Testing hypotheses on the dispersal and evolutionary history of freshwater mussels (Mollusca: Bivalvia: Unionidae)

The relationship between dispersal and differentiation of the European freshwater mussel Unio pictorum (Linnaeus, 1758) was studied with molecular genetic methods. Forty‐two populations from France, Italy and central Europe were analysed. Genetic relationships were assessed from the geographical distribution of allele frequencies at 17 enzyme loci. Neighbouring groups of populations show small to moderate mean genetic distances (0.020 < D_mean < 0.263). With a few exceptions the genetic affinities of the populations are the closest within the same drainage basin. In central Europe and Northern Italy genetic differences between drainage systems are relatively large. Populations from north‐eastern Italy are genetically similar to Danubian populations. Mussels from the islands of Corsica and Sardinia are more closely related to populations from the Italian peninsula than to French populations from the Rhône drainage system. Genetic relationships within U. pictorum from central Europe reflect palaeogeographical relationships between river systems during the Pliocene and Pleistocene. Literature data on two North American unionid species and one European fish species show the same relationship between genetic diversity and the history of drainage systems, although the correlations are less strong. In France and Italy this correspondence is much less evident. Population dynamic processes and human activities leading to populational bottlenecks might have obscured it.     

Population genetics and phylogeography of freshwater mussels in North America, Elliptio dilatata and Actinonaias ligamentina (Bivalvia: Unionidae)

Extrinsic and intrinsic forces combined shape the population structure of every species differently. Freshwater mussels are obligate parasites to a host fish during a juvenile stage (glochidia). Elliptio dilatata (ED) and Actinonaias ligamentina (AL) are co-occurring freshwater mussel taxa with similar North American distribution and share some potential host fish. Using mitochondrial DNA, we determined the genotypes of 190 + individuals from collection sites in at least two tributaries in the Lake Erie and Ohio River watersheds, along with the Ouachita and Strawberry rivers in the southeast. Both species had followed a stepping-stone model of dispersal, with greater pairwise genetic structure among collection sites of ED. Also, phylogeographical analysis for ED found significant geographical structuring of haplotype diversity. Overall, within-population variation increased significantly from north to south, with low genetic diversity in the Strawberry River. We calculated significant among-population structure for both species (ED: Phi(ST) = 0.62, P < 0.001; AL: Phi(ST) = 0.16, P < 0.001). Genetic analysis identified the Ouachita River as an area of significant reproductive isolation for both species. Results for AL indicated dispersal into northern areas from two genetically distinct glacial refugia, where results for ED indicated dispersal followed by low gene flow in northern areas. The conservation strategies for mussels that co-occur in the same 'bed' could be species specific. Species such as ED have management units on the population scale, where AL has a more homogeneous genetic structure across its range.     

Molecular genetics and speciation in Elliptio and its relationships to other taxa of North American Unionidae (Bivalvia)

The genetic relationships among 30 populations of 11 species and five genera of North American Unionidae were assessed by using standard allozyme procedures. Emphasis was on relationships among populations and species of Elliptio and Fusconaia. Multi-dimensional scaling based on a matrix of Nei's (1972) genetic distances substantiated the immunoelectrophoretic results of Davis&Fuller 1981, which demonstrated the distinct and divergent taxonomic groups Anodontinae, Margaritiferinae, and Ambleminae, plus die close relationship of Elliptio and Fusconaia , which justifies their inclusion within the same tribe. Genetic distance appears to increase regularly with time since divergence of taxa. The E. complanata species group is an apparently recent radiation and probably is actively radiating today. The I values among species of this group range from 0.90 to 0.99. Considerable heterozygosity, numerous polymorphic loci, and much interpopuladon phenotypic diversity was also recorded for this group. Some taxa mat have been considered synonyms are demonstrated to be valid species. Reasons for the low genetic distances among unionid taxa are discussed. Standard allozyme analyses are shown to be of great value for assessing relationships among unionid taxa.     

