The genetic basis of morphological differentiation in some Laminaria populations

Clinal differentiation of stipe traits in Laminaria (simplices) populations occurs on a gradient of exposure to wave action. Crossability tests and the production of fertile hybrids established interfertility between populations from the extremes of the morphological range. Reciprocal transplantation experiments between exposed and sheltered sites did not produce significant changes in stipe morphology. Quantitative genetic analysis of the degree of resemblance between relatives indicated a relatively high genetic component in the interpopulation differentiation. This is consistent with the results of the transplantation experiments. Within-population heritability estimates at the exposed site were low, perhaps because of intensive local stabilising selection which acts to reduce genetic variability. The question of conspecificity of L. saccharina and L. longicruris in view of these results is discussed.