SHORT COMMUNICATION Polymorphism of a genital organ under sexual selection in Monacha kuznetsovi from the Caucasus (Gastropoda: Hygromiidae) Marco T. Neiber | Frank Walther | Bernhard Hausdorf Center for Natural History, Zoological Museum, Universit€at Hamburg, Hamburg, Germany Correspondence Marco T. Neiber Email: mneiber@hotmail.de Funding information Volkswagen Foundation Contributing authors: Frank Walther (fw. walther@googlemail.com), Bernhard Hausdorf (hausdorf@zoologie.uni-hamburg. de) Abstract During mating the dart apparatus of the land snail group Helicoidea transfers an allohormone that increases paternity success. Thus, this organ is under sexual selec- tion. Despite this selective advantage, the dart sac has independently been lost in many lineages in the Helicoidea, presumably because natural selection counteracts sexual selection. However, the selective pressures that result in the loss of the organ have not been examined experimentally because most species show no varia- tion in the presence or absence of the dart sac. Using nuclear multilocus data and mitochondrial sequences, we show that a so far misidentified morph of the genus Monacha without appendicula, a homologue of the dart sac, is conspecific with M. kuznetsovi with appendicula. This is only the second case of polymorphy with regard to the presence or absence of the dart sac or its homologue in the Heli- coidea. Monacha kuznetsovi might therefore be a suitable model organism to study how the interplay between sexual selection and natural selection may affect the evolution of the genital organs. KEYWORDS AFLP, Caucasus, dart apparatus, Helicoidea, sexual selection 1 | INTRODUCTION The land snail group Helicoidea is characterized by the evolution of a dart apparatus that is usually equipped with one or more calcare- ous darts that are shot or pushed into the mating partner during courtship (Chase, 2007). The function of this behavior has long puz- zled biologists. It has been hypothesized that the dart is a nuptial gift of calcium for the production of the eggs (Charnov, 1979; Leonard, 1992), that dart shooting is a sexual signal and dart recipients select on dart shooting effectiveness (Landolfa, 2002; Leonard, 1992) or that the dart is used to manipulate the mating partner (Adamo & Chase, 1996). The latter hypothesis could be corroborated by experimental studies (Chase, 2007). It has been shown that the dart transfers an allohormone produced by the glands of the dart appara- tus, the glandulae mucosae, into the body of the recipient (Stewart, Wang, Koene, Storey, & Cummins, 2016). This allohormone inhibits the intake of the spermatophore into a digesting organ so that more sperm can reach the sperm-storing organ and fertilize eggs. The sex- ual conflict between dart shooter and dart receiver led to a co-evo- lutionary arms race resulting in a continuous improvement of the spermatophore receiving organs and the allohormone transfer, for example, by more complex surfaces of the darts (Koene & Schulen- burg, 2005). Because of the strong selective advantage, more effec- tive traits become quickly fixed. Thus, there is little variation of such genital characters within species. This is one reason, why differences in genital characters are often useful to distinguish species. It is less clear how an organ that is under strong sexual selection like the dart sacs might be reduced or lost. Actually, such losses have been documented in various lineages of the Helicoidea (Hirano, Kameda, Kimura, & Chiba, 2014; Neiber & Hausdorf, 2017; Neiber, Razkin, & Hausdorf, 2017; Wade, Hudelot, Davison, Naggs, & Mor- dan, 2007). Neiber and Hausdorf (2015) hypothesized that natural Accepted: 9 February 2018 DOI: 10.1111/jzs.12218 J Zool Syst Evol Res. 2018;1–6. wileyonlinelibrary.com/journal/jzs © 2018 Blackwell Verlag GmbH | 1