Sexual selection and simultaneous hermaphroditism among the Unionidae (Bivalvia: Mollusca)

Simultaneous hermaphroditism is an infrequent mode of reproduction among bivalves of the family Unionidae: only five of the 220 North American species are simultaneous hermaphrodites. However, hermaphroditic individuals of otherwise predominantly dioecious species have been encountered in 30 of I01 species examined. These hermaphroditic individuals as well as simultaneous hermaphrodites can exhibit considerable variability in the ratio of spermatogenic: oogenic tissue within the gonad, and the purposes of this paper are to determine the underlying causes of both this variability and the occurrence of occasional hermaphroditic individuals among dioecious species. Results indicate that the ratio of male: female gonodal tissue of a simultaneous hermaphrodite is bimodally distributed, and several hypotheses to account for this observation are presented. It is proposed that populations occurring in different habitats and under conditions of different individual density are subject to fundamentally different sexual selection pressures acting on ratios of allocation to male and female gametes. Occasional hermaphroditism among otherwise predominantly dioecious species was in this study associated with infection of the gonads by digenean trematodes. A model of sexual determination among unionids presented in this paper proposes that sex is determined by genetically controlled hormone levels: occasional hermaphrodites result from alterations in these hormone levels caused by developmental errors and trematodal infections which mimic the results of such errors. Predictions of this model are consistent with observed levels of variability in male: females gonadal tissue among occasional hermaphrodites.     

Population genetics of the freshwater mussel, <Emphasis Type="Italic">Amblema plicata</Emphasis> (Say 1817) (Bivalvia: Unionidae): Evidence of high dispersal and post-glacial colonization

Over 70% of North American freshwater mussel species (families Unionidae and Margaritiferidae) are listed as threatened or endangered. Knowledge of the genetic structure of target species is essential for the development of effective conservation plans. Because Ambelma plicata is a common species, its population genetic structure is likely to be relatively intact, making it a logical model species for investigations of freshwater mussel population genetics. Using mtDNA and allozymes, we determined the genotypes of 170+ individuals in each of three distinct drainages: Lake Erie, Ohio River, and the Lower Mississippi River. Overall, within-population variation increased significantly from north to south, with unique haplotypes and allele frequencies in the Kiamichi River (Lower Mississippi River drainage). Genetic diversity was relatively low in the Strawberry River (Lower Mississippi River drainage), and in the Lake Erie drainage. We calculated significant among-population structure using both molecular markers (A.p. Φ_st = 0.15, θ_st = 0.12). Using a hierarchical approach, we found low genetic structure among rivers and drainages separated by large geographic distances, indicating high effective population size and/or highly vagile fish hosts for this species. Genetic structure in the Lake Erie drainage was similar to that in the Ohio River, and indicates that northern populations were founded from at least two glacial refugia following the Pleistocene. Conservation of genetic diversity in Amblema plicata and other mussel species with similar genetic structure should focus on protection of a number of individual populations, especially those in southern rivers.     

Isolation and characterization of microsatellite loci in Alasmidonta heterodon (Bivalvia: Unionidae)

We developed 13 species‐specific microsatellite markers for the federally endangered Atlantic slope unionid Alasmidonta heterodon. Four to 18 alleles per locus were observed among 30 individuals. Observed heterozygosity throughout the loci ranged from 26.9 to 86.2% and averaged 63.6%. Estimates of individual pairwise genetic distances indicated that levels of genetic diversity among loci were sufficient to produce unique multilocus genotypes for all animals surveyed. Randomization tests showed that genotypes for this collection were consistent with Hardy–Weinberg expectations, and no significant linkage disequilibrium was observed between loci. These loci therefore appear suitable for population surveys, kinship assessment and other such applications.     

A new model for periostracum and shell formation in Unionidae (Bivalvia, Mollusca)

The periostracum in Unionidae consists of two layers. The outer one is secreted within the periostracal groove, while the inner layer is secreted by the epithelium of the outer mantle fold. The periostracum reaches its maximum thickness at the shell edge, where it reflects onto the shell surface. Biomineralization begins within the inner periostracum as fibrous spheruliths, which grow towards the shell interior, coalesce and compete mutually, originating the aragonitic outer prismatic shell layer. Prisms are fibrous polycrystalline aggregates. Internal growth lines indicate that their growth front is limited by the mantle surface. Transition to nacre is gradual. The first nacreous tablets grow by epitaxy onto the distal ends of prism fibres. Later growth proceeds onto previously deposited tablets. Our model involves two alternative stages. During active shell secretion, the mantle edge extends to fill the extrapallial space and the periostracal conveyor belt switches on, with the consequential secretion of periostracum and shell. During periods of inactivity, only the outer periostracum is secreted; this forms folds at the exit of the periostracal groove, leaving high-rank growth lines. Layers of inner periostracum are added occasionally to the shell interior during prolonged periods of inactivity in which the mantle is retracted.     

