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The objective of this study was to test, using a field experiment, the effect of genotypic diversity on productivity of farmed populations (Ancud and Chaica, Chile) of the domesticated red alga Agarophyton chilense (formerly known as Gracilaria chilensis), a species considered as economically important in Chile. Monoclonal and polyclonal (4 and 8 genotypes) subplots were outplanted into the mid intertidal in Metri Bay (Puerto Montt, Chile) during summer, a season in which A. chilense face higher temperatures (>18°C) and low nitrogen availability (<4.00 µmol). Ancud farm genotypes show higher growth rates in the monoclonal rather than the two polyclonal subplots. A similar tendency, yet not significant, was discernible in Chaica. In addition, whatever the population of origin of the thalli, no effect of genotypic diversity was detected neither on the agar yield and its quality, nor on the epiphyte load. Such unexpected results of a higher performance in plots with a lower genotypic diversity could be explained (a) by human-assisted selection for dominant-best-performing genotypes that could counterbalance the negative effect caused by the low genotypic diversity in farms and (b) by the fact that the organisms inhabiting the algal mats do not impact the fitness of their host. Overall, the results obtained here suggest that despite farm induced selection lead to impoverished pools of genotypes, they may also have a positive effect of on the resistance of farmed populations to seasonal stressors. However, whether this may have a secondary negative effect on the longer term in a fluctuating environment remains to be determined, but may be avoided by adopting strategy of selection favoring different genotypes in space and time, as implemented in forestry.

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Range and niche expansion are commonly associated with transitions to asexuality, polyploidy and hybridity (allopolyploidy) in plants. The ability of asexual polyploids to colonize novel habitats may be due to widespread generalist clones, multiple ecologically specialized clones, or may be a neutral by-product of multiple, independent origins of asexual polyploids throughout the range. We have quantified niche size and divergence for hawthorns of the Pacific Northwest using data from herbarium vouchers with known cytotypes. We find that all polyploid niches diverge from that of the diploid range, and allopolyploids have the broadest niches. Allotetraploids have the largest niche and the widest geographic distribution. We then assessed the genetic mechanism of range expansion by surveying the ecological and geographic distribution of genotypes within each cytotype from sites in which fine-scale habitat assessments were completed. We find no isolation by either geographic or ecological distance in allopolyploids, suggesting high dispersal and colonization ability. In contrast, autotriploids and diploids show patterns of isolation by geographic distance. We also compared the geographic and ecological distributions of clonal genotypes with those of randomly drawn sites of the most widespread cytotype. We found that most clones are geographically widespread and occur in a variety of habitats. We interpret these findings to suggest that patterns of range and niche expansion in Pacific Northwest Hawthorns may stem from these widespread, ecologically generalist clones of hybrid origin.

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PREMISE OF STUDY: The presence of genetic variation for traits that contribute to ecological range expansion can provide the potential for introduced taxa to evolve greater invasiveness. Genotypes that contribute to the spread of introduced range populations must have the ability to maintain fitness under changing environmental stress and competitive intensity. Previously, we identified a subset of genotypes in populations of the invasive annual Polygonum cespitosum that express consistently high reproductive fitness in diverse (shaded, dry, and resource-rich) conditions. Here, we investigated whether these broadly adaptive (High-Performance) genotypes also show a competitive advantage over conspecifics in full sun and/or shade. METHODS: We grew a population-balanced sample of 13 High-Performance and 13 'Control' genotypes in intraspecific competitive arrays, comprising all four possible combinations of High-Performance vs. Control target plants and competitive backgrounds, in both full sun and shaded glasshouse environments. KEY RESULTS: In full sun, High-Performance genotypes (1) better maintained growth and reproductive output despite competition and (2) more strongly suppressed growth and reproduction of target plants. However, genotypes did not differ significantly in shade. CONCLUSIONS: Competitive superiority in open conditions may contribute to increasing predominance of these broadly adapted genotypes in introduced-range Polygonum cespitosum populations, and hence to the evolution of greater invasiveness. This study provides insight into the role of genotypic variation for ecological traits in the range expansion of a contemporary plant invader. It also highlights how such variation can be differently expressed in alternative environments (gene by environment interaction).

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Invasion genetics is a relatively new discipline that investigates patterns of genetic variation in populations of invasive species and their ecological and evolutionary consequences. Evolutionary biologists have a long-standing interest in colonizing species, owing to their short life cycles and widespread distributions, but not until publication of The Genetics of Colonizing Species (1965), edited by H.G. Baker and G.L. Stebbins, was a synthesis on the genetics and evolution of colonizers available. Here, I make the case that the Baker and Stebbins volume is the foundational document for invasion genetics, and in conjunction with the increased use of genetic markers and development of invasion biology, resulted in the birth of this new field over the past two decades. I consider the historical origins and legacy of the Baker and Stebbins volume and review some of the key issues that were addressed. I provide biographical sketches of the two editors, emphasizing their contrasting backgrounds and personalities. I review examples from my own work on plant invasions that are relevant to issues discussed by contributors to the volume. These include the following: determinants of invasion success, life history trade-offs, generalist vs. specialist strategies, general-purpose genotypes, adaptive phenotypic plasticity, mating systems and the influence of bottlenecks on genetic variation. I conclude by posing several key questions in invasion genetics and argue that one of the main challenges that the area faces is to integrate experimental field studies of the ecology and demography of populations with the largely descriptive approaches that have tended to dominate most research to date.

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The nematode Howardula aoronymphium parasitizes several species of mushroom-feeding Drosophila. A survey of isofemale strains of H. aoronymphium and a 25-generation selection experiment revealed that this species does not comprise host races, and that it harbors little heritable variation for adaptation to specific hosts No tradeoffs in performance on the different host species were evident. General-purpose genotypes, which can utilize all host species, characterize H. aoronymphium. An important feature of the natural history of these nematodes-correlated epidemiology across host species-is postulated to be both a cause and a consequence of the evolution of general-purpose genotypes in this species.