Genetic diversity and migration patterns of the aquatic macrophyte Potamogeton malaianus in a potamo-lacustrine system

1. Previously, the Yangtze River connected thousands of shallow lakes which together formed a potamo-lacustrine system capable of sustaining a rich variety of submerged macrophytes.
2.  Potamogeton malaianus is one of the dominant submerged macrophytes in many lakes of this area. Genetic variation and population structure of P. malaianus populations from ten lakes in the potamo-lacustrine system were assessed using inter-simple sequence repeat markers.
3. Twelve primer combinations produced a total of 166 unambiguous bands of which 117 (70.5%) were polymorphic. Potamogeton malaianus exhibited a moderate level of population genetic diversity ( P _P = 70.5%, H _E = 0.163 and I =  0.255), as compared with that of plants in the same habitat and range. The main factors responsible for this moderate value were the plant's mixed breeding system (both sexual and asexual) and the hydrological connectivity among habitats.
4.  F statistics, calculated using different approaches, consistently revealed a moderate genetic differentiation among populations, contributing about 20% of total genetic diversity. An estimate of gene flow (using F _ST) suggested that gene flow played a more important role than genetic drift in the current population genetic structure of P. malaianus ( Nm  = 1.131).
5. The genetic diversity of P. malaianus did not increase downstream. A high level of linkage–disequilibrium at the whole population level suggested that metapopulation processes may affect genetic structure. The migration pattern of P. malaianus was best explained by a two-dimensional stepping stone model, indicating that bird-mediated dispersal could greatly influence gene movements among lakes.     

Restricted dispersal and genetic diversity in populations of an endangered montane lizard (Eulamprus leuraensis,Scincidae)

Many alpine species are under threat from global climate change, as their geographic ranges become increasingly fragmented and unsuitable. Understanding rates and determinants of gene flow among such fragmented populations, over historical as well as recent timescales, can help to identify populations under threat. It is also important to clarify the degree to which loss of local populations reduces overall genetic diversity within the taxon. The endangered Blue Mountains Water Skink (Eulamprus leuraensis) is restricted to <40 small swamps in montane south‐eastern Australia. Our analyses of seven microsatellite loci of 241 animals from 13 populations show strong geographic structure, with major genetic divergence even between populations separated by <0.5 km. Dispersal between populations is scarce, and appears to involve mostly males. Our analyses suggest potential recent bottleneck events in all the identified populations, and lower genetic diversity and population size parameter at lower‐elevation sites than at higher‐elevation sites. Management of this endangered taxon thus needs to treat most populations separately, because of their genetic distinctiveness and low rates of genetic exchange.     

Spatial structure inhibits the rate of invasion of beneficial mutations in asexual populations

Populations in spatially structured environments may be divided into a number of (semi-) isolated subpopulations due to limited offspring dispersal. Limited dispersal and, as a consequence, local competition could slow down the invasion of fitter mutants, allowing the short-term coexistence of ancestral genotypes and mutants. We determined the rate of invasion of beneficial mutants of Escherichia coli, dispersed to different degrees in a spatially structured environment during 40 generations, experimentally and theoretically. Simulations as well as experimental data show a decrease in the rate of invasion with increasingly constrained dispersal. When a beneficial mutant invades from a single spot, competition with the ancestral genotype takes place only along the edges of the growing colony patch. As the colony grows, the fitness of the mutant will decrease due to a decrease in the mutant's fraction that effectively competes with the surrounding ancestor. Despite its inherently higher competitive ability, increased intragenotype competition prevents the beneficial mutant from rapidly taking over, causing short-term coexistence of superior and inferior genotypes.     

Isolation by distance in a continuous population under stochastic demographic fluctuations

The local density of individuals is seldom uniform in space and time within natural populations. Yet, formal approaches to the process of isolation by distance in continuous populations have encountered analytical difficulties in describing genetic structuring with demographic heterogeneities, usually disregarding local correlations in the movement and reproduction of genes. We formulate exact recursions for probabilities of identity in continuous populations, from which we deduce definitions of effective dispersal () and effective density (D_e) that generalize results relating spatial genetic structure, dispersal and density in lattice models. The latter claim is checked in simulations where estimates of effective parameters obtained from demographic information are compared with estimates derived from spatial genetic patterns in a plant population evolving in a heterogeneous and dynamic habitat. The simulations further suggest that increasing spatio‐temporal correlations in local density reduce and generally decrease the product , with dispersal kurtosis influencing their sensitivity to density fluctuations. As in the lattice model, the expected relationship between the product and the genetic structure statistic a_r holds under fluctuating density, irrespective of dispersal kurtosis. The product D σ² between observed census density and the observed dispersal rate over one generation will generally be an upwardly biased (up to 400% in simulations) estimator of in populations distributed in spatially aggregated habitats.     

