ABSTRACT
Convergent evolution of echinoderm pluteus larva was examined from the standpoint of functional evolution of a transcription factor Ets1/2. In sea urchins, Ets1/2 plays a central role in the differentiation of larval skeletogenic mesenchyme cells. In addition, Ets1/2 is suggested to be involved in adult skeletogenesis. Conversely, in starfish, although no skeletogenic cells differentiate during larval development, Ets1/2 is also expressed in the larval mesoderm. Here, we confirmed that the starfish Ets1/2 is indispensable for the differentiation of the larval mesoderm. This result led us to assume that, in the common ancestors of echinoderms, Ets1/2 activates the transcription of distinct gene sets, one for the differentiation of the larval mesoderm and the other for the development of the adult skeleton. Thus, the acquisition of the larval skeleton involved target switching of Ets1/2. Specifically, in the sea urchin lineage, Ets1/2 activated a downstream target gene set for skeletogenesis during larval development in addition to a mesoderm target set. We examined whether this heterochronic activation of the skeletogenic target set was achieved by the molecular evolution of the Ets1/2 transcription factor itself. We tested whether starfish Ets1/2 induced skeletogenesis when injected into sea urchin eggs. We found that, in addition to ectopic induction of mesenchyme cells, starfish Ets1/2 can activate some parts of the skeletogenic pathway in these mesenchyme cells. Thus, we suggest that the nature of the transcription factor Ets1/2 did not change, but rather that some unidentified co-factor(s) for Ets1/2 may distinguish between targets for the larval mesoderm and for skeletogenesis. Identification of the co-factor(s) will be key to understanding the molecular evolution underlying the evolution of the pluteus larvae.
Subject(s)
Echinodermata/genetics , Evolution, Molecular , Proto-Oncogene Protein c-ets-1/genetics , Proto-Oncogene Protein c-ets-2/genetics , Animals , Cell Differentiation/genetics , Echinodermata/embryology , Echinodermata/growth & development , Embryo, Nonmammalian/embryology , Embryo, Nonmammalian/metabolism , Female , Gene Expression Regulation, Developmental , Gene Knockdown Techniques , Gene Transfer Techniques , In Situ Hybridization , Larva/genetics , Larva/growth & development , Male , Mesoderm/cytology , Mesoderm/metabolism , Phylogeny , Proto-Oncogene Protein c-ets-1/classification , Proto-Oncogene Protein c-ets-1/physiology , Proto-Oncogene Protein c-ets-2/classification , Proto-Oncogene Protein c-ets-2/physiology , Reverse Transcriptase Polymerase Chain Reaction , Sea Urchins/embryology , Sea Urchins/genetics , Sea Urchins/growth & development , Starfish/embryology , Starfish/genetics , Starfish/growth & developmentABSTRACT
One of the most important issues in asteroid phylogeny is the phylogenetic status of Paxillosida. This group lacks an anus and suckers on the tube feet in adults and does not develop the brachiolaria stage in early development. Two controversial hypotheses have been proposed for the phylogenetic status of Paxillosida, i.e., Paxillosida is primitive or rather specialized in asteroids. In this study, we determined the complete mitochondrial DNA nucleotide sequences from two paxillosidans (Astropecten polyacanthus and Luidia quinaria) and one forcipulatidan (Asterias amurensis). The mitochondrial genomes of the three asteroids were identical with respect to gene order and transcription direction, and were identical to the previously reported mitochondrial genomes of Asterina pectinifera (Valvatida) and Pisaster ochraceus (Forcipulatida) in this respect. Therefore, the comparison of genome structures was uninformative for the purposes of asteroid phylogeny. However, molecular phylogenetic analyses based on the amino acid sequences and the nucleotide sequences from the five asteroids supported the monophyly of the clade that included the two paxillosidans and Asterina. This suggests that the paxillosidan characters are secondarily derived ones.
Subject(s)
DNA, Mitochondrial/genetics , Echinodermata/classification , Echinodermata/genetics , Phylogeny , Animals , Base Composition , Base Sequence , Codon, Initiator/genetics , Codon, Terminator/genetics , Genome , Mitochondria/genetics , Molecular Sequence Data , Sequence AlignmentABSTRACT
Phylogenetic relationships among asteroids remain to be extremely controversial in spite of many morphological and molecular studies have been applied to this issue. In the present study, especially focusing on resolving the relationship of Asterina and Solasteridae, we reconstructed the molecular phylogenetic tree of asteroids using nuclear 18S rDNA. A close relationship between Asterina and Solasteridae, which has been suggested from analyses of mitochondrial 12S rDNA and 16S rDNA, is supported here by the nuclear 18S rDNA dataset. The support is even stronger when the sequences of mitochondrial rDNAs and nuclear 18S rDNA are combined as a total dataset. The independent support from both nuclear 18S rDNA and mitochondrial rDNAs strongly argues for a close relationship between the Asterina and Solasteridae.
