In order to better understand the alternation of generations that characterizes haploid–diploid life cycles, we assessed the existence of ecological differences between the two phases (haploid gametophyte and diploid tetraspor- ophyte) in Gracilaria chilensis, a rhodophyte with a typical Polysiphonia-type life cycle. We investigated the effect of light intensity and salinity on viability and growth of both phases at different ontogenetic stages: juveniles and adults. In our study, the survival of juvenile gametophytes (n) was higher than the survival of juvenile tetrasporophytes (2n) despite culture conditions; however, low salinity had greater effect on carpospores (2n) than on tetraspores (n). On the other hand, a complex interaction between salinity and light intensity within each life history phase generated observed differences between juvenile growth rates. Low light was shown to trigger early onset of alteration of the holdfast growing pattern. In addition, adult tetrasporophytes showed, despite the conditions, a faster vegetative growth than fe- male and male gametophytes. These differences between phases could have led to the complete dominance of tetra- sporophyte fragments of fronds observed in G. chilensis farms. We hypothesize that Chilean fishers could have un- knowingly selected for tetrasporophyte thalli during domes- tication of the species, thus enhancing the natural trend of tetrasporophytes dominance already present in estuarine natural populations of free-floating plants.
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