Fine-scale population structure and dispersal in Biomphalaria glabrata, the intermediate snail host of Schistosoma mansoni, in Venezuela

Biomphalaria glabrata is the main intermediate host of Schistosoma mansoni in America and one of the most intensely studied species of freshwater snails, yet very little is known about its population biology. Here, we used seven highly polymorphic microsatellite loci to analyse genetic diversity in the Valencia lake basin, which represents the core of the endemic area for schistosomiasis in Venezuela. Populations were sampled at short spatial scale (a few kilometres), both inside the lake and in ponds or rivers near the lake. Our results indicate that B. glabrata essentially cross-fertilizes, with little variation in selfing rates among populations. Our markers detected considerable genetic variation, with an average heterozygosity of 0.60. More diversity per population was found within than outside the lake, suggesting an influence of connectivity among populations on the levels of genetic diversity. A marked population structure was detected and lake populations were less structured than other populations. Most individuals were assigned to their population of origin using an assignment test. No strong demographic signal (e.g. bottleneck) was detected, though lake populations are likely to experience bottlenecks more frequently than the other populations analysed. Differences in gene flow therefore seem to play an important role in population differentiation and in the restoring of genetic diversity in demographically unstable populations.     

Development of 10 microsatellite loci in the pulmonate snail Biomphalaria kuhniana (Mollusca, Gastropoda)

We characterized 10 variable microsatellite loci in the freshwater snail Biomphalaria kuhniana, as well as conditions for multiplexing and co-loading sets of loci. Two to five alleles were detected per locus over the two studied populations in Venezuela. High inbreeding coefficients suggest high selfing rates. Cross-species amplification provided some variability at eight and three loci in the other species belonging to the Biomphalaria straminea complex (B. straminea and B. intermedia), but was unsuccessful in more divergent species.     

Phylogeography of Biomphalaria glabrata and B. pfeifferi, important intermediate hosts of Schistosoma mansoni in the New and Old World tropics

The historical phylogeography of the two most important intermediate host species of the human blood fluke Schistosoma mansoni, B. glabrata in the New World, and B. pfeifferi in the Old World, was investigated using partial 16S and ND1 sequences from the mitochondrial genome. Nuclear sequences of an actin intron and internal transcribed spacer (ITS)-1 were also obtained, but they were uninformative for the relationships among populations. Phylogenetic analyses based on mtDNA revealed six well-differentiated clades within B. glabrata: the Greater Antilles, Venezuela and the Lesser Antilles, and four geographically overlapping Brazilian clades. Application of a Biomphalaria-specific mutation rate gives an estimate of the early Pleistocene for their divergence. The Brazilian clades were inferred to be the result of fragmentation, due possibly to climate oscillations, with subsequent range expansion producing the overlapping ranges. Within the Venezuela and Lesser Antilles clade, lineages from each of these areas were estimated to have separated approximately 740 000 years ago. Compared to B. glabrata, mitochondrial sequences of B. pfeifferi are about 4x lower in diversity, reflecting a much younger age for the species, with the most recent common ancestor of all haplotypes estimated to have existed 880 000 years ago. The oldest B. pfeifferi haplotypes occurred in southern Africa, suggesting it may have been a refugium during dry periods. A recent range expansion was inferred for eastern Africa less than 100 000 years ago. Several putative species and subspecies, B. arabica, B. gaudi, B. rhodesiensis and B. stanleyi, are shown to be undifferentiated from other B. pfeifferi populations.     

Genetic diversity of Biomphalaria pfeifferi,the intermediate host of Schistosoma mansoni in Shamva district,Zimbabwe: role on intestinal schistosomiasis transmission

Molecular Biology Reports - The fresh water snail Biomphalaria pfeifferi is the intermediate host for Schistosoma mansoni, which causes human intestinal schistosomiasis in Zimbabwe. Despite the...     

Microsatellites in the freshwater snail Bulinus globosus (Gastropoda: Planorbidae) from Zanzibar

Six microsatellite loci were isolated and characterized in Bulinus globosus, a freshwater snail with a wide distribution throughout sub‐Saharan Africa. Bulinus globosus is an intermediate host of Schistosoma haematobium, the causative agent of human urinary schistosomiasis. Microsatellites were tested using 32 snails from four populations collected from Pemba and Unguja islands of Zanzibar. The microsatellite loci displayed relatively low levels of variation, with between two and five alleles per locus. F_ST estimates indicate that gene flow is low, as has previously been suggested for other species of Bulinus.     

