Articles Online (Volume 9, Issue 3)

Preface

Hox and ParaHox Genes in Evolution, Development and Genomics

David E.K. Ferner

Page 63–64


Review

The Origin of Patterning Systems in Bilateria—Insights from the Hox and ParaHox Genes in Acoelomorpha

Eduardo Moreno, Jon Permanyer, Pedro Martinez

Hox and ParaHox genes constitute two families of developmental regulators that pattern the Anterior-Posterior body axis in all bilaterians. The members of these two groups of genes are usually arranged in genomic clusters and work in a coordinated fashion, both in space and in time. While the mechanistic aspects of their action are relatively well known, it is still unclear how these systems evolved. For instance, we still need a proper model of how the Hox and ParaHox clusters were assembled over time. This problem is due to the shortage of information on gene complements for many taxa (mainly basal metazoans) and the lack of a consensus phylogenetic model of animal relationships to which we can relate our new findings. Recently, several studies have shown that the Acoelomorpha most probably represent the first offshoot of the Bilateria. This finding has prompted us, and others, to study the Hox and ParaHox complements in these animals, as well as their activity during development. In this review, we analyze how the current knowledge of Hox and ParaHox genes in the Acoelomorpha is shaping our view of bilaterian evolution.

Page 65–76


Review

Evolution of Invertebrate Deuterostomes and Hox/ParaHox Genes

Tetsuro Ikuta

Transcription factors encoded by Antennapedia-class homeobox genes play crucial roles in controlling development of animals, and are often found clustered in animal genomes. The Hox and ParaHox gene clusters have been regarded as evolutionary sisters and evolved from a putative common ancestral gene complex, the ProtoHox cluster, prior to the divergence of the Cnidaria and Bilateria (bilaterally symmetrical animals). The Deuterostomia is a monophyletic group of animals that belongs to the Bilateria, and a sister group to the Protostomia. The deuterostomes include the vertebrates (to which we belong), invertebrate chordates, hemichordates, echinoderms and possibly xenoturbellids, as well as acoelomorphs. The studies of Hox and ParaHox genes provide insights into the origin and subsequent evolution of the bilaterian animals. Recently, it becomes apparent that among the Hox and ParaHox genes, there are significant variations in organization on the chromosome, expression pattern, and function. In this review, focusing on invertebrate deuterostomes, I first summarize recent findings about Hox and ParaHox genes. Next, citing unsolved issues, I try to provide clues that might allow us to reconstruct the common ancestor of deuterostomes, as well as understand the roles of Hox and ParaHox genes in the development and evolution of deuterostomes.

Page 77–96


Review

Hox Gene Clusters of Early Vertebrates: Do They Serve as Reliable Markers for Genome Evolution?

Shigehiro Kuraku

Hox genes, responsible for regional specification along the anteroposterior axis in embryogenesis, are found as clusters in most eumetazoan genomes sequenced to date. Invertebrates possess a single Hox gene cluster with some exceptions of secondary cluster breakages, while osteichthyans (bony vertebrates) have multiple Hox clusters. In tetrapods, four Hox clusters, derived from the so-called two-round whole genome duplications (2R-WGDs), are observed. Overall, the number of Hox gene clusters has been regarded as a reliable marker of ploidy levels in animal genomes. In fact, this scheme also fits the situations in teleost fishes that experienced an additional WGD. In this review, I focus on cyclostomes and cartilaginous fishes as lineages that would fill the gap between invertebrates and osteichthyans. A recent study highlighted a possible loss of the HoxC cluster in the galeomorph shark lineage, while other aspects of cartilaginous fish Hox clusters usually mark their conserved nature. In contrast, existing resources suggest that the cyclostomes exhibit a different mode of Hox cluster organization. For this group of species, whose genomes could have differently responded to the 2R-WGDs from jawed vertebrates, therefore the number of Hox clusters may not serve as a good indicator of their ploidy level.

Page 97–103


Review Article

16S rDNA-Based Phylogeny of Non-Symbiotic Bacteria of Entorno-pathogenic Nematodes from Infected Insect Cadavers

M. Razia, R. KarthikRaja, K. Padmanaban, P. Chellapandi, S. Sivaramakrishnan

Using 16S rDNA gene sequencing technique, three different species of non-symbiotic bacteria of entomopatho-genic nematodes (EPNs) (Steinernema sp. and Heterorhabditis sp.) were isolated and identified from infected insect cadavers {Galleria mellonella larvae) after 48-hour post infections. Sequence similarity analysis revealed that the strains SRK3, SRK4 and SRK5 belong to Ochrobactrum cytisi, Schineria larvae and Ochrobactrum anthropi, respectively. The isolates O. anthropi and S. larvae were found to be associated with Heterorhabditis indica strains BDU-17 and Yer-136, respectively, whereas O. cytisi was associated with Steinernema siamkayai strain BDU-87. Phenotypically, temporal EPN bacteria were fairly related to symbiotic EPN bacteria (Photorhabdus and Xenorhabdus genera). The strains SRK3 and SRK5 were phylogeographically similar to several non-symbionts and contaminated EPN bacteria isolated in Germany (LMG3311T) and China (X-14), while the strain SRK4 was identical to the isolates of S. larvae (Ll/57, Ll/58, Ll/68 and L2/11) from Wohlfahrtia magnifica in Hungary. The result was further confirmed by RNA secondary structure and minimum energy calculations of aligned sequences. This study suggested that the non-symbionts of these nematodes are phylogeographically diverged in some extent due to phase variation. Therefore, these strains are not host-dependent, but environment-specific isolates.

Page 104–112


Review Article

Genetic Flux Between H1 and H2 Haplotypes of the 17q21.31 Inversion in European Population

Libin Deng, Xiaoli Tang, Xiangwen Hao, Wei Chen, Jiari Lin, Yangyu Yu, Dake Zhang, Changqing Zeng

The chromosome 17q21.31 inversion is a 900-kb common structural polymorphism found primarily in European population. Although the genetic flux within inversion region was assumed to be considerable suppressed, it is still unclear about the details of genetic exchange between the H1 (non-inverted sequence) and H2 (inverted sequence) haplotypes of this inversion. Here we describe a refined map of genetic exchanges between pairs of gene arrangements within the 17q21.31 region. Using HapMap phase II data of 1,546 single nucleotide polymorphisms, we successfully deduced 96 H1 and 24 H2 haplotypes in European samples by neighbor-joining tree reconstruction. Furthermore, we identified 15 and 26 candidate tracts with reciprocal and non-reciprocal genetic exchanges, respectively. In all 15 regions harboring reciprocal exchange, haplotypes reconstructed by clone sequencing did not support these exchange events, suggesting that such signals of exchange between two sister chromosomes in certain heterozygous individual were caused by phasing error regions. On the other hand, the finished clone sequencing across 4 of 26 tracts with non-reciprocal genetic flux confirmed that this kind of genetic exchange was caused by gene conversion. In summary, as crossover between pairs of gene arrangements had been considerably suppressed, gene conversion might be the most important mechanism for genetic exchange at 17q21.31.

Page 113–118