BackgroundThe common long-arm octopus (Octopus minor) is found in mudflats of subtidal zones and faces numerous environmental challenges. The ability to adapt its morphology and behavioral repertoire to diverse environmental conditions makes the species a promising model for understanding genomic adaptation and evolution in cephalopods.FindingsThe final genome assembly of O. minor is 5.09 Gb, with a contig N50 size of 197 kb and longest size of 3.027 Mb, from a total of 419 Gb raw reads generated using the Pacific Biosciences RS II platform. We identified 30,010 genes; 44.43% of the genome is composed of repeat elements. The genome-wide phylogenetic tree indicated the divergence time between O. minor and Octopus bimaculoides was estimated to be 43 million years ago based on single-copy orthologous genes. In total, 178 gene families are expanded in O. minor in the 14 bilaterian species.ConclusionsWe found that the O. minor genome was larger than that of closely related O. bimaculoides, and this difference could be explained by enlarged introns and recently diversified transposable elements. The high-quality O. minor genome assembly provides a valuable resource for understanding octopus genome evolution and the molecular basis of adaptations to mudflats.

The Forkhead box (Fox) gene family is an evolutionarily ancient gene family named after the Drosophila melanogaster forkhead gene (fkh). Fox genes are highly conserved transcription factors critical for embryogenesis and carcinogenesis. In the current study, we report a whole‐genome survey of Fox genes and their expression patterns in the leech Helobdella austienesis. Phylogenetic analysis suggests that some Fox genes of leeches are correlated with other Lophotrochozoa and vertebrate Fox genes. Here we have performed semiquantitative reverse transcription polymerase chain reaction and whole‐mount in situ hybridization of Fox genes throughout the embryonic development of H. austinensis. We found that each one of the leech Fox genes (FoxA1, FoxA3, FoxC, FoxL2, FoxO1, and FoxO2) is expressed in a specific set of cells or tissue type. From Stages 9–11, Hau‐FoxA1 was expressed in the foregut of the anterior region, and Hau‐FoxL2 was expressed in mesodermal muscle fiber. Hau‐FoxA3 was temporally expressed in the ventral neuroectoderm. At Stage 11, Hau‐FoxC was expressed in the foregut. Hau‐FoxO genes have a ubiquitous expression. Our results provide more insight on the evolutionary linkage and role of the Fox gene function in Bilateria.

Echiura is one of the most intriguing major subgroups of phylum Annelida because, unlike most other annelids, echiuran adults lack metameric body segmentation. Urechis unicinctus lives in U-shape burrows of soft sediments. Little is known about the molecular mechanisms underlying the development of U. unicinctus. Herein, we overviewed the developmental process from zygote to juvenile U. unicinctus using immunohistochemistry and F-actin staining for the nervous and muscular systems, respectively. Through F-actin staining, we found that muscle fibers began to form in the trochophore phase and that muscles for feeding were produced first. Subsequently, in the segmentation larval stage, the transversal muscle was formed in the shape of a ring in an anterior-to-posterior direction with segment formation, as well as a ventromedian muscle for the formation of a ventral nerve cord. After that, many muscle fibers were produced along the entire body and formed the worm-shaped larva. Finally, we investigated the spatiotemporal expression of Uun_st-mhc, Uun_troponin I, Uun_calponin, and Uun_twist genes found in U. unicinctus. During embryonic development, the striated and smooth muscle genes were co-expressed in the same region. However, the adult body wall muscles showed differential gene expression of each muscle layer. The results of this study will provide the basis for the understanding of muscle differentiation in Echiura.

The eye of a cephalopod is a well-known example of convergent evolution and resembles the vertebrate eye. Although cephalopods and vertebrates exhibit similar eye form and function, they differ in visual origin and structure. The common long-arm octopus (Octopus minor) is a good model system in evolutionary and developmental studies due to its highly centralized nervous system, shorter life cycle, and specific camera-type eyes that contribute to convergence with vertebrate eye. Lens-containing eyes represent a significant improvement of simple eye and have evolved by convergent mechanisms, a variety of lenses and corneas containing diverse crystallin. The diversity and taxon-specificity of lens crystallin is indicative of convergent evolution of crystallin roles. Previous studies have focused on morphological, ontogenetic and phylogenetic analysis of crystallin to understand the evolution of lens-containing eyes. However, little is known about the functional analysis of taxon-specific crystallin genes at the molecular level in the eye of O. minor. Using an embryonic staging system of Octopus minor as a model system, we investigated fifteen genomes and the structure of eye by immunohistochemistry, phalloidin staining and the three-dimensional structures. We also obtained the crystallin-related genes (i.e., α-, S-, and Ω-crystallin) from the transcriptome data of O. minor. Subsequent molecular phylogenetic analysis based on these genes revealed a distinct divergence pattern among the three gene classes and further suggested the evidence supporting the taxon-specific convergent evolutionary trend. We analyzed the expression pattern of crystallin genes via in situ hybridization during developmental stages. All crystallin genes are commonly expressed in the lentigenic cells of ciliary body. The α-crystallin found in cephalopods was also expressed at the peripheral region of the lens including ciliary body, suggesting a possible role in lens formation in cephalopods. This study will provide information on the eye development of O. minor and support the typical models of convergent evolution by demonstrating independent recruitment of different types of proteins to fulfill their unique visual role.