Geometrical and crystallographic constraints determine the self-organization of shell microstructures in Unionidae (Bivalvia: Mollusca)

Unionid shells are characterized by an outer aragonitic prismatic layer and an inner nacreous layer. The prisms of the outer shell layer are composed of single-crystal fibres radiating from spheruliths. During prism development, fibres progressively recline to the growth front. There is competition between prisms, leading to the selection of bigger, evenly sized prisms. A new model explains this competition process between prisms, using fibres as elementary units of competition. Scanning electron microscopy and X-ray texture analysis show that, during prism growth, fibres become progressively orientated with their three crystallographic axes aligned, which results from geometric constraints and space limitations. Interestingly transition to the nacreous layer does not occur until a high degree of orientation of fibres is attained. There is no selection of crystal orientation in the nacreous layer and, as a result, the preferential orientation of crystals deteriorates. Deterioration of crystal orientation is most probably due to accumulation of errors as the epitaxial growth is suppressed by thick or continuous organic coats on some nacre crystals. In conclusion, the microstructural arrangement of the unionid shell is, to a large extent, self-organized with the main constraints being crystallographic and geometrical laws.     

Genetics of the peripheral populations of the alpine bullhead, Cottus poecilopus (Scorpaeniformes, Cottidae) in Poland

The alpine bullhead ( Cottus poecilopus ) in Poland is highly polymorphic ( P for examined populations in the range of 0.0–0.45, mean P  = 0.11, P for species 0.64) in seven out of 11 loci studied ( Pgd-1, Mdh-1, Ck-1, Gpd-1, Gpd-2, Gpi-1, Gpi-2 ). However, they reveal very low heterozygosity (mean H _o = 0.007, mean H _e = 0.019), which may result from population subdivisions, small genetically effective size ( N _e) of some populations and frequent inbreeding, the latter being probably mediated through polygyny. The index of genetic differentiation ( G _ST) for the Polish metapopulation equals 0.48, which is relatively high. By far, the strongest are genetic differences between the Sudeten-Carpathian group and the Hańcza population. Gene flow between the Sudeten-Carpathian group and the Hańcza population is very limited and their divergence may have started as early as the last (Eemian) interglacial period.     

Molecular phylogenetic analysis of tropical freshwater mussels (Mollusca: Bivalvia: Unionoida) resolves the position of Coelatura and supports a monophyletic Unionidae

In previous molecular phylogenetic analyses of the freshwater mussel family Unionidae (Bivalvia: Unionoida), the Afrotropical genus Coelatura had been recovered in various positions, generally indicating a paraphyletic Unionidae. However that result was typically poorly supported and in conflict with morphology-based analyses. We set out to test the phylogenetic position of Coelatura by sampling tropical lineages omitted from previous studies. Forty-one partial 28S nuclear rDNA and partial COI mtDNA sequences (1130 total aligned nucleotides) were analyzed separately and in combination under both maximum parsimony and likelihood, as well as Bayesian inference. There was significant phylogenetic incongruence between the character sets (partition homogeneity test, p < 0.01), but a novel heuristic for comparing bootstrap values among character sets analyzed separately and in combination illustrated that the observed conflict was due to homoplasy rather than separate gene histories. Phylogenetic analyses robustly supported a monophyletic Unionidae, with Coelatura recovered as part of a well-supported Africa–India clade (= Parreysiinae). The implications of this result are discussed in the context of Afrotropical freshwater mussel evolution and the classification of the family Unionidae.     

Unravelling the systematics of Nodularia (Bivalvia,Unionidae) species from eastern Russia

Conservation efforts have been hindered by data deficient conservation status assessments, especially due to taxonomic problems. This is especially true for many eastern Russian species of freshwater mussels, where distinct classification systems have complicated their delimitation and identification. Nodularia is a widespread eastern Asian freshwater mussel genus, present from Vietnam in the south to the Magadan region in eastern Russia in the north. The number of recognized species in the genus Nodularia in eastern Russia has been inflated over the last several decades due to the use of a typological species concept, the so-called 'Comparatory Method'. This method uses a single diagnostic character for species delimitation, i.e., the arc of maximal convexity of the shell's outline. Using this classification system, 10 species were recognized for far eastern Russia under the genus Nodularia Conrad, 1853, divided into three subgenera: Nodularia s. str., Amurunio and Magadaninaia. Since it is not supported by any other classification methods, the current comparatory classification is rejected by many Russian and international scientists, who only recognize a single species for that region, i.e., Nodularia douglasiae. Therefore, the aim of the present study is to clarify the taxonomy and systematics of the Nodularia genus in far eastern Russia and adjacent territories, using a multiple dataset approach that combines distribution with detailed conchological and anatomical analyses with morphometry and COI barcoding molecular techniques. All analyses performed in this study support the existence of a single Nodularia species in eastern Russia, i.e., N. douglasiae.     

