Evolution of CUT class homeobox genes: insights from the genome of the amphioxus, Branchiostoma floridae

CUT class homeobox genes, including CUX/CASP, ONECUT, SATB and COMPASS family genes, are known to exhibit diverse features in the homeodomain and the domain architecture. Furthermore, the intron/exon organization of CUX/CASP is different between vertebrates and protostomes, and SATB genes are only known for vertebrates, whereas COMPASS genes have only been found in protostomes. These observations suggest a complex evolutionary history for the CUT class homeobox genes, but the evolution of CUT class homeobox genes in the lineage to vertebrates remained largely unknown. To obtain clearer insights into this issue, we searched the genome of amphioxus, Branchiostoma floridae, a lower chordate, for CUT class homeobox genes by extensive BLAST survey and phylogenetic analyses. We found that the genome of Branchiostoma floridae encodes each single orthologue of CUX/CASP, ONECUT, and COMPASS, but not the SATB gene, and one atypical CUT gene likely specific to this species. In addition, the genomic structure of the amphioxus CUX/CASP gene turned out to be protostome-type, but not vertebrate-type. Based on these observations, we propose a model in which SATB is suggested to evolve at the expense of COMPASS and this change, together with the structural change in CUX/CASP, is supposed to take place in the lineage to vertebrates after divergence of the amphioxus and vertebrate ancestors. The present study provides an example of dramatic evolution among homeobox gene groups in the vertebrate lineage and highlights the ancient character of amphioxus, retaining genomic features shared by protostomes.     

Excitation-contraction coupling in a pre-vertebrate twitch muscle: The myotomes of Branchiostoma lanceolatum

Summary The segmented trunk muscle (myotome muscle) of the lancelet (Branchiostoma lanceolatum), a pre-vertebrate chordate, was studied in order to gain information regarding the evolution of excitation-contraction (EC) coupling.Myotome membrane vesicles could be separated on isopycnic sucrose gradients into two main fractions, probably comprising solitary microsomes and diads of plasma membrane and sarcoplasmic reticulum, respectively. Both fractions bound the dihydropyridine PN 200/110 and the phenylalkylamine (–)D888 (devapamil) while specific ryanodine binding was observed in the diad preparation only. Pharmacological effects on Ca²⁺ currents measured under voltage-clamp conditions in single myotome fibers included a weak block by the dihydropyridine nifedipine and a shift of the voltage dependences of inactivation and restoration to more negative potentials by (–)D888. After blocking the Ca²⁺ current by cadmium in voltage-clamped single fibers, the contractile response persisted and a rapid intramembrane charge movement could be demonstrated. Both responses exhibited a voltage sensitivity very similar to the one of the voltage-activated Ca²⁺ channels.Our biochemical and electrophysiological results indicate that the EC coupling mechanism of the protochordate myotome cell is similar to that of the vertebrate skeletal muscle fiber: Intracellular Ca²⁺ release, presumably taking place via the ryanodine receptor complex, is under control of the cell membrane potential. The sarcolemmal Ca²⁺ channels might serve as voltage sensors for this process.We thank Drs. H.Ch. Lüttgau and L.M.G. Heilmeyer, Jr. for stimulating discussions during the work, Dr. N.R. Brandt for helpful suggestions, and Drs. A.H. Caswell and M. Michalak for their generous gifts of antibodies. We also thank Ms. P. Goldmann, Mr. R. Schwalm, and Mr. U. Siemen for technical support and Ms. E. Linnepe for editorial help. This work was supported by grant G1 72/1-5 of the Deutsche Forschungsgemeinschaft. R. Benterbusch was recipient of a scholarship by the Studienstiftung des Deutschen Volkes.     

The amphioxus (Branchiostoma floridae) genome contains a highly diversified set of G protein-coupled receptors

Background

When natural hybridization occurs at sites where the hybridizing species differ in abundance, the pollen load delivered to the rare species should be predominantly from the common species. Previous authors have therefore proposed a hypothesis on the direction of hybridization: interspecific hybrids are more likely to have the female parent from the rare species and the male parent from the common species. We wish to test this hypothesis using data of plant hybridizations both from our own experimentation and from the literature.

Results

By examining the maternally inherited chloroplast DNA of 6 cases of F1 hybridization from four genera of plants, we infer unidirectional hybridization in most cases. In all 5 cases where the relative abundance of the parental species deviates from parity, however, the direction is predominantly in the direction opposite of the prediction based strictly on numerical abundance.

Conclusion

Our results show that the observed direction of hybridization is almost always opposite of the predicted direction based on the relative abundance of the hybridizing species. Several alternative hypotheses, including unidirectional postmating isolation and reinforcement of premating isolation, were discussed.     

