Heterosis is common and inbreeding depression absent in natural populations of Arabidopsis thaliana

The importance of genetic drift in shaping patterns of adaptive genetic variation in nature is poorly known. Genetic drift should drive partially recessive deleterious mutations to high frequency, and inter‐population crosses may therefore exhibit heterosis (increased fitness relative to intra‐population crosses). Low genetic diversity and greater genetic distance between populations should increase the magnitude of heterosis. Moreover, drift and selection should remove strongly deleterious recessive alleles from individual populations, resulting in reduced inbreeding depression. To estimate heterosis, we crossed 90 independent line pairs of Arabidopsis thaliana from 15 pairs of natural populations sampled across Fennoscandia and crossed an additional 41 line pairs from a subset of four of these populations to estimate inbreeding depression. We measured lifetime fitness of crosses relative to parents in a large outdoor common garden (8,448 plants in total) in central Sweden. To examine the effects of genetic diversity and genetic distance on heterosis, we genotyped parental lines for 869 SNPs. Overall, genetic variation within populations was low (median expected heterozygosity = 0.02), and genetic differentiation was high (median F_ST = 0.82). Crosses between 10 of 15 population pairs exhibited significant heterosis, with magnitudes of heterosis as high as 117%. We found no significant inbreeding depression, suggesting that the observed heterosis is due to fixation of mildly deleterious alleles within populations. Widespread and substantial heterosis indicates an important role for drift in shaping genetic variation, but there was no significant relationship between fitness of crosses relative to parents and genetic diversity or genetic distance between populations.     

Investigating the effects of topography and clonality on genetic structuring within a large Norwegian population of Arabidopsis lyrata

Background and Aims

The gene flow through pollen or seeds governs the extent of spatial genetic structure in plant populations. Another factor that can contribute to this pattern is clonal growth. The perennial species Arabidopsis lyrata ssp. petraea (Brassicaceae) is a self-incompatible, clonal species found in disjunctive populations in central and northern Europe.

Methods

Fourteen microsatellite markers were employed to study the level of kinship and clonality in a high-altitude mountain valley at Spiterstulen, Norway. The population has a continuous distribution along the banks of the River Visa for about 1·5 km. A total of 17 (10 m × 10 m) squares were laid out in a north–south transect following the river on both sides.

Key Results

It is shown that clonal growth is far more common than previously shown in this species, although the overall size of the genets is small (mean diameter = 6·4 cm). Across the whole population there is no indication of isolation by distance, and spatial genetic structure is only visible on fine spatial scales. In addition, no effect of the river on the spatial distribution of genotypes was found.

Conclusions

Unexpectedly, the data show that populations of small perennials like A. lyrata can behave like panmictic units across relatively large areas at local sites, as opposed to earlier findings in central Europe.     

Within-population spatial variation in pollinator visitation rates, pollen limitation on seed set, and flower longevity in an alpine species

Pollen limitation through insufficient pollen deposition on stigmas caused by too infrequent pollinator visitation may influence the reproductive outcome of plants. In this study we investigated how pollinator visitation rate, the degree of pollen limitation, and flower longevity varied spatially among three sites at different altitudes within a population of the dwarf shrub Dryas octopetala L. in alpine southern Norway. Significant pollen limitation on seed set only occurred at the mid-elevation site, while seed set at the other sites appeared to be mainly resource limited, thus indicating a spatial variation in pollen limitation. There was no association between the spatial variation in the extent of pollen limitation and pollinator visitation rate to flowers. However, pollinator visitation rates were related to flower longevity of Dryas; sites with low visitation rates had long-lived flowers and vice versa. Thus, our results suggest within-population spatial co-variation between pollinator visitation rates, pollen limitation, and a developmental response to these factors, flower longevity.     

DHA induces ER stress and growth arrest in human colon cancer cells: associations with cholesterol and calcium homeostasis

Polyunsaturated fatty acids (PUFAs) are normal constituents of the diet, but have properties different from other fatty acids (e.g., through generation of signaling molecules). N-3 PUFAs reduce cancer cell growth, but no unified mechanism has been identified. We show that docosahexaenoic acid (DHA; 22:6 n-3) causes extensive changes in gene expression patterns at mRNA level in the colon cancer cell line SW620. Early changes include unfolded protein response (UPR) and increased levels of phosphorylated eIF2alpha as verified at protein level. The latter is considered a hallmark of endoplasmic reticulum (ER) stress and is abundantly present already after 3 h. It may coordinate many of the downstream changes observed, including signaling pathways for cell cycle arrest/apoptosis, calcium homeostasis, cholesterol metabolism, ubiquitination, and proteasomal degradation. Also, eicosapentaenoic acid (EPA), but not oleic acid (OA), induced key mediators of ER stress and UPR at protein level. Accumulation of esterified cholesterol was not compensated for by increased total levels of cholesterol, and mRNAs for cholesterol biosynthesis as well as de novo synthesis of cholesterol were reduced. These results suggest that cytotoxic effects of DHA are associated with signaling pathways involving lipid metabolism and ER stress.     

