One of the most fascinating questions in evolutionary biology is the evolution and maintenance of sexual reproduction. Although sexual reproduction is associated with major costs (cost of males, cost of meiosis etc.) this reproduction mode is widespread and common in both animal and plant kingdom. In the recent years a variety of theories and hypothesis has been developed to explain this so-called paradox of sex. Among them the most prominent ones claim the advantages of sex in preventing the accumulation of deleterious mutations (Deterministic mutation hypothesis, Muller's ratchet) a well as in adapting to fluctuating environments, especially escaping from coevolving parasites (Red Queen).
We try to examine the evolutionary advantages of sex and asex using the freshwater planarian flatworm Schmidtea polychroa as a model system (Figure 1). This simultaneously hermaphroditic species contains sexual and parthenogenetic biotypes. Both are morphological indistinguishable, but differ in ploidy level: sexuals are diploid, while parthenogens are polyploid (triploid or tetraploid). Parthenogens are sperm-dependent, that means allosperm is required to trigger embryo development. Both reproductive types co-exist in the some lakes in Southern Europe, but parthenogens also occur in the absence of sexuals, which is the case for Western and Central Europe. Research focus is to examine the persistence of parthenogenesis in purely parthenogenetic populations and the maintenance of sexual reproduction in mixed sexual and parthenogenetic populations.

One way to understand sexual reproduction is to investigate parthenogenesis in more detail. Here we ask the question of how parthenogenesis can persist despite of its assumed disadvantages. One possibility is that parthenogens show cryptic forms of sexual reproduction combining the advantages of sex and asex. Using microsatellites as genetic markers and karyological techniques we could confirm the existence of multiple sexual processes in a parthenogenetic population (Figure 2). Additional modelling revealed that in total 12% of the offspring are sexually produced. This is important, as sexual processes in parthenogens in the absence of sexual conspecifics have never been reported at least in these proportions. With this system we aim to study the adaptive significance of occasional sex.

A more traditional way to study evolutionary advantages of sexual reproduction is to compare coexisting sexual and parthenogenetic forms. Here we look for empirical evidence for the so-called pluralistic approach, a combination of mutational based and environmental models of sex. In a lake in Northern Italy, where sexuals and parthenogens coexist, parthenogens are found to have higher embryo mortality rates, which is likely to reflect an accumulation of deleterious mutations. At the same time, parthenogens are more often and heavily infected with parasites, in agreement with the Red Queen hypothesis. How these two findings act together and may increase their impact on the maintenance of sex is another focus of our research

Bruvo R, Schulenburg H, Storhas M, Michiels NK (2007) Synergism between mutational meltdown and Red Queen in parthenogenetic biotypes of the freshwater planarian Schmidtea polychroa. Oikos 116: 313-323.

D'Souza TG, Michiels NK. Genetic signatures of occasional sex in parthenogenetic subpopulations of the freshwater planarian Schmidtea polychroa. Freshwater Biology 51: 1890-1900.

D'Souza TG, Schulte RD, Schulenburg H, Michiels NK. Paternal inheritance in parthenogenetic forms of the planarian Schmidtea polychroa. Heredity 97: 97-101.

D'Souza TG, Storhas M, Michiels NK (2005) The effect of polyploidy on fitness in a parthenogenetic flatworm. Biological Journal of Linnean Society 85: 191-198.

D'Souza TG, Storhas M, Schulenburg H, Beukeboom LW, Michiels NK (2004) Occasional sex in an "asexual" polyploid hermaphrodite. Proceedings of the Royal Society London B. 271:1001-1007.

Pongratz N, Storhas M, Carranza S, Michiels NK (2003) Phylogeography of competing sexual and parthenogenetic forms of a freshwater flatworm: patterns and explanations. BMC Evolutionary Biology 3:23.

Michiels NK, Beukeboom LW, Pongratz N, Zeitlinger J (2001) Parthenogenetic flatworms have more symbionts than their coexisting, sexual conspecifics. Journal of Evolutionary Biology 14(1): 110-119.

Storhas MG, Weinzierl RP, Michiels NK (2000) Paternal sex in parthenogenetic planarians: a tool to investigate the accumulation of deleterious mutations. Journal of Evolutionary Biology 13(1): 1-8.

Weinzierl RP, Schmidt P, Michiels NK (1999) High fecundity and low fertility in parthenogenetic planarians. Invertebrate Biology 118(2): 87-94.

Weinzierl RP, Berthold K, Beukeboom LW, Michiels NK (1998) Reduced male allocation in a parthenogenetic hermaphrodite (Dugesia polychroa, Tricladida, Platyhelminthes). Evolution 52(1): 109-115.

Thomas D'Souza (PI), Beatriz Sanchez Navarro, Nico Michiels