RESUMEN
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.
RESUMEN
In terrestrial plants, it is well known that genetic diversity can affect responses to abiotic and biotic stress and have important consequences on farming. However, very little is known about the interactive effects of genetic and environmental factors on seaweed crops. We conducted a field experiment on Gracilaria chilensis to determine the effect of heterozygosity and nutrient addition on two southern Chilean farms: Ancud and Chaica. In addition to growth rate and productivity, we measured photosynthetic responses, photosynthetic pigment concentration (chlorophyll a and phycobiliproteins), C:N ratio (C:N), and epiphytic load. Nutrient addition affected the growth rate, productivity, phycobilin, and C:N content, but not the epiphytic load. These results were independent of the heterozygosity of the strains used in the experiments. Interestingly, depending on the sampled sites, distinct photosynthetic responses (i.e., maximal quantum yield, Fv /Fm , and maximal electron transport rate, ETRmax ) to nutrient addition were observed. We propose that thallus selection over the past few decades may have led to ecological differentiation between G. chilensis from Chaica and Ancud. The lack of effect of heterozygosity on growth and physiological responses could be related to the species domestication history in which there is a limited range of genetic variation in farms. We suggest that the existing levels of heterozygosity among our thalli is not sufficient to detect any significant effect of genetic diversity on growth or productivity in Metri bay, our experimental site located close to the city of Puerto Montt, during summer under nitrogen limiting conditions.