Rapid displacement of a monoecious plant lineage is due to pollen swamping by a dioecious relative.
(25/998)
Interspecific hybridization is recognized as a potentially destructive process that represents a major threat to biodiversity. The rate of population displacement by hybridization can be rapid, but underlying mechanisms are often obscure. One hypothesis is that a species may be driven to extinction by interspecific gene flow, or pollen swamping, when hybrids are inviable or sterile. Here, we document the rapid movement of two zones of contact between monoecious hexaploid and dioecious diploid populations of the wind-pollinated plant Mercurialis annua (Euphorbiaceae) in northeastern and northwestern Spain, where diploids have displaced hexaploids by about 80 and 200 km, respectively, over a period of four decades. By using experimental mating arrays, we show that hybridization is highly asymmetrical in favor of the diploids, mainly because they disperse substantially more pollen, as expected in a comparison between an obligate outcrosser and a facultative selfer. Self-fertilization, which is expected to reduce the proportion of sterile hybrids produced in mixed ploidy populations, allowed the hexaploids to avoid the effects of pollen swamping only slightly, and in a density-dependent manner. Our results thus provide a mechanistic explanation for the rapid movement of both contact zones of M. annua in Spain. (+info)
Partition of the Botrytis cinerea complex in France using multiple gene genealogies.
(26/998)
In micro-organisms biodiversity is often underestimated because relevant criteria for recognition of distinct evolutionary units are lacking. Phylogenetic approaches have been proved the most useful in fungi to address this issue. Botrytis cinerea, a generalist fungus causing gray mold, illustrates this problem. It long has been thought to be a single variable species. Recent population genetics studies have shown that B. cinerea is a species complex. However conflicting partitions were proposed. To identify the most relevant partitions within the B. cinerea complex we used a multiple-gene genealogies approach. We sequenced portions of four nuclear genes, of which genealogies congruently clustered into two well supported groups corresponding to Groups I and II previously described, indicating that they represent phylogenetic species. Estimates of migration rates and genetic differentiation showed that these groups had been isolated for a long time, without detectable gene flow. This was confirmed by the high number of polymorphic sites fixed within each group. The genetic diversity was lower within Group I, as revealed by DNA polymorphism and vegetative incompatibility tests. Groups I and II exhibited phenotypic differences in their phenology, host range, size of asexual spores and vegetative compatibility. All these morphological and molecular aspects suggest that B. cinerea Groups I and II may be different cryptic species, isolated for a long time. Phylogenies and molecular analyzes of variance revealed no genetic structure according to the other suggested partitions for the B. cinerea complex (i.e., among host plants, between strains with and without transposable elements, nor between strains responsible for noble rot and gray mold. This suggests that recombination regularly occurs, or occurred until recently, within B. cinerea Group II. This also was supported by recombination rates at each locus. Multiple-gene genealogies showed their utility by providing a relevant partition criterion for the B. cinerea complex. (+info)
DNA microsatellite characterization of the jaguar (Panthera onca) in Colombia.
(27/998)
The Colombian jaguar population is thought to contain two different subspecies, Panthera onca centralis and Panthera onca onca. The genetic structure of this population was evaluated using 12 microsatellite loci (n = 62 samples). In addition, 22 jaguar DNA samples from Guatemala, Paraguay, Peru, Bolivia, Venezuela and Brazil were analyzed for these microsatellite loci (n = 84 samples). The results of this study indicate six primary themes. First, the levels of gene diversity were very high. Second, the majority of the loci analyzed showed an absence of Hardy-Weinberg equilibrium, probably due to the Wahlund effect (= population subdivision). Third, several microsatellite loci showed significant heterogeneity between the two supposed subspecies in the country. Nevertheless, gene flow was present between them, and heterogeneity was relatively low, although the assignment analyses showed good classification of the jaguars studied into their respective subspecies. Fourth, the long-term historical effective population sizes were calculated through a maximum likelihood procedure for single and multi-step mutation models. Fifth, seven out of twelve DNA microsatellites studied significantly deviated from a single-step mutation model. However, the overall mean multi-step mutation percentage for these 12 DNA microsatellites was only 6%. Therefore, 94% of mutations were uni-step. Sixth, no bottleneck events were detected in the Colombian jaguar population overall. (+info)
Regional heterogeneity and gene flow maintain variance in a quantitative trait within populations of lodgepole pine.
(28/998)
Genetic variation is of fundamental importance to biological evolution, yet we still know very little about how it is maintained in nature. Because many species inhabit heterogeneous environments and have pronounced local adaptations, gene flow between differently adapted populations may be a persistent source of genetic variation within populations. If this migration-selection balance is biologically important then there should be strong correlations between genetic variance within populations and the amount of heterogeneity in the environment surrounding them. Here, we use data from a long-term study of 142 populations of lodgepole pine (Pinus contorta) to compare levels of genetic variation in growth response with measures of climatic heterogeneity in the surrounding region. We find that regional heterogeneity explains at least 20% of the variation in genetic variance, suggesting that gene flow and heterogeneous selection may play an important role in maintaining the high levels of genetic variation found within natural populations. (+info)
Origins and evolution of the Europeans' genome: evidence from multiple microsatellite loci.
