predicts an influence of the mating group size on sex allocation. We have recently confirmed this prediction in an experiment where we raised Macrostomum lignano, in different group sizes (pairs, triplets, quartets, octets) and in different enclosure sizes (small and large). This design allows to distinguish between effects of group size and density. The results suggest that the mating group size is a function of the group size, and not of the density (Schärer & Ladurner 2003). Based on this finding we have since tested another prediction from sex allocation theory, namely that hermaphrodites may adjust their sex allocation to variable environmental conditions. This ability may be one of the key advantages of hermaphroditism over gonochorism, because gonochorists can only adjust their sex allocation to future environments. We again raised worms in small and large groups until they had established their sex allocation for that environment. They were then transferred to a contrasting environment and we checked if they readjusted their sex allocation to the new situation in the expected way. The results suggest that the worms do have this kind of phenotypic flexibility in sex allocation (Brauer, Schärer & Michiels 2007).
Resource Availability
The amount of resources available to a simultaneous hermaphrodite are predicted to influence its sex allocation decision, but there is very little experimental evidence for this. Individuals that have few resources are predicted to be more male biased, which is reflected in many cases of size-dependent sex allocation. We raised worms to maturity on a wide range of food levels and determined the resulting sex allocation patterns. The results are in agreement with these predictions (Vizoso & Schärer 2007).
Conflicts over sex allocation
The high flexibility in sex allocation exhibited by hermaphrodites poses a potential problem. Different genetic entities may disagree over the optimal sex allocation of an individual, and may therefore try to manipulate its sex allocation to their own benefit. We are currently performing a range of experiments, which aim at testing for such conflicts a) between the mating partners and b) between different genetic entities within an individual (e.g. nuclear vs. cytoplasmic genes).


this page was last updated on Tuesday, December 22, 2015