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A Phylogenetic Supertree of the Hammerhead Sharks

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A Phylogenetic Supertree of the Hammerhead Sharks Zoological Studies 46(1): 6-11 (2007) A Phylogenetic Supertree of the Hammerhead Sharks (Carcharhiniformes: Sphyrnidae) Mauro José Cavalcanti Departamento de Vertebrados, Museu Nacional/UFRJ, Quinta da Boa Vista, São Cristóvão, 20940-040, Rio de Janeiro, RJ, Brazil (Accepted December 29, 2005) Mauro José Cavalcanti (2007) A phylogenetic supertree of the hammerhead sharks (Carcharhiniformes, Sphyrnidae). Zoological Studies 46(1): 6-11. In this study, 5 hammerhead shark phylogenies (based on mor- phological, isozyme, and mtDNA sequence data) were combined to generate a complete, well-resolved com- posite phylogeny for the Sphyrnidae using matrix representation with parsimony. The resulting supertree con- tains all 8 known sphyrnid species and represents the best estimate of the combined relationships presented in the original 5 source trees. This supertree will provide a useful framework for further phylogenetic comparative studies of sphyrnid evolution, ecology, and biogeography. http://zoolstud.sinica.edu.tw/Journals/46.1/6.pdf Key words: Composite trees, Matrix representation with parsimony, Phylogeny, Sphyrna, Eusphyra. Hammerhead sharks (family Sphyrnidae) surface of the head (lateral line and ampullae of are recognized as a highly derived, monophyletic Lorenzini). Only a few of these hypotheses have group in the order Carcharhiniformes, character- been empirically tested, and none of them has ized by the presence of a dorsoventrally com- been placed within a phylogenetic framework. pressed and laterally expanded pre-branchial Clearly, what is needed to understand the evolu- head, known as the cephalofoil (Gilbert 1967, tion of the cephalofoil within the Sphyrnidae, and Compagno 1988). The morphology of the the diversification of the clade itself, is a compara- cephalofoil differs greatly among species, from tive approach (Harvey and Pagel 1991). However, evenly rounded in the bonnethead shark (Sphyrna the lack of a complete and well-resolved phyloge- tiburo) to very wide and narrow in the winghead ny for the sphyrnids, based on all the available evi- shark (Eusphyra blochii). dence afforded by morphological and molecular The evolution of the peculiar head shape of datasets, is a major obstacle to explaining what hammerhead sharks has been the subject of much factors might have driven the evolution of the debate, and several hypotheses have been cephalofoil. advanced to explain the adaptive significance of Morphological and molecular data have yield- this unusual head morphology. It has been sug- ed very different phylogenetic trees for hammer- gested that the cephalofoil evolved from the slight- head sharks. The most complete phylogenetic ly flattened head typical of many carcharhinid hypothesis based on morphological characters sharks (Compagno 1988), which acts as a bow- and anatomical structures (Compagno 1988) indi- plane that provides hydrodynamic lift and increas- cates that evolution in this family has been charac- es maneuverability (Nakaya 1995, Kajiura et al. terized by a change from a plesiomorphic condi- 2003). Other hypotheses for the evolution of the tion of small size, inshore dwelling, and bottom cephalofoil (Kajiura 2001) invoke potential advan- feeding, to a more-derived condition of large, tages of spacing sensory structures at the lateral pelagic piscivores. On the other hand, according ends of the head (eyes and nostrils) or across the to a hypothesis based on mtDNA sequence data *To whom correspondence and reprint requests should be addressed. E-mail:[email protected] 6 Cavalcanti -- Sphyrnid Supertree 7 (Martin 1993), the large pelagic species are more of the previous studies were concerned with mam- , primitive than the small demersal species, and malian supertrees and, to the author s knowledge, there is a trend towards the evolution of the only 1 fish supertree has appeared before in the lit- cephalofoil along the evolution of the clade. This erature (Mank et al. 2005). sequence of divergence is precisely the opposite In this paper, all available published phyloge- from that suggested by the hypothesis based on nies of the Sphyrnidae were assembled to gener- morphological data. ate a composite phylogeny for hammerhead Comparisons among phylogenies obtained by sharks using the technique of matrix representa- different techniques or based on different datasets tion with parsimony (Baum 1992, Ragan 1992). can be performed by constructing consensus trees The resulting supertree will provide a useful frame- (Rohlf 1982). Consensus trees, introduced by work for further phylogenetic comparative studies Adams (1972), allow the combination of informa- of sphyrnid evolution, ecology, and biogeography. tion from 2 (or more) phylogenetic trees into a sin- gle tree that represents the information in the set of trees being compared, but this procedure is only MATERIALS AND METHODS possible if the species sets present in the different phylogenies are the same. Source trees Another possibility, developed more recently, is to construct a composite phylogeny or All phylogenetic source trees available for the “supertree”from source trees derived from sepa- Sphyrnidae (Fig. 1) were obtained from the litera- rate morphological and molecular datasets, based ture (Gilbert 1967, Compagno 1988, Lavery 1992, on the principle that the phylogenies can be con- Naylor 1992, Martin 1993), following the protocol verted in data matrices which, after combination, proposed by Bininda-Emonds et al. (2004). The can be subjected to parsimony analysis phylogenetic hypothesis of Gilbert (1967) was (Sanderson et al. 1998, Bininda-Emonds et al. included as a “seed” tree, even though it was 2002). This procedure has the advantage of not not built using a formal cladistic analysis, as rec- requiring that all species be shared among the ommended by Purvis (1995) and Bininda-Emonds several studies being compared. As the method and Sanderson (2001) to obtain complete taxo- combines existing phylogenetic information in the nomic coverage and ensure sufficient overlap form of trees, supertrees potentially resolve many among the source trees, and therefore to improve of the problems associated with other character- the resolution of the composite phylogeny. When based methods (e.g., the absence of homologous a given bibliographic source presented several characters and incompatible data types). phylogenetic trees for the same dataset, obtained Supertree methods are capable of synthesizing the with different methods, a strict consensus tree results of separate studies into more-comprehen- (Rohlf 1982) was initially computed among the sive, larger-scale phylogenies that are useful for trees, for inclusion in the procedure of supertree comparative and macroevolutionary studies and construction. The trees compiled from the litera- which represent major steps towards building the ture were stored in the NEXUS format (Maddison Tree of Life (Soltis and Soltis 2001, Bininda- et al. 1997) using the program, TreeView, vers. Emonds et al. 2002, Page 2004). In addition to 1.6.6 (Page 1996) for further analysis. helping synthesize hypotheses of phylogenetic relationships among larger sets of taxa, supertrees Data analysis can suggest optimal strategies for taxon sampling (either for future supertree construction or for To construct the composite phylogeny, each experimental design issues such as the choice of source tree was recoded as a binary matrix using outgroups for cladistic analyses) and can reveal the Supertree program, vers. 0.85b (Salamin et al. emerging patterns in the large knowledge base of 2002), after the procedure proposed by Baum phylogenies currently in the literature (Sanderson (1992) and Ragan (1992). The Baum/Ragan et al. 1993). method for constructing supertrees can be used Many supertrees have already been pub- whether the source trees are compatible or not. lished for various groups of organisms, sometimes The minimum requirement for including a source with associated comparative or macroevolutionary tree in supertree construction is that it shares 2 or analysis (see Bininda-Emonds et al. 2002 for more taxa with at least 1 other source tree. These references on individual studies). However, many matrices are then combined and analyzed with 8 Zoological Studies 46(1): 6-11 (2007) parsimony to generate the composite phylogeny or al. 2005). “supertree” (see Sanderson et al. 1998, Bininda- Four separate analyses were performed, Emonds et al. 2002, Bininda-Emonds 2004, and using different weighting schemes of the matrix Page 2004 for details on supertree construction elements and allowing or prohibiting reversals using matrix representation with parsimony). The (Bininda-Emonds and Bryant 1998, Bininda- resulting MRP dataset had 8 taxa and 23“charac- Emonds and Sanderson 2001). The matrix ele- ters”. As all source trees were rooted, an all-zero ments were weighted in proportion to their clade hypothetical outgroup (the“MRP outgroup”) was support on each source tree (Bininda-Emonds and added to the data matrix to polarize the subse- Bryant 1998, Bininda-Emonds and Sanderson quent parsimony analysis, preserving the rooting 2001). information of the source trees (Bininda-Emonds et The degree of support of the composite phy- Eusphyra blochii Sphyrna tiburo (a) (b) Sphyrna mokarran Sphyrna corona Sphyrna zygaena Sphyrna media Sphyrna lewini Sphyrna tudes Sphyrna tiburo Sphyrna mokarran Sphyrna media Sphyrna zygaena Sphyrna corona Sphyrna lewini Sphyrna tudes Eusphyra blochii Eusphyra blochii Sphyrna tiburo (c) (d) Sphyrna tudes Sphyrna
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