(29/998)
There is general agreement that the current European gene pool is mainly derived from Palaeolithic hunting-gathering and Neolithic farming ancestors, but different studies disagree on the relative weight of these contributions. We estimated admixture rates in European populations from data on 377 autosomal microsatellite loci in 235 individuals, using five different numerical methods. On average, the Near Eastern (and presumably Neolithic) contribution was between 46 and 66%, and admixture estimates showed, with all methods, a strong and significant negative correlation with distance from the Near East. If the assumptions of the model are approximately correct, i.e. if the Basques' and Near Easterners' genomes represent a good approximation to the Palaeolithic and Neolithic settlers of Europe, respectively, these results imply that half or more of the Europeans' genes are descended from Near Eastern ancestors who immigrated in Europe 10000 years ago. If these assumptions are incorrect, our results show anyway that clinal variation is the rule in the Europeans' genomes and that lower estimates of Near Eastern admixture obtained from the analysis of single markers do not reflect the patterns observed at the genomic level. (+info)
Iran: tricontinental nexus for Y-chromosome driven migration.
(30/998)
Due to its pivotal geographic position, present day Iran likely served as a gateway of reciprocal human movements. However, the extent to which the deserts within the Iranian plateau and the mountain ranges surrounding Persia inhibited gene flow via this corridor remains uncertain. In order to assess the magnitude of this region's role as a nexus for Africa, Asia and Europe in human migrations, high-resolution Y-chromosome analyses were performed on 150 Iranian males. Haplogroup data were subsequently compared to regional populations characterized at similar phylogenetic levels. The Iranians display considerable haplogroup diversity consistent with patterns observed in populations of the Middle East overall, reinforcing the notion of Persia as a venue for human disseminations. Admixture analyses of geographically targeted, regional populations along the latitudinal corridor spanning from Anatolia to the Indus Valley demonstrated contributions to Persia from both the east and west. However, significant differences were uncovered upon stratification of the gene donors, including higher proportions from central east and southeast Turkey as compared to Pakistan. In addition to the modulating effects of geographic obstacles, culturally mediated amalgamations consistent with the diverse spectrum of a variety of historical empires may account for the distribution of haplogroups and lineages observed. Our study of high-resolution Y-chromosome genotyping allowed for an in-depth analysis unattained in previous studies of the area, revealing important migratory and demographic events that shaped the contemporary genetic landscape. (+info)
Genetic rescue of an insular population of large mammals.
(31/998)
Natural populations worldwide are increasingly fragmented by habitat loss. Isolation at small population size is thought to reduce individual and population fitness via inbreeding depression. However, little is known about the time-scale over which adverse genetic effects may develop in natural populations or the number and types of traits likely to be affected. The benefits of restoring gene flow to isolates are therefore also largely unknown. In contrast, the potential costs of migration (e.g. disease spread) are readily apparent. Management for ecological connectivity has therefore been controversial and sometimes avoided. Using pedigree and life-history data collected during 25 years of study, we evaluated genetic decline and rescue in a population of bighorn sheep founded by 12 individuals in 1922 and isolated at an average size of 42 animals for 10-12 generations. Immigration was restored experimentally, beginning in 1985. We detected marked improvements in reproduction, survival and five fitness-related traits among descendants of the 15 recent migrants. Trait values were increased by 23-257% in maximally outbred individuals. This is the first demonstration, to our knowledge, of increased male and female fitness attributable to outbreeding realized in a fully competitive natural setting. Our findings suggest that genetic principles deserve broader recognition as practical management tools with near-term consequences for large-mammal conservation. (+info)
Genealogical concordance and the specific status of Peromyscus sejugis.
(32/998)
Peromyscus sejugis, a peripheral isolate of Peromyscus maniculatus, is a threatened taxon endemic to 2 small islands in the Sea of Cortes. Although its insularity makes the specific recognition of P. sejugis inherently problematic, resolution of this problem has important conservation implications. To evaluate the specific validity and evolutionary history of P. sejugis, we compared sequence variation (ND3/ND4L/ND4) in mtDNA for both island populations of P. sejugis with that for 8 populations of P. maniculatus from mainland Baja California. Each island population of P. sejugis had a single haplotype (0.7% sequence divergence), whereas 11 different haplotypes (mean sequence divergence = 0.68%) were obtained for the populations of P. maniculatus. The mean sequence divergence between the populations of the 2 species was 2.0%. Nested clade analysis supports the conclusion that P. sejugis is an insular isolate of P. maniculatus from mainland Baja California. Although our analysis confirms a low level of mtDNA divergence between P. sejugis and P. maniculatus from Baja California, the genealogical concordance of morphological, chromosomal, microsatellite, and mtDNA haplotype distinctiveness supports the conclusion that the 2 island populations of P. sejugis constitute independent evolutionarily significant units and together represent a phylogenetic species distinct from the P. maniculatus from Baja California. (+info)