GENETIC DIVERGENCE CORRELATES WITH MORPHOLOGICAL AND ECOLOGICAL SUBDIVISION IN THE DEEP-WATER ELK KELP, PELAGOPHYCUS PORRA (PHAEOPHYCEAE)

Abstract

Pelagophycus porra (Leman) Setchell has a narrow distribution confined to deep water from the Channel Islands off the southern California coast to central Baja California, Mexico. Distinct morphotypes are consistently correlated with distinctive habitats, that is, windward exposures characterized by strong water motion and rocky substrates, and sheltered areas with soft substrates found on the lee sides of the islands. We tested the hypothesis that morphologically and ecologically distinct forms reflect genetically distinct stands. Individuals representing populations from three islands and the mainland were compared using RFLP analyses of the nuclear rDNA internal transcribed 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 of trnL intron (241 bp) sequences among nine individuals from seven populations were identical with the exception of a CATAGT insert in two adjacent stands. A RAPD analysis of 24 individuals from nine populations (4 windward and 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% were variable. Neighbor joining analysis produced a well-resolved tree with moderately high bootstrap support in which windward and leeward populations were easily distinguished. The lack of divergence in both the fast evolving nuclear rDNA-ITS and the chloroplast trnL intron does not support the morphotypes as different species. However, the compartmentalized differentiation shown in the RAPD data clearly points to isolation. This, and previous ecological studies that demonstrate habitat specificity suggest that leeward stands probably comprise a species in situ nascendi.