Genetic diversity and population structure of the threatened Bliss Rapids snail (Taylorconcha serpenticola)

1. The Bliss Rapids snail is a federally listed yet poorly known small caenogastropod which lives in the Snake River drainage (main stem river and spring‐fed tributaries) of south‐central Idaho. The construction of three large dams along this portion of the Snake River during the 20th century is thought to have fragmented a single, ancestral population of this species into genetically isolated subunits that are vulnerable to extinction. We assessed variation of 11 microsatellite loci within and among 29 samples (820 snails) from across the entire range of the Bliss Rapids snail to assess genetic structure and test whether habitat fragmentation resulting from dam construction has impacted population connectivity. 2. The overall F_ST (0.15133, P < 0.05) and pairwise comparisons among samples (384/406 significant) indicated extensive population subdivision in general. A consistent trend of isolation by distance trend was not detected by Mantel tests. We found no evidence of reduced genetic diversity attributable to segmentation of the Snake River, and genetic variation among portions of drainage separated by the dams was not significant. Population structuring in spring–tributary habitats was considerably greater than in the main stem river as evidenced by differences in F_ST (0.18370, 0.06492) and the number of private alleles detected (16, 4), and by the results of an assignment test (69.4%, 58.7% correctly classified to sample of origin) and Bayesian genetic clustering algorithm. 3. Our results provide no evidence that dam construction has genetically impacted extant populations of the Bliss Rapids snail. We speculate that the generally weaker genetic structuring of riverine populations of this species is a result of passive dispersal within the water column, which may enable occasional passage through the dams. The somewhat stronger structuring observed in a portion of the river (Shoshone reach) which receives discharge from many springs may be due to local mixing of main stem and more highly differentiated tributary populations. Our findings parallel recent, genetically based studies of other western North American freshwater gastropods that also demonstrate complex population structure that conflicts with traditional concepts of dispersal ability and sensitivity to putative barriers.     

Observations on the Anopheles gambiae complex in the Senegal River Basin, West Africa

1. Three sibling species of mosquitoes of the Anopheles gambiae complex are found in the Senegal River Basin: An. melas Theobald, An. gambiae Giles and An. arabiensis Patton. 2. An. melas is restricted to the river delta and environs where saltwater breeding places are present. 3. An. gambiae and An. arabiensis are sympatric in the study area; An. arabiensis predominates in coastal zones where it breeds also during the dry season; An. gambiae predominates in inland areas where breeding is mostly restricted to the rainy season (July-September). 4. An. arabiensis is chromosomally polymorphic all over the study area, with much variation of inversion frequencies, particularly for the 2Ra arrangement. 5. An. gambiae is characterized by a very high frequency of the 2Rb-2La inversion arrangement which is typical of the Savanna chromosomal form.     

Direct and delayed effects of low temperature on the freshwater snail Indoplanorbis exustus (Deshayes, 1934), intermediate host of schistosomias

The direct and delayed effect of short-term exposure to low temperature (17.5 °C) on growth, reproduction and longevity of adolescent and mature freshwater Indoplanorbis exustus Deshyaes (Mollusca: Gastropoda) was studied in the laboratory. We found that growth and reproduction were retarded, but were enhanced when later they were returned to ambient temperature.     

Swimming against the current: genetic structure, host mobility and the drift paradox in trematode parasites

Life-cycle characteristics and habitat processes can potentially interact to determine gene flow and genetic structuring of parasitic species. In this comparative study, we analysed the genetic structure of two freshwater trematode species with different life histories using cytochrome c oxidase I gene (COI) sequences and examined the effect of a unidirectional river current on their genetic diversity at 10 sites along the river. We found moderate genetic structure consistent with an isolation-by-distance pattern among subpopulations of Coitocaecum parvum but not in Stegodexamene anguillae. These contrasting parasite population structures were consistent with the relative dispersal abilities of their most mobile hosts (i.e. their definitive hosts). Genetic diversity decreased, as a likely consequence of unidirectional river flow, with increasing distance upstream in C. parvum, which utilizes a definitive host with only restricted mobility. The absence of such a pattern in S. anguillae suggests that unidirectional river flow affects parasite species differently depending on the dispersal abilities of their most mobile host. In conclusion, genetic structure, genetic diversity loss and drift are stronger in parasites whose most mobile hosts have low dispersal abilities and small home ranges. An additional prediction can be made for parasites under unidirectional drift: those parasites that stay longer in their benthic intermediate host or have more than one benthic intermediate hosts would have relatively high local recruitment and hence increased retention of upstream genetic diversity.     

