Genetic divergence correlates with morphological and ecological subdivision in the deep-water elk kelp, Pelagophycus porra (phaeophyceae)

Pelagophycus porra (Leman) Setchell has a narrow distribution confined to d eep water from the Channel Islands off the southern California coast to cen tral Baja California, Mexico. Distinct morphotypes are consistently correla ted with distinctive habitats, that is, windward exposures characterized by strong water motion and rocky substrates, and sheltered areas with soft su bstrates found on the lee sides of the islands. We tested the hypothesis th at morphologically and ecologically distinct forms reflect genetically dist inct stands. Individuals representing populations from three islands and th e mainland were compared using RFLP analyses of the nuclear rDNA internal t ranscribed spacers (ITS1 and ITS2), chloroplast trnL (UAA) intron sequences , and random amplified polymorphic DNA (RAPDs). No variation was found in a survey of 20 restriction sites of ITS1 (ca. 320 base pair [bp]) and ITS2 ( ca. 360 bp) among individuals from six populations. Likewise, comparisons o f trnL intron (241 bp) sequences among nine individuals from seven populati ons were identical with the exception of a CATAGT insert in two adjacent st ands. A RAPD analysis of 24 individuals from nine populations (4 windward a nd 5 leeward) using 16 primers generated 166 bands. Thirty-eight percent of the bands did not vary, 16% were unique to a given individual, and 46% wer e variable. Neighbor joining analysis produced a well-resolved tree with mo derately high bootstrap support in which windward and leeward populations w ere easily distinguished. The lack of divergence in both the fast evolving nuclear rDNA-ITS and the chloroplast trnL intron does not support the morph otypes as different species. However, the compartmentalized differentiation shown in the RAPD data clearly points to isolation. This, and previous eco logical studies that demonstrate habitat specificity suggest that leeward s tands probably comprise a species in situ nascendi.