Genetic differences between two growth-forms of Lithophyllum margaritae (Rhodophyta) in Baja California Sur, Mexico

Unattached, nongeniculate, coralline algae or rhodoliths exhibit a range of morphological variability seemingly dependent on environmental factors. Rhodoliths have an extensive fossil record, and environmentally dependent characteristics make them potentially reliable paleoindicators. Species of the rhodolith-forming genus Lithophyllum Philippi in Baja California Sur, Mexico were recently consolidated into one species. Under the new classification, L. margaritae (Hariot) Heydrich consists of several growth forms presumably reflecting local environmental conditions. We examined the genetic structure of four populations of this species using amplified fragment length polymorphisms (AFLP) to characterize the extent of genetic variation associated with foliose and fruticose growth forms. AFLP band sharing analysis revealed that foliose growth forms exhibited consistently higher intrapopulation similarities (0.75-0.85) than fruticose growth forms (similarity range, 0.55-0.67). This trend was also evident in comparisons of geographically isolated populations. These data indicate that the two morphologies are genetically distinct and that genetic exchange between foliose and fruticose growth forms of L. margaritae may be limited. Consequently, rhodolith growth forms appear to be the result of an interplay between both genetic makeup and environmental conditions.