Annual production of two Nova Scotian populations of Nucella lapillus (L.)

Summary One population of Nucella lapillus, under conditions of food shortage, decreased in biomass with an annual production of 5.5 kcal per m². The second population, with plenty of food, gained biomass and the annual production was 16.7 kcal per m².Individuals grew faster, while attaining sexual maturity and terminating growth at a larger size in the second population. In both populations, energy produced as gametes exceeded the energy produced due to growth. A 2.56 cm female was estimated to produce 46.6 egg capsules (about 1 kcal) a year but the output of males was unknown and assumed to be half that of females. The sex ratio was equal.     

Population genetics of marine species: the interaction of natural selection and historically differentiated populations

High gene flow, particularly as mediated by larval dispersal, has usually been viewed as sufficient to limit geographic isolation as a major source of population differentiation among marine species. Despite the general observation of relatively little geographic variation among populations of high dispersal marine species many cases of divergence have been observed and natural selection has usually been invoked to explain geographic divergence. Detailed study of several allozyme polymorphisms provided additional evidence that selection may be the predominant force that determines genetic divergence in marine systems. There is, however, growing evidence that marine species with high dispersal are more subdivided than originally thought. The use of multi-locus approaches and the application of molecular techniques have provided new insight into the nature of population divergence in marine species. I argue that (1) many species, which were formerly thought to be unstructured, are in fact subdivided into genetically discrete groups, (2) it is often the case that genetically subdivided populations have distinct evolutionary histories, (3) in many cases, natural selection is the consequence of introgression between these groups, and (4) the combination of molecular assays of both nuclear and mitochondrial DNA and allozyme loci provides the best approach to understanding the evolutionary dynamics of these interacting populations.     

The genetics of California populations of Geukensia demissa (Dillwyn) (Mollusca): further evidence on the selective importance of leucine aminopeptidase variation in salinity acclimation

A study of the population genetics of the estuarine bivalve Geukensia demissa (Dillwyn) in southern California marshes revealed significant spatial and temporal variation in leucine aminopeptidase-1 (LAP-1) gene frequencies which were correlated with environmental salinity. S California populations also generally show a deficit of heterozygotes at the LAP-1 locus with the degree of this deficit also correlated with environmental salinity. The results of this study are similar to previous studies on Mytilus edulis Linné where it has been proposed that antagonistic directional selection in low vs. high salinity environments acts to maintain the LAP-1 polymorphism. However, a general similarity between the genetic patterns for LAP-1 and other loci in G. demissa in S California makes a strict selectionist interpretation difficult. A similar study of G. demissa in San Francisco Bay failed to detect heterogeneity in allele frequencies or significant heterozygote deficits over gradients in environmental salinity.     

Molecular phylogeny,taxonomy, and distribution of French Unio species (Bivalvia,Unionidae)

A plethora of unionid names was established in the nineteenth century by the “Nouvelle Ecole”. Although naiad morphological plasticity is well documented, the currently recognized fauna, with 17 species and subspecies included in the French checklist for the Unio genus, is still based upon morphological characters only. Insights have been provided from molecular data elsewhere in Europe and North Africa, but the French fauna remains unstudied. We present a molecular phylogeny of the Unio genus in France based on COI, 16S and 28S genes; taking up all available data in Europe plus 273 specimens collected in all main French drainages. The results show that there are either three valid species in France, with U. pictorum and U. mancus synonymized, or five, with the subspecies U. crassus courtillieri elevated to species level. Subspecies were generally not recovered, which questions the evolutionary units tacitly implied by subspecific names. Although sampling topotypes is the most reliable way to evaluate the status of a nominal subspecies, major human-induced changes in aquatic hydrosystems challenge the method. Nevertheless, operational taxonomy has to rely on ground-truthed data and we propose to reduce the actual number of valid taxa in France to the seven observed operational taxonomic units.     

