Land snails of Leptopoma Pfeiffer, 1847 in Sabah, Northern Borneo (Caenogastropoda: Cyclophoridae): an analysis of molecular phylogeny and variations in shell form due to geography

Leptopoma is a species rich genus with approximately 100 species documented according to shell morphology and animal anatomy. Many of the Leptopoma species are described in terms of shell size, shape, sculpture and colour patterns of a small number of examined materials. However, the implications of the inter- and intra-species variations in shell form to the taxonomy of Leptopoma species and the congruency of its current shell based taxonomy with its molecular phylogeny are still unclear. Over the last decade, more than 900 collection lots consisting of more than 4000 Leptopoma specimens have been obtained in Sabah and deposited in BORNEENSIS at Universiti Malaysia Sabah. Access to this collection gave us the opportunity to examine the geographical variations in shell forms and the phylogenetic relationship of Leptopoma species in Sabah. The phylogenetic relationship of three Leptopoma species was first estimated by performing maximum likelihood and Bayesian analysis based on mitochondrial genes (16S and COI) and nuclear gene (ITS-1). After this, a total of six quantitative shell characters (i.e. shell height, shell width, aperture height, aperture width, shell spire height, and ratio of shell height and width) and three qualitative shell characters (i.e. shell colour patterns, spiral ridges, and dark ring band in aperture) of the specimens were mapped across the phylogenetic tree and tested for phylogenetic signals. Data on shell characters of Leptopoma sericatum and Leptopoma pellucidum from two different locations (i.e. Balambangan Island and Kinabatangan) where both species occurred sympatrically were then obtained to examine the geographical variations in shell form. The molecular phylogenetic analyses suggested that each of the three Leptopoma species was monophyletic and indicated congruence with one of the shell characters (i.e. shell spiral ridges) in the current morphological-based classification. Other qualitative and quantitative shell characters were incongruent with the Leptopoma species phylogeny. Although the geographical variation analyses suggested some of the shell characters indicating inter-species differences between the two Leptopoma species, these also pointed to intra-species differences between populations from different locations. This study provides an initiation to resolve the taxonomy conundrum for the remaining 100 little known Leptopoma species from other regions and highlights a need to assess variations in shell characters before they could be used in species classification.