The relative importance of local conditions and regional processes in structuring aquatic plant communities

1. The structure of biological communities reflects the influence of both local environmental conditions and processes such as dispersal that create patterns in species’ distribution across a region. 2. We extend explicit tests of the relative importance of local environmental conditions and regional spatial processes to aquatic plants, a group traditionally thought to be little limited by dispersal. We used partial canonical correspondence analysis and partial Mantel tests to analyse data from 98 lakes and ponds across Connecticut (northeastern United States). 3. We found that aquatic plant community structure reflects the influence of local conditions (pH, conductivity, water clarity, lake area, maximum depth) as well as regional processes. 4. Only 27% of variation in a presence/absence matrix was explained by environmental conditions and spatial processes such as dispersal. Of the total explained, 45% was related to environmental conditions and 40% to spatial processes. 5. Jaccard similarity declined with Euclidean distance between lakes, even after accounting for the increasing difference in environmental conditions, suggesting that dispersal limitation may influence community composition in the region. 6. The distribution of distances among lakes where species occurred was associated with dispersal‐related functional traits, providing additional evidence that dispersal ability varies among species in ways that affect community composition. 7. Although environmental and spatial variables explained a significant amount of variation in community structure, a substantial amount of stochasticity also affects these communities, probably associated with unpredictable colonisation and persistence of the plants.     

Spatial correlations at different spatial scales are themselves highly correlated in isolation by distance processes

Although many properties of spatial autocorrelation statistics are well characterized, virtually nothing is known about possible correlations among values at different spatial scales, which ultimately would influence how inferences about spatial genetics are made at multiple spatial scales. This article reports the results of stochastic space-time simulations of isolation by distance processes, having a very wide range of amounts of dispersal for plants or animals, and analyses of the correlations among Moran's I-statistics for different mutually exclusive distance classes. In general, the stochastic correlations are extremely large (>0.90); however, the correlations bear a complex relationship with level of dispersal, spatial scale and spatial lag between distance classes. The correlations are so large that any existing or conceived statistical method that employs more than one distance class (or spatial scale) should not ignore them. This result also suggests that gains in statistical power via increasing sample size are limited, and that increasing numbers of assayed loci generally should be preferred. To the extent that sampling error for real data sets can be treated as white noise, it should be possible to account for stochastic correlations in formulating more precise statistical methods. Further, while the current results are for isolation by distance processes, they provide some guidance for some more complex stochastic space-time processes of landscape genetics. Moreover, the results hold for several popular measures other than Moran's I. In addition, in the results, the signal to noise ratios strongly decreased with distance, which also has several implications for optimal statistical methods using correlations at multiple spatial scales.     

Effect of dispersal and nutrient availability on the competitive ability of toxin-producing yeast

The ecological role of interference competition through toxin production is not well understood. In particular, it is unclear under what conditions the benefits of toxic killing outweigh the metabolic costs involved. A killer advantage has been suggested to rely on local competitive interactions where the benefits of killing accrue to the toxin producer preferentially, but this notion has little empirical support. In addition, contrasting predictions exist about the effect of resource abundance on the benefits of toxin production; this benefit should either be highest when resources are abundant and metabolic costs are relatively low or when resources are scarce and toxic killing is a 'last resort strategy' to obtain nutrients. Here, we test these predictions for one aspect of competitive ability, that is, the ability of toxin producers to invade a population of sensitive non-producers from a low initial frequency. We use competition experiments between isogenic K1 toxin-producing and non-producing strains of Saccharomyces cerevisiae, where we manipulate dispersal under two extreme nutrient conditions: one environment with and the other without replenishment of nutrients. We find that toxin production is beneficial when dispersal is limited under both nutrient conditions, but only when resources are abundant these outweigh its cost and allow invasion of the producer.