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ABSTRACT Nucleotide sequences from the 18S subunit of ribosomal RNA were determined for 9 species of decapod crustaceans: 1 from the suborder Dendrobranchiata (Penaeus aztecus) and 8 from the suborder Pleocyemata, representing 4 different infraorders (Procaris ascensionis and Palaemoneteskadiakensis, Caridea; Stenopus hispidus, Stenopodidea; Procambarus leonensis, P. paeninsulanus, and P. youngi, Astacidea; Callinectes sapidus, Brachyura). The total aligned set of nucleotides consisted of sequences ranging in length from 869-1721 bases. Comparison of sequences among species revealed that (1) the nucleotide sequences of the 3 species of Procambarus are virtually identical, differing in only 3 of more than 1,500 nucleotides; (2) variation is not evenly distributed across the molecule but follows a repeated pattern of conserved regionvariable region-highly variable region; (3) the transversion: transition ratio varies from 0.67― 1.52, with a mean of 0.987 ± 0.042 across all species; (4) Penaeus aztecus differs from the other species in the sequence of a highly conserved region; (5) there is sufficient variation for phylogenetic analysis; and (6) the variation is phylogenetically informative to infraorder or possibly superfamily level. Phylogenetic relationships were inferred by parsimony analysis, and confidence intervals were estimated by bootstrapping. With the brine shrimp Artemia salina as an outgroup, the following results were obtained: Penaeus aztecus comes off the tree first, followed by a node leading to the remaining decapods; next is a branch leading to a node uniting Procaris ascensionis and Palaemonetes kadiakensis; the next branch to come off the tree leads to Stenopus hispidus, which is followed by a node uniting Procambarus leonensis and Callinectes sapidus. Bootstrap analyses suggest that both the node uniting P. kadiakensis and P. ascensionis and that uniting P. leonensis and C. sapidus are valid. Similar results were obtained with only decapod species using P. aztecus as the outgroup. An invariants/operator-metrics analysis supports a P. kadiakensis/P. ascensionis and a S. hispidus/P. leonensis clade at P < 0.006; adding C. sapidus supports the placement of S. hispidus with reptant taxa.


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