Background Evolutionary conflicts of interest between the sexes often lead to co-evolutionary arms races consisting of repeated arisal of traits advantageous for one sex but harmful to the other sex, and counter-adaptations by the latter. the first comprehensive comparative analysis of reproductive organ characteristics in simultaneous hermaphrodites. Moreover, it strongly suggests that co-evolutionary arms races can result from sexual conflict in these organisms and play a key role in the evolution of hermaphroditic mating Polygalasaponin F IC50 systems. Background Evolutionary conflicts of interest between the sexes have been convincingly exhibited in species with individual sexes . These sexual conflicts often give rise to traits that are advantageous for one sex but harmful to the other. If these detrimental effects are counteracted, a co-evolutionary arms Polygalasaponin F IC50 race may ensue in which harmful characteristics and corresponding counter-adaptations arise repeatedly . Such antagonistic interactions can bring about major changes in the mating behaviour, genital morphology, gametes and seminal products, potentially leading to speciation . Similar arms races seem to occur in hermaphrodites, contrary to Darwin’s conviction  that sexual selection cannot act in hermaphroditic organisms. In fact, theoretical modeling indicates that these processes can become even more extreme in hermaphroditic species (N.K. Michiels and J.M. Koene, unpublished data), mainly because within one mating simultaneous hermaphrodites gain paternity (male fitness) which can outweigh the loss in female fitness. Here we investigate the evolution of a most peculiar reproductive behaviour that occurs in simultaneously Rabbit polyclonal to OGDH hermaphroditic land snails (Stylommatophora), the “shooting” of a so-called love-dart into the mating partner. Several explanations have been offered for the evolution of the enigmatic dart shooting behaviour. The dart is made of calcium carbonate and has therefore been proposed to serve as a nuptial gift of calcium for the production of eggs [4,5]. However, in Cantareus aspersus (previously Helix aspersa) the dart does not contain enough calcium to significantly contribute to egg production and darts are only rarely incorporated by the recipient . Likewise, in other investigated species darts are even retained by shooters to be reused on the next mate [[7,8], J.M. Koene and S. Chiba, unpublished data). Therefore two other hypotheses have been put forward. In the female choice hypothesis the dart represents a sexual signal and recipients select on dart shooting effectiveness [5,9]. The important prediction of this hypothesis is that this can only be beneficial for the recipient if the dart is usually shot consistently by individuals (assuming that shooting ability is usually heritable). Assessments in C. aspersus do not support this, because dart shooting of individually-identified animals in consecutive matings is usually unpredictable (G-test: N = 29 snails, df = 1, G = 6.745, P < 0.01; J.M. Koene, unpublished data). Besides, in Arianta arbustorum dart shooting seems to be an optional component of courtship . In the last hypothesis, the dart is used to manipulate the mating partner and can thus cause a sexual conflict . This latter hypothesis seems most consistent with findings in the common garden snail C. aspersus, the species in which dart shooting and copulatory behaviour has been extensively studied. During courtship, the stylophore (dart sac) is usually everted and the single calcareous dart is usually pierced into the partner. Both mating partners normally shoot a dart before their penises are simultaneously intromitted. During intromission, spermatophores are exchanged and transferred into the partner’s receiving organ, either directly into the bursa copulatrix or in an associated diverticulum (Physique ?(Figure1).1). To avoid digestion in the spermatophore receiving organ (SRO), sperm have to actively swim out via the spermatophore’s tail (formed by the flagellum of the penis) into the vaginal duct to reach the sperm storage site (the spermathecae) . Previous work on C. aspersus has exhibited that this shooting of the love-dart serves to introduce Polygalasaponin F IC50 an allohormone [13-15], produced by associated glands, into the blood of the partner . This allohormone inhibits digestion of sperm , which results in more of the donated sperm reaching the spermathecae  and fertilizing eggs [19,20]. This manipulation of the sperm storage process caused by darts can have a negative effect on the recipient’s (reproductive) fitness because Polygalasaponin F IC50 of interference with cryptic female choice. Physique 1 Schematic morphological drawing of the reproductive morphology of a land snail with one dart and a diverticulum. The love-dart (D) is usually produced and stored in the stylophore (S, often called dart sac) and shot by a Polygalasaponin F IC50 forceful eversion of.