Molecular phylogeny of Candidula (Geomitridae) land snails inferred from mitochondrial and nuclear markers reveals the polyphyly of the genus

Abstract

The genus Candidula (Geomitridae), consisting of 28 species in Western Europe as currently described, has a disjunct distribution in the Iberian Peninsula, Italy, the Balkans, the Aegean Islands, and one species on the Canary Islands. Although the genus is seemingly well defined by characters of the reproductive system, the relationships within the genus are still unclear and some authors have indicated a possible subgeneric division based on the internal morphology of the dart sac. Despite substantial phylogenetic incongruence, we present a well-resolved molecular phylogeny of Candidula based on two mitochondrial genes (COI and 16S rRNA), the nuclear rDNA region (5.8S rNRA + ITS2 + 28S rRNA) and seven additional nuclear DNA regions developed specifically for this genus (60SL13, 60SL17, 60SL7, RPL14, 40SS6, 60SL9, 60SL13a), in total 5595 bp. Six reciprocally monophyletic entities including Candidula species were recovered, grouping into two major clades. The incorporation of additional geomitrid genera allowed us to unequivocally demonstrate the polyphyly of the genus Candidula. One major clade grouped species from southern France and Italy with the widely distributed species C. unifasciata. The second major clade grouped all the species from the Iberian Peninsula, including C. intersecta and C. gigaxii. Candidula ultima from the Canary Islands was recovered as separated lineage within the latter clade and related to African taxa. The six monophyla were defined as six new genera belonging to different tribes within the Helicellinae. Thus, we could show that similar structures of the stimulatory apparatus of the genital system in different taxa do not necessarily indicate a close phylogenetic relationship in the Geomitridae. More genera of the family are needed to clarify their evolutionary relationships, and to fully understand the evolution of the stimulatory apparatus of the genital system within the Geomitridae.

Keywords: Candidula; Dart sac; Helicoidea; Land snails; Molecular phylogeny; Polyphyly.