Journal of Applied Phycology 11: 241–246, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 241 Strain selection and genetic variation in Gracilaria chilensis (Gracilariales, Rhodophyta) I. Meneses & B. Santelices * Departamento de Ecolog´ıa, Facultad de Ciencias Biol´ ogicas, Pontifica Universidad Cat´ olica de Chile, Casilla 114-D, Santiago, Chile ( * Author for correspondence) Received 30 October 1998; revised January 1999; accepted 26 January 1999 Key words: DNA polymorphism, Gracilaria chilensis, genetic variability, strain selection Abstract Strain selection processes in seaweed often have assumed that sterile clones could be maintained for long periods in a diversity of environments without major genetic changes. However, clonal species such as Gracilaria chilensis exhibit intra-clonal variation in performance and ongoing studies suggest such changes may be due to rapid changes in DNA composition associated with growth, via mitotic recombinations. Therefore performance of a given ramet in this type of seaweed should be understood as the dynamic outcome of rapid reactions between the environment and the changing genotype of the selected strain. To evaluate this idea, we measured changes in genetic variability, as detected by DNA-fragment polymorphism using RAPDs-PCR, exhibited by clones of G. chilensis after two transfers to different environmental conditions (from field to controlled laboratory conditions and from the laboratory to large-scale tank culture). The transfer to laboratory conditions reduced the frequency of low similarity values and increased the frequency of intermediate similarity values in DNA banding patterns, suggesting the branchlets produced under controlled laboratory conditions have less genetic variability (evaluated as total DNA polymorphism) than plants recently collected in the field. Tank incubation reduced the total range of similarity and significantly increased the frequency of high similarity values. Results thus suggest the dynamic of genetic changes in vegetative clones of Gracilaria chilensis that is fast and strongly affected by the external environment. Introduction Vegetative propagation through thallus fragmentation is an advantageous aspect of commercial algal spe- cies. Simple thallus fragments are likely to grow and spread faster than spores or other types of microscopic propagules and the features selected in one particular strain are maintained by growing self-replicating frag- ments, which are supposedly identical genetic units. However, strain selection processes in seaweed (see Santelices, 1992 for review) often have assumed that: (a) selection processes could be completed under en- vironmental conditions (e.g., laboratory conditions or small tank facilities) not necessarily representative of the farm conditions where the selected strain is to be propagated and (b) that sterile clones could be main- tained for long periods of time without major changes in their genetic make up. Current conceptual and empirical knowledge of strain selection and genetic variation of clones sug- gests that these two assumptions may not be valid. Since a particular gene combination is selected under a given set of environmental conditions, the optimal performance of the selected strain is intimately re- lated to the specific culture conditions under which selection was performed. Strain selection ultimately means the fixation of particular gene combinations to be later propagated intensively and extensively in a given mariculture setting. Given the close relationship between strain performance and culture conditions