Genetically distinct island populations of the Egyptian vulture (Neophron percnopterus)

The Egyptian vulture (Neophron percnopterus) is a species in decline throughout Europe, with the largest remaining breeding populations found in northern Spain. Iberian Peninsula populations of this species (about 1000 pairs) migrate to Africa in winter, while small populations in both the Canary and Balearic Islands (less than 40 pairs in each case) are apparently sedentary. We found that Egyptian vultures from both of these island groups were significantly differentiated from Iberian Peninsula populations (R _ST = 0.065–0.129, p = 0.000–0.007), using nine microsatellite loci isolated in a related species, the bearded vulture. The greatest degree of genetic differentiation was observed between the two island groups (R _ST = 0.279, p = 0.000). These island populations were more distinct from mainland groups than was a small sample of a well-defined separate subspecies from India (N. p. ginginianus; R _ST = 0.083–0.091, p = 0.023–0.024). This implies that these two island populations have been isolated from peninsular populations for many generations, despite the long-distance migration capabilities of the species. In constrast, populations within the Peninsula were not differentiated from one another at these microsatellite loci (R _ST = $-$0.004–0.007, p = 0.442–0.675). Introductions of Egyptian vultures from the larger northern breeding groups might therefore be appropriate in southern Spain, if necessary, but mainland birds should not be introduced to the islands if the genetic distinctiveness of these groups is to be preserved. Independent conservation plans are urgently required to protect these two island populations from extinction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Allometric and morphological characteristics of Tonicella marmorea (Fabricius, 1780) populations (Mollusca: Polyplacophora: Ischnochitonidae)

Allometric and morphological characteristics of population samples of Tonicella marmorea from three sea lochs on the west coast of Scotland are described. The suitability of allometric relationships as taxonomic criteria are investigated by statistical comparisons of various regression constants, scatter diagrams and comparisons of curvature indices. Morphological characteristics such as ctenidia number and numbers of notches in the head and tail valves proved too variable to be regarded as diagnostic but they are useful taxonomic indicators. The need for all taxonomic characteristics to be derived from population samples is stressed and the use of valve and girdle sculpturing together with radula structure is emphasized.     

Genetic variation in relation to demography of peripheral pool frog populations (Rana lessonae)

Summary Genetic variation in an isolated northern metapopulation of the pool frog (Rana lessonae) in Sweden was compared to that of Central European populations using enzyme electrophoresis and literature data. Of the 31 loci scored, two (EST-2 andIDH-2) were polymorphic while no variation occurred in seven of the eight loci which are polymorphic in Central European populations.The heterozygosity level of the Swedish pool frogs is very low compared to that of other anuran populations, but their mean proportion of fertilized eggs within egg masses (97.5%) was not lower than in more heterozygous species, and their body size-specific fecundity did not differ from that of Polish conspecifics. The low genetic variability of the Swedish pool frogs is discussed in relation to features of the local populations such as size (N), calculated effective size (N _e ) reproductive success and probable history. It is concluded that long-term strong fluctuations in population size caused by reporductive failure in cold years have contributed more to the low genetic variability than could a single founder event due to a recent introduction by man.     

Phenotypic variation and stress resistance in core and peripheral populations of Hordeum spontaneum

The phenotypic variation and response of plants to water stress were studied in a field trial in populations of wild barley, Hordeum spontaneum Koch. from Israel and Turkmenistan. Populations from the species distributional core and periphery were compared and contrasted for phenotypic variation in 16 phenological and morphological traits. The peripheral populations (six) were found to be phenotypically more variable and more resistant to water stress than core populations (12). The association of water-stress resistance with high phenotypic variability gives support to the hypothesis that populations that are genetically more variable are better adapted or pre-adapted to environmental changes and are thus valuable for conservation.     

New Zealand Recent Hyriidae (Mollusca: Bivalvia: Unionida)

The New Zealand freshwater mussels are taxonomically revised and compared with key Australian and South American taxa using DNA sequence data. Three living species are recognised: Echyridella menziesii, Echyridella aucklandica and Echyridella onekaka. Cucumerunio websteri websteri and Cucumerunio websteri delli, are treated as junior synonyms of Unio aucklandica, which is transferred from Cucumerunio (and Hyridella) to Echyridella. Lectotypes are designated for Unio waikarensis and Unio hochstetteri, which are illustrated together with primary type specimens of other relevant taxa. The type locality of U. aucklandica is restricted to the catchment of the Kawakawa River.