A phylogenomic perspective on annelid evolution with emphasis on the evolution of bloodfeeding in leeches (Clitellata, Hirudinida)
Annelida Lamarck, 1809 embodies over 17,000 species of segmented worms such as leeches, earthworms, lugworms, sandworms and clamworms. The phylum has traditionally been divided into two main orders: Sedentaria Lamarck, 1818 and Errantia Andouin & Milne Edwards, 1834, yet Sedentaria is seldom recovered as monophyletic and Errantia has only been recovered as monophyletic in one study. Recently, a large amino acid data set of expressed sequence tags (EST’s) was created for Annelida and its close allies. The phylogenetic analyses of this data set, based on Bayesian inference and Maximum Likelihood estimations, recovered both Errantia and Sedentaria as monophyletic groups. Herein, I assess whether or not this hypothesis is also recovered by a nucleotide representation of the amino acids, and if the result is general across optimality criteria. Whereas parsimony analyses of the largest molecular character data set compiled for Annelida fails to recover Sedentaria or Errantia as monophyletic entities, re-analysis of the original amino acid data set does recover Errantia as monophyletic but with low support. In conjunction with previous studies, the analyses presented here suggest that the phylogenetic hypotheses of relationships both within Annelida, and between the phylum and its constituent taxa are still unstable and that finding suitable data for resolving this is an important yet problematic issue. Although medicinal leeches have long been used as treatment for various ailments because of their potent anticoagulation factors, the debates regarding the evolution of bloodfeeding and the ancestral feeding preference of leeches are still contentious. Moreover, neither the full diversity of salivary components that inhibit coagulation, much less the evolutionary selection acting on them, has been thoroughly investigated in a comparative manner. To address these questions, the full genome of the non-bloodfeeding leech Helobdella robusta was screened for anticoagulation factors. In addition, EST libraries from salivary glands of two species of medicinal leeches, Hirudo verbana and Aliolimnatis fenestrata, were constructed. For H. robusta, a total of eight loci matching leech antiplatelet proteins and positioned as a tandem array, were recovered with significantly low e-values, suggesting that this leech possesses ancestrally-inherited anticoagulants. In the medicinal leeches, expression of salivary peptides greatly exceed expectations and also suggest the feasibility of identifying the important active sites of the proteins through selective pressure analyses. Although symbiotic associations between bacteria and leeches are well documented, several questions concerning the function of the bacterial symbionts and their phylogenetic positioning still remain. To address this and other issues, herein I characterize and annotate a large subset of the genome of Reichenowia parasitica, an phaproteo-bacterial endosymbiont of the freshwater leech Placobdella parasitica. Results suggest that R. parasitica possesses genes coding for proteins related to nitrogen fixation, iron/vitamin B translocation and plasmid survival, and that the bacterium interacts with its host in part by transmembrane signaling. The phylogenetic analyses support the nesting of R. parasitica within the plant-inhabiting Rhizobiaceae, as sister to a group containing Agrobacterium and Rhizobium species.
xiii, 180 p. : ill. (some col.)
Annelida., Leeches., Phylogeny., Evolution., Blood feeding.