Testing the role of the Himalaya Mountains as a dispersal barrier in freshwater gastropods (Gyraulus spp.)

The Himalayas are considered to constitute a biogeographical boundary between the Palaearctic and Oriental regions. However, this mountain range does not form a uniform barrier because several large river valleys deeply cut into the Himalayas, crossing drainage divides and potentially forming dispersal corridors, particularly for freshwater organisms. In the present study, the effectiveness of these corridors is tested for the first time based on molecular data, using the amphi‐Himalayan freshwater gastropod genus Gyraulus as a model group. Based on a broad spatial sampling from the Himalayas and the Tibetan Plateau, phylogenetic analyses were performed using mitochondrial DNA sequence data. The resulting phylogeny shows that northern and southern Himalayan clades are geographically distinct (i.e. they exclusively consist of either northern or southern specimens). This pattern suggests a lack of gene flow across the Himalayas probably at least since the late Pliocene/early Pleistocene. Successful dispersal and/or establishment of gastropods might have been impeded by geographical and ecological features. Instead of direct dispersal across the range, multiple colonizations of regions north and south of the Himalayas from extralimital areas have to be assumed. The Himalayas thus represent a very strong dispersal barrier for freshwater snails, and probably for other taxa as well. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 526–534.     

Water relations and osmotic pressure in Biomphalaria pfeifferi (Krauss), Biomphalaria glabrata (Say) and Helisoma trivolvis (Say) (Gastropoda : Planorbidae) in response to cationic alterations of the medium

The percentage of water in the total living body weight is restricted within a narrow range for each species, even in snails cultured under differing controlled ionic concentrations in the medium. The water level in Helisoma trivolvis is consistently higher than in Biomphalaria pfeifferi and B. glabrata under the same culture conditions.The whole cationic fraction of the total osmoconcentration of the haemolymph is maintained at a constant hypertonic level, even though individual cation levels vary with the cationic composition of the surrounding medium.     

Structure of the extracellular hemoglobin of Biomphalaria glabrata

The hemoglobin of Biomphalaria glabrata was purified to homogeneity by gel filtration column followed by anion exchange chromatography. The dissociation products were analyzed by a 5–15% gradient polyacrylamide gel electrophoresis containing sodium dodecyl sulfate (SDS-PAGE) giving a band of 270 kDa and a band of 180 kDa after reduction with β-mercaptoethanol. The same profile was obtained in a 3.5% agarose gel electrophoresis containing SDS (SDS-AGE) but showed additional bands of higher molecular weight. These bands were proposed to be monomers, dimers and trimers, since they showed a good correlation in a plot of R_f versus log M_r. After partial reduction in a two-dimensional SDS-AGE, the proposed monomers and dimers produced two and four bands, respectively, likely indicating one to four chains crosslinked by disulfide bridges. Digestion with four different proteases yielded several equivalent fragments with molecular weights multiples of its minimum molecular weight (17.7 kDa). The circular dichroism spectrum of the protein showed a characteristic high α-helix content (70%). It was proposed that this hemoglobin is a pentamer with a molecular weight of aproximately 1.8×10³ kDa, assembled by five 360-kDa subunits, each formed by two 180-kDa chains linked in pairs by disulfide bridges and each of these chains, in turn, comprised by ten heme binding domains linked in tandem. These data are compared to the published information for other planorbid extracellular hemoglobins.     

Musculature of the penial complex: A new criterion in unravelling the phylogeny of Hygrophila (Gastropoda: Pulmonata)

The taxonomy of freshwater pulmonates (Hygrophila) has been in a fluid state warranting the search for new morphological criteria that may show congruence with molecular phylogenetic data. We examined the muscle arrangement in the penial complex (penis and penis sheath) of most major groups of freshwater pulmonates to explore to which extent the copulatory musculature can serve as a source of phylogenetic information for Hygrophila. The penises of Acroloxus lacustris (Acroloxidae), Radix auricularia (Lymnaeidae), and Physella acuta (Physidae) posses inner and outer layers of circular muscles and an intermediate layer of longitudinal muscles. The inner and outer muscle layers in the penis of Biomphalaria glabrata consist of circular muscles, but this species has two intermediate longitudinal layers separated by a lacunar space, which is crossed by radial and transverse fibers. The muscular wall of the penis of Planorbella duryi is composed of transverse and longitudinal fibers, with circular muscles as the outer layer. In Planorbidae, the penial musculature consists of inner and outer layers of longitudinal muscles and an intermediate layer of radial muscles. The penis sheath shows more variation in muscle patterns: its muscular wall has two layers in A. lacustris, P. acuta, and P. duryi, three layers in R. auricularia and Planorbinae and four layers in B. glabrata. To trace the evolution of the penial musculature, we mapped the muscle characters on a molecular phylogeny constructed from the concatenated 18S and mtCOI data set. The most convincing synapomorphies were found for Planorbinae (inner and outer penis layers of longitudinal muscles, three-layered wall of the penis sheath). A larger clade coinciding with Planorbidae is defined by the presence of radial muscles and two longitudinal layers in the penis. The comparative analysis of the penial musculature appears to be a promising tool in unraveling the phylogeny of Hygrophila.