Cladistic analysis of Medusozoa and cnidarian evolution

A cladistic analysis of 87 morphological and life history characters of medusozoan cnidarians, rooted with Anthozoa, results in the phylogenetic hypothesis (Anthozoa (Hydrozoa (Scyphozoa (Staurozoa, Cubozoa)))). Staurozoa is a new class of Cnidaria consisting of Stauromedusae and the fossil group Conulatae. Scyphozoa is redefined as including those medusozoans characterized by strobilation and ephyrae (Coronatae, Semaeostomeae, and Rhizostomeae). Within Hydrozoa, Limnomedusae is identified as either the earliest diverging hydrozoan lineage or as the basal group of either Trachylina (Actinulida (Trachymedusae (Narcomedusae, Laingiomedusae))) or Hydroidolina (Leptothecata (Siphonophorae, Anthoathecata)). Cladistic results are highly congruent with recently published phylogenetic analyses based on 18S molecular characters. We propose a phylogenetic classification of Medusozoa that is consistent with phylogenetic hypotheses based on our cladistic results, as well as those derived from 18S analyses. Optimization of the characters presented in this analysis are used to discuss evolutionary scenarios. The ancestral cnidarian probably had a sessile biradial polyp as an adult form. The medusa is inferred to be a synapomorphy of Medusozoa. However, the ancestral process (metamorphosis of the apical region of the polyp or lateral budding involving an entocodon) could not be inferred unequivocally. Similarly, character states for sense organs and nervous systems could not be inferred for the ancestral medusoid of Medusozoa. Additional key words: Staurozoa, Scyphozoa, Hydrozoa, Cubozoa Cnidarian species are often used as model organisms in evolutionary studies (of development, cell biology, gene families, etc.) because Cnidaria diverged relatively early in the history of Metazoa. The presumption in these studies is that cnidarian species may exhibit relatively underived character states that can be compared with those of other animals, particularly bilaterians. As an example, a recent study made inferences about the ancestral paired domain (a conserved DNA-binding domain present in developmental control genes) in animals based on the condition of Pax genes in a hydrozoan cnidarian (Sun et al. 2001). Though character states in cnidarians are possibly ancient, features under study in a given cnidarian species may certainly be derived at some level within Cnidaria and to assume the opposite could be misleading. Thus, the utility of comparative studies is enhanced as more taxa are investigated (Lowe et al. 2002) and as knowledge of phylogeny increases. In the case of Cnidaria, neither its position within Metazoa nor the relationa Author for correspondence. E-mail: [email protected] ships among its component groups are known with precision (Marques 1996; Nielsen et al. 1996; Jenner & Schram 1999; Nielsen 2001; Collins 2002). Prior studies focusing on relationships among cnidarian classes contain many contradictions (SalviniPlawen 1978, 1987; Petersen 1990; Bridge et al. 1992; Bridge et al. 1995; Schuchert 1993; Odorico & Miller 1997; Marques 2001; Nielsen 2001; Collins 2002). However, in the last decade Anthozoa has emerged as the most likely sister group of the remaining cnidarians, first dubbed Tesserazoa (Salvini-Plawen 1978), but more commonly known as Medusozoa (Petersen 1979). Monophyly of the medusozoans is particularly well supported by their shared possession of linear mtDNA, in contrast to anthozoans and other metazoans (Bridge et al. 1992). To move toward a consensus view of cnidarian phylogeny that will enlighten further comparative studies using cnidarians, we present a cladistic analysis of 87 morphological and life history characters to generate a hypothesis of relationships among medusozoan cnidarians. We compare the cladistic results with those of recent phylogenetic analyses of Medusozoa using molecular characters (Collins 24 Marques & Collins 2002), and present a classification that is consistent with phylogenetic hypotheses derived from both analyses. Finally, we make inferences about the evolution of morphological and life history traits within Cnidaria based on our cladistic analysis.