Identification of rDNA-specific non-LTR retrotransposons in Cnidaria

Ribosomal RNA genes are abundant repetitive sequences in most eukaryotes. Ribosomal DNA (rDNA) contains many insertions derived from mobile elements including non-long terminal repeat (non-LTR) retrotransposons. R2 is the well-characterized 28S rDNA-specific non-LTR retrotransposon family that is distributed over at least 4 bilaterian phyla. R2 is a large family sharing the same insertion specificity and classified into 4 clades (R2-A, -B, -C, and -D) based on the N-terminal domain structure and the phylogeny. There is no observation of horizontal transfer of R2; therefore, the origin of R2 dates back to before the split between protostomes and deuterostomes. Here, we in silico identified 1 R2 element from the sea anemone Nematostella vectensis and 2 R2-like retrotransposons from the hydrozoan Hydra magnipapillata. R2 from N. vectensis was inserted into the 28S rDNA like other R2, but the R2-like elements from H. magnipapillata were inserted into the specific sequence in the highly conserved region of the 18S rDNA. We designated the Hydra R2-like elements R8. R8 is inserted at 37 bp upstream from R7, another 18S rDNA-specific retrotransposon family. There is no obvious sequence similarity between targets of R2 and R8, probably because they recognize long DNA sequences. Domain structure and phylogeny indicate that R2 from N. vectensis is the member of the R2-D clade, and R8 from H. magnipapillata belongs to the R2-A clade despite its different sequence specificity. These results suggest that R2 had been generated before the split between cnidarians and bilaterians and that R8 is a retrotransposon family that changed its target from the 28S rDNA to the 18S rDNA.     

Horizontal transfer of non-LTR retrotransposons in vertebrates

Since their discovery in family Bovidae (bovids), Bov-B LINEs, believed to be order-specific SINEs, have been found in all ruminants and recently also in Viperidae snakes. The distribution and the evolutionary relationships of Bov-B LINEs provide an indication of their origin and evolutionary dynamics in different species. The evolutionary origin of Bov-B LINE elements has been shown unequivocally to be in Squamata (squamates). The horizontal transfer of Bov-B LINE elements in vertebrates has been confirmed by their discontinuous phylogenetic distribution in Squamata (Serpentes and two lizard infra-orders) as well as in Ruminantia, by the high level of nucleotide identity, and by their phylogenetic relationships. The direction of horizontal transfer from Squamata to the ancestor of Ruminantia is evident from the genetic distances and discontinuous phylogenetic distribution of Bov-B LINE elements. The ancestral snake lineage (Boidae) has been recognized as a possible donor of Bov-B LINE elements to Ruminantia. The timing of horizontal transfer has been estimated from the distribution of Bov-B LINE elements in Ruminantia and the fossil data of Ruminantia to be 40–50mya. The phylogenetic relationships of Bov-B LINE elements from the various Squamata species agrees with that of the species phylogeny, suggesting that Bov-B LINE elements have been stably maintained by vertical transmission since the origin of Squamata in the Mesozoic era. This revised version was published online in July 2006 with corrections to the Cover Date.     

The ingi and RIME non-LTR retrotransposons are not randomly distributed in the genome of Trypanosoma brucei

The ingi (long and autonomous) and RIME (short and nonautonomous) non--long-terminal repeat retrotransposons are the most abundant mobile elements characterized to date in the genome of the African trypanosome Trypanosoma brucei. These retrotransposons were thought to be randomly distributed, but a detailed and comprehensive analysis of their genomic distribution had not been performed until now. To address this question, we analyzed the ingi/RIME sequences and flanking sequences from the ongoing T. brucei genome sequencing project (TREU927/4 strain). Among the 81 ingi/RIME elements analyzed, 60% are complete, and 7% of the ingi elements (approximately 15 copies per haploid genome) appear to encode for their own transposition. The size of the direct repeat flanking the ingi/RIME retrotransposons is conserved (i.e., 12-bp), and a strong 11-bp consensus pattern precedes the 5'-direct repeat. The presence of a consensus pattern upstream of the retroelements was confirmed by the analysis of the base occurrence in 294 GSS containing 5'-adjacent ingi/RIME sequences. The conserved sequence is present upstream of ingis and RIMEs, suggesting that ingi-encoded enzymatic activities are used for retrotransposition of RIMEs, which are short nonautonomous retroelements. In conclusion, the ingi and RIME retroelements are not randomly distributed in the genome of T. brucei and are preceded by a conserved sequence, which may be the recognition site of the ingi-encoded endonuclease.     