Substrate complexation and aggregation influence the cyclodextrin glycosyltransferase (CGTase) catalyzed synthesis of alkyl glycosides

Bacillus macerans cyclodextrin glycosyltransferase (CGTase) was used to convert dodecyl-β-maltoside (DDM) to dodecyl-β-maltooctaoside (DDMO) using α-cyclodextrin (α-CD) or starch as glycosyl donors. At 300 mM α-CD, varied DDM concentration and 60 °C, the reaction obeyed Michaelis-Menten kinetics with a K_m value of 18 mM and a V_max value of 100 U/mg enzyme. However, at 25 mM α-CD the reaction rate decreased with increasing DDM concentration (5-50 mM), and when the α-CD concentration was varied at fixed DDM concentration an S shaped curve was obtained. The deviations from Michaelis-Menten kinetics were interpreted as being caused by formation of inclusion complexes between α-CD and DDM and by micellation of DDM. To achieve a high reaction rate, a high concentration of free α-CD is necessary, since α-CD in the form of a complex has low reactivity. When starch is used as glycosyl donor in the CGTase catalyzed alkyl glycoside elongation reaction, it is thus important to choose reaction conditions under which the cyclization of starch to α-CD is efficient.     

Genetic structure in a fragmented Northern Hemisphere rainforest: large effective sizes and high connectivity among populations of the epiphytic lichen Lobaria pulmonaria

An extraordinary diversity of epiphytic lichens is found in the boreal rainforest of central Norway, the highest-latitude rainforest in the world. These rainforest relicts are located in ravine systems, and clear cutting has increased the distance between remaining patches. We hypothesized that the relatively small lichen populations in the remaining forest stands have suffered a depletion of genetic diversity through bottlenecks and founder events. To test this hypothesis, we assessed genetic diversity and structure in the populations of the tripartite lichen Lobaria pulmonaria using eight SSR loci. We sampled thalli growing on Picea abies branches and propagules deposited in snow at three localities. Contrary to expectations, we found high genetic diversity in lichen and snow samples, and high effective sizes of the studied populations. Also, limited genetic differentiation between populations, high historical migration rates, and a high proportion of first generation immigrants were estimated, implying high connectivity across distances <30km. Almost all genetic variation was attributed to variation within sites; spatial genetic structures within populations were absent or appeared on small scales (5-10m). The high genetic diversity in the remaining old boreal rainforests shows that even relict forest patches might be suitable for conservation of genetic diversity.     

Contrasting Patterns of Genetic Structuring in Natural Populations of Arabidopsis lyrata Subsp. petraea across Different Regions in Northern Europe

Level and partitioning of genetic diversity is expected to vary between contrasting habitats, reflecting differences in strength of ecological and evolutionary processes. Therefore, it is necessary to consider processes acting on different time scales when trying to explain diversity patterns in different parts of species'' distributions. To explore how historical and contemporary factors jointly may influence patterns of genetic diversity and population differentiation, we compared genetic composition in the perennial herb Arabidopsis lyrata ssp. petraea from the northernmost parts of its distribution range on Iceland to that previously documented in Scandinavia. Leaf tissue and soil were sampled from ten Icelandic populations of A. lyrata. Seedlings were grown from soil samples, and tissue from above-ground and seed bank individuals were genotyped with 21 microsatellite markers. Seed bank density in Icelandic populations was low but not significantly different from that observed in Norwegian populations. While within-population genetic diversity was relatively high on Iceland (H _E = 0.35), among-population differentiation was low (F _ST = 0.10) compared to Norwegian and Swedish populations. Population differentiation was positively associated with geographical distance in both Iceland and Scandinavia, but the strength of this relationship varied between regions. Although topography and a larger distribution range may explain the higher differentiation between mountainous Norwegian relative to lowland populations in Sweden, these factors cannot explain the lower differentiation in Icelandic compared to Swedish populations. We propose that low genetic differentiation among Icelandic populations is not caused by differences in connectivity, but is rather due to large historical effective population sizes. Thus, rather than contemporary processes, historical factors such as survival of Icelandic lineages in northern refugia during the last glacial period may have contributed to the observed pattern.     

Disentangling the genetic origin of Heracleum persicum (Apiaceae) in Europe: multiple introductions from multiple source populations

Biological Invasions - Unraveling the origin and colonization history of invasive plants is a long-standing challenge in evolutionary and conservation biology. The knowledge of the origin of the...