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THE EVOLUTION OF PARASITISM IN THE RED ALGAE: MOLECULAR COMPARISONS OF ADELPHOPARASITES AND THEIR HOSTS 1

In several groups of parasites including insect, flowering plant, fungal, and red algal parasites, morphological similarities of the parasites and their specific hosts have led to hypotheses that these parasites evolved from their hosts. But these conclusions have been criticized because the morphological features shared by parasite and host may be the result of convergent evolution. In this study, we examine the hypothesis, originally put forth by Setchell, that adelphoparasitic red algae, that is, parasitic red algae that are morphologically very similar to their hosts, evolved from their specific red algal hosts. Rather than comparing morphological features of parasites and hosts, small‐subunit 18S nuclear ribosomal DNA and the internal transcribed spacer regions (ITSs) of the nuclear ribosomal repeat are compared for five parasites, their hosts, and related nonhosts from four red algal orders. These comparisons reveal that each of these adelphoparasites has evolved either directly from the host on which it is currently found, or it evolved from some other taxon that is closely related to the modern host. The parasites Gardneriella tuberifera, Rhodymeniocolax botryoides, and probably Gracilariophila oryzoides evolved from their respective hosts Sarcodiotheca gaudichaudii, Rhodymenia pacifica, and Gracilariopsis lemaneiformis, respectively. The parasite Faucheocolax attenuata evolved from either Fauchea laciniata or Fauchea fryeana and subsequently radiated onto the other host species. Presently this parasite is found on both hosts. Lastly, some parasitic genera such as Plocamiocolax are polyphyletic in their origins. A species of Plocamiocolax from an Antarctic Plocamium cartilagineum appears to have evolved from its host whereas the common Plocamiocolax pulvinata that occurs along the west coast of North America likely evolved from Plocamium violaceum and radiated secondarily onto its present day host, Plocamium cartilagineum.