Populations In Sympatry Research Articles

Genetic structure of cetacean populations in sympatry, parapatry, and mixed assemblages: implications for conservation policy

Many cetaceans have a wide distribution in one or more oceans, and in some species, individuals migrate over an extensive range. When breeding sites are geographically distant from foraging grounds, as for a number of baleen whales, genetic stocks may be geographically isolated during a breeding season and together in a mixed assemblage during a separate feeding season. These assemblages can be spatially and temporally dynamic and pose a special problem for managers when whales are hunted on feeding grounds (as is typical). For other species seasonal migrations are less pronounced, but the same effect of locally mixing genetic stocks can develop through other mechanisms. Examples are described where intraspecific foraging specializations appear to be important in limiting gene flow between sympatric and parapatric populations. It is proposed that learning could be important in the generation and maintenance of these specializations. The effective conservation of genetic diversity in these species will require further data on both spatial and temporal components of population genetic structure.

A mtDNA analysis of spatiotemporal distribution of two sympatric larval populations of rainbow smelt (Osmerus mordax) in the St. Lawrence River estuary, Quebec, Canada

Ecological isolation through resource partitioning is invoked as a major factor for explaining the persistence of genetically distinct yet closely related sympatric populations. Two genetically distinct sympatric populations of anadromous rainbow smelt (Osmerus mordax) exist in the middle estuary of the St. Lawrence River. The persistence of these coexisting populations in sympatry is in conflict with current theoretical concepts predicting population richness. In the present study, we performed mtDNA PCR-RFLP analysis of 922 larvae from 33 sampling stations to test the hypothesis that the larvae belonging to the two sympatric smelt populations of the St. Lawrence middle estuary are spatially segregated and that such segregation may promote the persistence of the populations. Results clearly revealed spatial homogeneity in the relative distribution of larvae from the two populations. Consequently, they did not support our working hypothesis that larvae belonging to the two sympatric smelt populations are spatially segregated. Two alternative explanations may account for the lack of spatial partitioning observed here. Competition may not be important enough to promote resource partitioning at the larval stage. Alternatively, resource partitioning occurs, but not spatially. This study also demonstrated that the effect of historical events may have been as important as contemporary ecological settings in determining genetic population structure in smelt.

Geographic distribution, species displacement, and niche overlap for lake trout and bull trout in mountain lakes

Indigenous lacustrine populations of bull trout (Salvelinus confluentus) and lake trout (S. namaycush) are spatially separated within the southern part of the zone of distributional overlap (northern Montana, southwestern Alberta, and east-central British Columbia). In this area, lake trout occurred primarily in mountain lakes of 1032–1500 m elevation, while bull trout were found primarily in lakes between 1500 and 2200 m. Introductions of lake trout in the twentieth century and data obtained from beyond the study area indicated that both fishes can establish significant allopatric populations (more than 5% of the catch) in large, deep lakes (>8 ha in area and >8 m deep) over a wide elevation range. We tested the hypothesis that lake trout displace or exclude bull trout from lakes by determining the outcome of introductions of lake trout into two lakes that supported indigenous bull trout. Lake trout were introduced into Bow Lake in 1964, and by 1992 the bull trout population was decimated there and in another lake (Hector) situated 15 km downstream. Thus, lake trout can displace bull trout and may prevent bull trout from becoming established in certain low-elevation lakes. Population age-structure analyses also suggest that lake trout adversely affected bull trout. Bull trout populations in sympatry with lake trout, including the one extirpated from Hector Lake, had few old fish (18% were more than 5 years old; N = 40 fish from three lakes) compared with allopatric populations (49% were more than 5 years old; N = 235 fish from seven lakes). Niche overlap and the potential for competition between the two char species were substantial. In lakes with trophic structure ranging from simple to complex, bull trout and lake trout fed on similar foods and had similar ecological efficiencies (growth rates). Predation by lake trout on bull trout was not documented during the study.

Evolutionary role of gametophytic selection in the domestication of Pennisetum thyphoides (pearl millet): a two-locus asymmetrical model

Gametophytic competition, demonstrated in pearl millet, has been shown to promote homogametic fertilization, especially in the case of wild/cultivated pollen mixtures. Therefore, gametophytic selection can reduce interbreeding between the two forms of pearl millet. A theoretical two-locus asymmetrical model was built in order to evaluate the potential role of pollen competition in the control of gene flow between wild and cultivated pearl millet populations in sympatry. Pollen selection coefficients, estimated in a previous experiment, were input in this model. It was shown that the wild/cultivated hybrid percentage in the cultivated field can be strongly reduced in the presence of strong gametophytic selection. The hybrid percentages obtained from this model were very similar to those observed in natural conditions with pollen migration rates from wild populations ranging between 10 and 40 per cent according to the selective pollen coefficient. It may be concluded from this that pollen competition can contribute to the continuing phenotypic integrity of cultivated forms even when there is an abundance of wild forms adjacent to the field. However, in most cases, an initial divergence between wild and cultivated populations at the homogamy locus, is required for pollen competition to reduce hybridization. This requirement makes it doubtful whether this aspect of reproductive isolation between wild and cultivated forms was involved in domestication.

Gene Flow Among Three Races of European Corn Borers (Lepidoptera: Pyralidae) in New York State

In New York state, there are three races of European corn borer moths, which are characterized principally by differences in voltinism and the sex pheromone communication system. One race is bivoltine, with females producing and males responding to a 99:1 E / Z isomeric ratio of ∆11-14:0Ac's as the sex pheromone. Two races, one univoltine and one bivoltine, produce and respond to a 3:97 E / Z blend of ∆11-14:OAc's. The races are referred to as BE, UZ, and BZ to denote these differences. Analyses by gas chromatography of field-collected females indicate that there is significant hybridization between the Z and E races when in sympatry, with hybrid females producing a 68:32 ratio of E / Z 11-14:OAc. Gene flow among the races was investigated using differences in triose phosphate isomerase (TPI) allozyme frequencies of moths from sites sampled across the state. The fixed nature of the Tpi-1 allele in the BE race and increases in the Tpi-1 allele frequencies in UZ populations in sympatry with the BE race support the hypothesis that gene flow following hybridization is unidirectional from BE populations into the Z populations. In addition, the TPI genotypes of the 10 hybrid females analyzed were consistent with hybridization occurring by E males mating with Z females.