Absence Of Dispersal Barriers Research Articles

Agriculture creates subtle genetic structure among migratory and nonmigratory populations of burrowing owls throughout North America.

Population structure across a species distribution primarily reflects historical, ecological, and evolutionary processes. However, large‐scale contemporaneous changes in land use have the potential to create changes in habitat quality and thereby cause changes in gene flow, population structure, and distributions. As such, land‐use changes in one portion of a species range may explain declines in other portions of their range. For example, many burrowing owl populations have declined or become extirpated near the northern edge of the species' breeding distribution during the second half of the 20th century. In the same period, large extensions of thornscrub were converted to irrigated agriculture in northwestern Mexico. These irrigated areas may now support the highest densities of burrowing owls in North America. We tested the hypothesis that burrowing owls that colonized this recently created owl habitat in northwestern Mexico originated from declining migratory populations from the northern portion of the species' range (migration‐driven breeding dispersal whereby long‐distance migrants from Canada and the United States became year‐round residents in the newly created irrigated agriculture areas in Mexico). We used 10 novel microsatellite markers to genotype 1,560 owls from 36 study locations in Canada, Mexico, and the United States. We found that burrowing owl populations are practically panmictic throughout the entire North American breeding range. However, an analysis of molecular variance provided some evidence that burrowing owl populations in northwestern Mexico and Canada together are more genetically differentiated from the rest of the populations in the breeding range, lending some support to our migration‐driven breeding dispersal hypothesis. We found evidence of subtle genetic differentiation associated with irrigated agricultural areas in southern Sonora and Sinaloa in northwestern Mexico. Our results suggest that land use can produce location‐specific population dynamics leading to subtle genetic structure even in the absence of dispersal barriers.

Ecological Divergence with Gene Flow in a Thermophilic Cyanobacterium.

How ecological diversity is maintained and distributed within populations is a longstanding question in microbial ecology. In the thermophilic cyanobacterium Synechococcus B', high observed levels of recombination are predicted to maintain ecological variation despite the simultaneous action of diverse selective pressures on different regions of the genome. To investigate ecological diversity in these bacteria, we directly isolated laboratory strains of Synechococcus B' from samples collected along the thermal gradients of two geothermal environments in Yellowstone National Park. Extensive recombination was evident for a multi-locus sequence data set, and, consequently, our sample did not exhibit the sequence clustering expected for distinct ecotypes evolving by periodic clonal selection. Evidence for local selective sweeps at specific loci suggests that sweeps may be common but that recombination is effective for maintaining diversity of unlinked genomic regions. Thermal performance for strain growth was positively associated with the temperature of the environment, indicating that Synechococcus B' populations consist of locally adapted ecological specialists that occupy specific thermal niches. Because this ecological differentiation is observed despite the absence of dispersal barriers among sites, we conclude that these bacteria may freely exchange much of the genome but that barriers to gene flow exist for loci under direct temperature selection.

Only true pelagics mix: comparative phylogeography of deepwater bathybatine cichlids from Lake Tanganyika

In the absence of dispersal barriers, species with great dispersal ability are expected to show little, if at all, phylogeographic structure. The East African Great Lakes and their diverse fish faunas provide opportunities to test this hypothesis in pelagic fishes, which are presumed to be highly mobile and unrestricted in their movement by physical barriers. Here, we address the link between panmixis and pelagic habitat use by comparing the phylogeographic structure among four deepwater cichlid species of the tribe Bathybatini from Lake Tanganyika. We show that the mitochondrial genealogies (based on the most variable part or the control region) of the four species are very shallow (0.8–4% intraspecific divergence across entire distribution ranges) and that all species experienced recent population growth. A lack of phylogeographic structure in the two eupelagic species, Bathybates fasciatus and B. leo, was consistent with expectations and with findings in other pelagic cichlid species. Contrary to expectations, a clear phylogeographic structure was detected in the two benthopelagic species, B. graueri and Hemibates stenosoma. Differences in genetic diversity between eupelagic and benthopelagic species may be due to differences in their dispersal propensity, mediated by their respective predatory niches, rather than precipitated by external barriers to dispersal.

Phytoplankton functional response to spatial and temporal differences in a cold and oligotrophic lake

We explored phytoplankton communities from a functional perspective in two connected but hydro-morphologically distinct basins during the ice-free period in the cold and oligotrophic Lake Tovel, Italy. Despite the absence of dispersal barriers, we expected a clear separation of the phytoplankton communities between basins based on their physical distinctions: a shallow and a deep basin with low and high water residence time, respectively. To investigate seasonal succession and spatiality, taxa were classified according to their functional groups (FGs) and traits (FTs). Relationships between functional classifications and environmental parameters were assessed by non-metric multidimensional scaling integrated with cluster analysis. Clustering of FGs and FTs was complementary and reflected different hydrological conditions of each basin: (i) more stable conditions and higher functional variation across seasons in the deep basin and (ii) less stable conditions and lower functional variation across seasons in the shallow basin. Phytoplankton functional composition evidenced how local conditions selected for corresponding functional attributes adapted to each basin’s environment. These results, together with the presence of rare, cold-tolerant taxa such as Pseudotetraedriella kamillae and Stephanocostis chantaicus, highlighted the need for long-term phytoplankton studies, and the creation of a FG that includes cold-adapted, oligotrophic taxa.

Assessing host specificity of obligate ectoparasites in the absence of dispersal barriers

Host specificity is a characteristic property of parasite-host associations and often is high among those involving obligate or permanent parasites. While many parasites are highly host-specific under natural conditions, specificity may break down in the absence of dispersal barriers. We tested the host specificity of obligate and permanent blood-feeding bat parasites (Hemiptera: Polyctenidae) under experimental conditions where parasite dispersal barriers had been removed. Under these conditions, parasites not only readily accepted a secondary host species but also remained there when a primary host was immediately available. Experiments with bat bugs and observations of streblid bat flies suggest that specificity may at least temporarily break down when dispersal barriers are removed. To affect long-term coevolutionary patterns, such transfers would necessarily entail the establishment of viable parasite populations on secondary host species.

New Mysida (Crustacea) in the genera Amblyops and Pseudomma from the Iceland Basin

Five species of Erythropinae are described from the Iceland Basin. Amblyops trisetosa and A. spinifera are shown to have extended their species range from the Bay of Biscay to Iceland. Pseudomma maasaki and P. islandicum are described as new species. A full description of P. antarcticum, previously reported only from Antarctic waters, is presented, including a first description of the male pleopods. The extended northward distribution of the previously described species coupled with P. islandicum bearing close resemblance to Antarctic Pseudomma species indicates the absence of dispersal barriers for bottom-living mysids in the Atlantic deep sea east of the mid-Atlantic Ridge.