BfCBR: a cannabinoid receptor ortholog in the cephalochordate Branchiostoma floridae (Amphioxus)

A gene encoding an ortholog of vertebrate CB(1)/CB(2) cannabinoid receptors was recently identified in the urochordate Ciona intestinalis (CiCBR; [Elphick, M.R., Satou, Y., Satoh, N., 2003. The invertebrate ancestry of endocannabinoid signalling: an orthologue of vertebrate cannabinoid receptors in the urochordate Ciona intestinalis. Gene 302, 95-101.]). Here a cannabinoid receptor ortholog (BfCBR) has been identified in the cephalochordate Branchiostoma floridae. BfCBR is encoded by a single exon and is 410 amino acid residue protein that shares 28% sequence identity with CiCBR and 23% sequence identity with human CB(1) and human CB(2). The discovery of BfCBR and CiCBR and the absence of cannabinoid receptor orthologs in non-chordate invertebrates indicate that CB(1)/CB(2)-like cannabinoid receptors originated in an invertebrate chordate ancestor of urochordates, cephalochordates and vertebrates. Furthermore, analysis of the relationship of BfCBR and CiCBR with vertebrate CB(1) and CB(2) receptors indicates that the gene/genome duplication that gave rise to CB(1) and CB(2) receptors occurred in the vertebrate lineage. Identification of BfCBR, in addition to CiCBR, paves the way for comparative analysis of the expression and functions of these proteins in Branchiostoma and Ciona, respectively, providing an insight into the ancestral functions of cannabinoid receptors in invertebrate chordates prior to the emergence of CB(1) and CB(2) receptors in vertebrates.     

Systematic investigation of Amphioxus (Branchiostoma floridae) microRNAs

Ascaris lumbricoides and Ascaris suum are parasitic nematodes living in the small intestine of humans and pigs, and can cause the disease ascariasis. For long, there has been controversy as to whether the two ascaridoid taxa represent the same species due to their significant resemblances in morphology. However, the complete mitochondrial (mt) genome data have been lacking for A. lumbricoides in spite of human and animal health significance and socio-economic impact globally of these parasites. In the present study, we sequenced the complete mt genomes of A. lumbricoides and A. suum (China isolate), which was 14,303 bp and 14,311 bp in size, respectively. The identity of the mt genomes was 98.1% between A. lumbricoides and A. suum (China isolate), and 98.5% between A. suum (China isolate) and A. suum (USA isolate). Both genomes are circular, and consist of 36 genes, including 12 genes for proteins, 2 genes for rRNA and 22 genes for tRNA, which are consistent with that of all other species of ascaridoid studied to date. All genes are transcribed in the same direction and have a nucleotide composition high in A and T (71.7% for A. lumbricoides and 71.8% for A. suum). The AT bias had a significant effect on both the codon usage pattern and amino acid composition of proteins. Phylogenetic analyses of A. lumbricoides and A. suum using concatenated amino acid sequences of 12 protein-coding genes, with three different computational algorithms (Bayesian analysis, maximum likelihood and maximum parsimony) all clustered in a clade with high statistical support, indicating that A. lumbricoides and A. suum was very closely related. These mt genome data and the results provide some additional genetic evidence that A. lumbricoides and A. suum may represent the same species. The mt genome data presented in this study are also useful novel markers for studying the molecular epidemiology and population genetics of Ascaris.     

APE-type non-LTR retrotransposons: determinants involved in target site recognition

Non-long terminal repeat (Non-LTR) retrotransposons represent a diverse and widely distributed group of transposable elements and an almost ubiquitous component of eukaryotic genomes that has a major impact on evolution. Their copy number can range from a few to several million and they often make up a significant fraction of the genomes. The members of the dominating subtype of non-LTR retrotransposons code for an endonuclease with homology to apurinic/apyrimidinic endonucleases (APE), and are thus termed APE-type non-LTR retrotransposons. In the last decade both the number of identified non-LTR retrotransposons and our knowledge of biology and evolution of APE-type non-LTR retrotransposons has increased tremendously.     

Evolution of target specificity in R1 clade non-LTR retrotransposons

Although most non-long terminal repeat (non-LTR) retrotransposons are inserted throughout the host genome, many non-LTR elements in the R1 clade are inserted into specific sites within the target sequence. Four R1 clade families have distinct target specificity: R1 and RT insert into specific sites of 28S rDNA, and TRAS and SART insert into different sites within the (TTAGG)(n) telomeric repeats. To study the evolutionary history of target specificity of R1-clade retrotransposons, we have screened extensively novel representatives of the clade from various insects by in silico and degenerate polymerase chain reaction (PCR) cloning. We found four novel sequence-specific elements; Waldo (WaldoAg1, 2, and WaldoFs1) inserts into ACAY repeats, Mino (MinoAg1) into AC repeats, R6 into another specific site of the 28S rDNA, and R7 into a specific site of the 18S rDNA. In contrast, several elements (HOPE, WISHBm1, HidaAg1, NotoAg1, KagaAg1, Ha1Fs1) lost target sequence specificity, although some of them have preferred target sequences. Phylogenetic trees based on the RT and EN domains of each element showed that (1) three rDNA-specific elements, RT, R6, and R7, diverged from Waldo; (2) the elements having similar target sequences are phylogenetically related; and (3) the target specificity in the R1 clade was obtained once and thereafter altered and lost several times independently. These data indicate that the target specificity in R1 clade retroelements has changed during evolution and is more divergent than has been speculated so far.