Genetic diversity, fixation and differentiation of the freshwater snail Biomphalaria pfeifferi (Gastropoda, Planorbidae) in arid lands

The freshwater snail Biomphalaria pfeifferi is the main intermediate host of human intestinal Bilharziasis. It is widely distributed in Africa, Madagascar and middle-eastern countries, and its habitat includes wetlands, and arid to semi-arid areas. Based on analysis of 18 microsatellites, we investigated reference allelic variation among 30 populations of B. pfeifferi from three drainage basins in Dhofar, Oman (the eastern limit of its distribution). This is an arid to semi-arid region, with a 9,000-year history of very low rainfall, but is subject to unpredictable and destructive flash floods. In this context we showed that genetic fixation was very high compared to genetic differentiation which was moderate and, that, relative to B. pfeifferi populations from wetlands, the populations in Dhofar show evidence of lower levels of genetic diversity, a higher degree of genetic fixation, a quasi-absence of migration, and a higher level of genetic drift. Despite the extreme conditions in the Dhofar habitat of this species, it is able to survive because of its very high self-fertilization (approaching 100 %) and fecundity rates.     

Low genetic diversity and limited genetic structure across the range of the critically endangered Mexican howler monkey (Alouatta palliata mexicana)

Genetic diversity provides populations with the possibility to persist in ever-changing environments, where selective regimes change over time. Therefore, the long-term survival of a population may be affected by its level of genetic diversity. The Mexican howler monkey (Alouatta palliata mexicana) is a critically endangered primate restricted to southeast Mexico. Here, we evaluate the genetic diversity and population structure of this subspecies based on 83 individuals from 31 groups sampled across the distribution range of the subspecies, using 29 microsatellite loci. Our results revealed extremely low genetic diversity (H_O = 0.21, H_E = 0.29) compared to studies of other A. palliata populations and to other Alouatta species. Principal component analysis, a Bayesian clustering method, and analyses of molecular variance did not detect strong signatures of genetic differentiation among geographic populations of this subspecies. Although we detect small but significant F_ST values between populations, they can be explained by a pattern of isolation by distance. These results and the presence of unique alleles in different populations highlight the importance of implementing conservation efforts in multiple populations across the distribution range of A. p. mexicana to preserve its already low genetic diversity. This is especially important given current levels of population isolation due to the extreme habitat fragmentation across the distribution range of this primate.     

Discriminating coho salmon (Oncorhynchus kisutch) populations within the Fraser River, British Columbia, using microsatellite DNA markers

Three microsatellite loci were used to examine genetic variation among 16 coho salmon ( Oncorhynchus kisutch ) populations within the Fraser River drainage system, in British Columbia, Canada. Each locus was highly polymorphic with 30 alleles at the Ots 101 locus, 15 alleles at the Ots 3 locus and 38 alleles at the Ots 103 locus. Average observed heterozygosities were 86.1%, 70%, and 56.1%, respectively. With the exception of the Dunn and Lemieux River populations, Chi-square tests and F _ST values indicated that all populations had significantly different allele frequencies. Two distinct population groups within the Fraser River drainage were observed. Lower Fraser River populations were strongly differentiated from populations spawning in the upper Fraser River, which includes the Thompson River (a tributary flowing into the upper Fraser) and the portion of the Fraser River beyond the precipitous Fraser River canyon. This regional population structure may have resulted from colonization of the upper and lower Fraser River regions by different founder populations following Pleistocene glaciation, and be maintained by adaptive differences between the two groups of coho salmon. Coho salmon populations in the upper Fraser and Thompson River drainages form an evolutionarily significant unit (ESU) of importance for conservation of biodiversity in coho salmon. Microsatellite DNA loci show promise as technically simple and highly informative genetic markers for coho salmon population management.     

