Ostracoda Phylogeny Based On Phylotranscriptomics And Mitochondrial Genome

Ostracods(Crustacea,Ostracoda)are widely distributed in various water bodies from deep sea to vernal pools all around the world as one of the most ancient Arthropods and process the most abundant fossil records and many living fossils.And more and more phylogenomic studies illustrate that Ostracoda,Mystacocarida,Branchiura and Pentastomida constitute a clade namely Oligostraca as a sister group to the rest of Pancrustaceans which makes Ostracoda an extraordinary group in Arthropods' or Pancrustaceans' phylogenetic and evolutionary research.While as the most diverse group in Oligostraca,Ostracods' monophyly,relationship with the other Oligostracans and phylogeny inside themselves are still controversial.Here we newly sequenced 36 Ostracods transcriptomes from subclass Podocopa and Myodocopa,integrating all the published Oligostracan transcriptomes,r DNA and morphological characters,and built the most inclusive matrices so far for Ostracods phylogeny.With the combination of Ostracods mitochondrial genome and fossil records,a comprehensive Ostracods phylogenetic analysis was conducted.The main results and conclusions of the current study are listed as follows:1.Oligostraca with Ostracods are sister to the rest of PancrustaceansPancrustacea phylogenetic studies with a special focus on Ostracods and their relatives strongly support Oligostraca consisting of Ostracoda,Mystacocarida,Branchiura and Pentastomida to be a sister group to the rest of Pancrustacea.Pancrustacea comprised Oligostraca,Multicrustacea(Malacostraca,Thecostraca and Copepoda)and Allotriocarida(Cephalocarida,Branchiopoda,Hexapoda and Remipedia).2.Ostracoda are not monophyletic with Podocopa being close to Mystacocarida who may lost their carapaces during evolutionAs we got Ostracoda monophyly in the phylogenetic analysis with main Pancrustacea clades,matrices for Oligostraca with only Chelicerates and Myriapods as outgroups were analyzed.We got consistent support for Ostracoda polyphyly with Podocopa and Myodocopa not grouping together.Mystacocarida always form sister group with Podocopa,which is supported by molecular phylogeny,morphological characters and Ontogeny.Mystacocarida may derive from a bivalved ancestor and lost their carapaces during evolution.3.New support for phylogenetic relations within Podocopa and MyodocopaWithin Podocopa,as Darwinulocopina are sister to Platycopida,here we suggest to promote Darwinulocopina to Darwinulocopida as the fourth Order of Podocopa.Bairdiocopina are closer to Cytherocopina than to Cypridocopina.Taxonomical Candonidae are not monophyly,here we accept the proposal to promote subfamilies Candoininae,Cyclocypridinae and Paracypridinae to family levels.Within Myodocopa,Halocyprida consisting of Cladocopina and Halocypridina are sister to Myodocopida.Within Mydocopida,Cypridinoidea are sister to Cylindroleberidoidea and Philomedidae are not monophyly.4.The invasive history of Ostracods from marine to fresh water habitat was rebuilt based on molecular phylogeny and fossil recordsAll myodocopans inhabit in the ocean and have never entered non-marine habitat.Podocopans entered fresh water environment independently at least 9 times,which happened 1 time in(Darwinulocopida + Platycopida)clade,1 time in Cypridocopina clade and 7 times in Cytherocopina clade.There are at least twice secondary returning to marine habitat within Cypridocopina namely within Pontocypridoidea and Paracypridinae.5.Both Mitochondrial genome phylogeny and Mitochondrial genome structure do not support Ostracoda monophylyAs the rapidly evolutionary rate,mitochondrial genome is not suitable for high levels phylogeny(among classes or subphylum).The comparison of mitochondrial genomes of Oligostracans and mitochondrial genome phylogeny imply that Ostracoda are not monophyly.The duplicated control region evolved convergently in Myodocopa and Branchiura.Mitochondrial genome gene assembly is highly conserved in Arthropods with the random shift may happen at family level or below.