Translocations maintain genetic diversity and increase connectivity in sea otters,Enhydra lutris

Sea otters, Enhydra lutris, were once abundant along the nearshore areas of the North Pacific. The international maritime fur trade that ended in 1911 left 13 small remnant populations with low genetic diversity. Subsequent translocations into previously occupied habitat resulted in several reintroduced populations along the coast of North America. We sampled sea otters between 2008 and 2011 throughout much of their current range and used 19 nuclear microsatellite markers to evaluate genetic diversity, population structure, and connectivity between remnant and reintroduced populations. Average genetic diversity within populations was similar: observed heterozygosity 0.55 and 0.53, expected heterozygosity 0.56 and 0.52, unbiased expected heterozygosity 0.57 and 0.52, for reintroduced and remnant populations, respectively. Sea otter population structure was greatest between the Northern and Southern sea otters with further structuring in Northern sea otters into Western, Central, and Southeast populations (including the reintroduced populations). Migrant analyses suggest the successful reintroductions and growth of remnant groups have enhanced connectivity and gene flow between populations throughout many of the sampled Northern populations. We recommend that future management actions for the Southern sea otter focus on future reintroductions to fill the gap between the California and Washington populations ultimately restoring gene flow to the isolated California population.     

Population genetic structure of the lemon shark (Negaprion brevirostris) in the western Atlantic: DNA microsatellite variation

DNA microsatellite markers were used to characterize the population genetic structure of the lemon shark, Negaprion brevirostris, in the western Atlantic. This study demonstrates for the first time the usefulness of microsatellites to study population genetic structure and mating systems in the Chondricthyes. Lemon sharks (mostly juveniles) were sampled non-destructively from four locations, Gullivan Bay and Marquesas Key in Florida, Bimini, Bahamas, and Atol das Rocas, Brazil. At least 545 individuals were genotyped at each of four dinucleotide loci. The number of alleles per locus ranged from 19 to 43, and expected heterozygosities ranged from 0.69 to 0.90. Relatively little genetic structure was found in the western Atlantic, with small but significant values for estimators of F(ST) and R(ST) among populations, theta (0.016) and rho (0.026), respectively. No sharp discontinuities were found between the Caribbean sites and Brazil, and most alleles were found at all four sites, indicating that gene flow occurs throughout the western Atlantic with no evidence for distinct stocks.     

Anopheles pharoensis and transmission of Plasmodium falciparum in the Senegal River delta, West Africa

Abstract. 1. Anopheles pharoensis Theobald was found to be the prevalent man-biting anopheline mosquito in the central area of the Senegal River delta.
2. Blood-fed females of An.pharoensis were obtained during September-December 1987 from mosquito bednets in the village of Souhlloul, near the Boundoum dam, 70 km NE of St Louis.
3. Dried mosquito specimens were identified morphologically and each thorax processed using monoclonal antibody against the circum-sporozoite protein of Plasmodium falciparum.
4. Five An.pharoensis out of 912 examined were sporozoite positive, while ninety-eight An.gambiae Giles sensu lato were all negative. This finding strongly supports the local importance of An.pharoensis as a malaria vector.
5. Successful use of pyrethroid-impregnated bednets against malaria transmission in this situation has helped to achieve more than 90% reduction of malaria prevalence.     

湖北海棠的等位酶变异和遗传多样性研究

采用超薄平板微型聚丙烯酰胺等电聚焦电泳方法对湖北海棠（Malus hupehensis)的9个野生居群和2个人工栽培居群的等位酶变异和遗传多样性进行了初步研究。通过对12个酶系统29个酶位点的检测,结果表明湖北海棠有25个酶位点的等位基因频率分布差异,,有10个居群发现稀有等位基因,并有11个（37．9％）重复位点;湖北海棠的遗传多样性水平很高,等位基因平均数A＝2．127,多态位点百分率P＝74．927,平均预期杂合度He=0.376;居群间的基因分化系数GST＝0．224。与其他苹果属植物相比,湖北海棠具有中等丰富的遗传变异水平。居群间的基因流仅为Nm=0.866,表明遗传漂变是影响居群遗传变异和遗传结构的一个重要因素。