Differential expression of conserved germ line markers and delayed segregation of male and female primordial germ cells in a hermaphrodite, the leech helobdella.

In sexually reproducing animals, primordial germ cells (PGCs) are often set aside early in embryogenesis, a strategy that minimizes the risk of genomic damage associated with replication and mitosis during the cell cycle. Here, we have used germ line markers (piwi, vasa, and nanos) and microinjected cell lineage tracers to show that PGC specification in the leech genus Helobdella follows a different scenario: in this hermaphrodite, the male and female PGCs segregate from somatic lineages only after more than 20 rounds of zygotic mitosis; the male and female PGCs share the same (mesodermal) cell lineage for 19 rounds of zygotic mitosis. Moreover, while all three markers are expressed in both male and female reproductive tissues of the adult, they are expressed differentially between the male and female PGCs of the developing embryo: piwi and vasa are expressed preferentially in female PGCs at a time when nanos is expressed preferentially in male PGCs. A priori, the delayed segregation of male and female PGCs from somatic tissues and from one another increases the probability of mutations affecting both male and female PGCs of a given individual. We speculate that this suite of features, combined with a capacity for self-fertilization, may contribute to the dramatically rearranged genome of Helobdella robusta relative to other animals

Trichoplax, the simplest known animal, contains an estrogen-related receptor: Implications for the evolution of vertebrate and invertebrate estrogen receptors

Although, as their names imply, vertebrate and invertebrate estrogen receptors [ERs] and estrogen-related receptors [ERRs] are related transcription factors, their evolutionary relationships to each other are not fully understood. We searched recently sequenced genome of _Trichoplax_, the simplest known animal, and genomes from three lophotrochozoans: _Capitella_, a worm, _Helobdella robusta_, a leech, and _Lottia gigantea_, a snail, to elucidate the origins and evolution of ERs and ERRs. BLAST found an ERR in _Trichoplax_, but no ER. BLAST searches of the lophotrochozaons found ERRs in all three and invertebrate ERs in _Capitella_ and _Lottia_, but not in _Helobdella_. These database searches and a phylogenetic analyses indicate that invertebrate ERs arose in a protostome, and vertebrate ERs arose later in deuterostome

First record of a potentially introduced leech, Helobdella aff. robusta Shankland, Bissen & Weisblat, 1992 (Hirudinea, Glossiphoniidae), in Japan

Helobdella aff. robusta Shankland, Bissen & Weisblat, 1992 is recorded for the first time from Japan. The haplotype of the cytochrome c oxidase subunit I (COI) gene sequence of a specimen from Japan is identical to that of one collected in California, USA. The dorsal pigmentation pattern of the Japanese specimens is different from that of H. robusta inhabiting North America, but nonetheless, most of their morphological characteristics are con-sistent with the diagnostic features of this species. This species is considered to be of New World origin, and the Japanese population was likely established from a recent introduction

Stochastic WNT signaling between nonequivalent cells regulates adhesion but not fate in the two-cell leech embryo

AbstractBackground: In the leech Helobdella robusta, an annelid worm, the early pattern of cell divisions is stereotyped. The unequal first cleavage yields cells AB and CD, which differ in size, cytoplasmic inheritance, normal fate, and developmental potential.Results: Here we report a dynamic and transcription-independent pattern of WNT signaling in the two-cell stage of H. robusta. Surprisingly, HRO-WNT-A is first expressed in a stochastic manner, such that either AB or CD secretes the protein in each embryo. This stochastic phase is followed by a deterministic phase during which first AB, then CD expresses HRO-WNT-A. When contact between the cells is reduced or eliminated, both AB and CD express HRO-WNT-A simultaneously. Finally, bathing embryos in anti-HRO-WNT-A antibody during first cleavage reduces the adhesion between cells AB and CD.Conclusions: Our findings show that the stochastic phase of HRO-WNT-A signaling in the two-cell stage of Helobdella is negatively regulated by cell-cell contact and that this early signaling affects cell adhesion without affecting cell fate. We speculate that the primordial function of wnt class genes may have been to regulate cell-cell adhesion and that the nuclear signaling components of the wnt pathway arose later in association with the evolution of diverse cell types

D- and L-lactate dehydrogenases during invertebrate evolution

Background: The L-lactate and D-lactate dehydrogenases, which are involved in the reduction of pyruvate to L(-)-lactate and D(+)-lactate, belong to evolutionarily unrelated enzyme families. The genes encoding L-LDH have been used as a model for gene duplication due to the multiple paralogs found in eubacteria, archaebacteria, and eukaryotes. Phylogenetic studies have suggested that several gene duplication events led to the main isozymes of this gene family in chordates, but little is known about the evolution of L-Ldh in invertebrates. While most invertebrates preferentially oxidize L-lactic acid, several species of mollusks, a few arthropods and polychaetes were found to have exclusively D-LDH enzymatic activity. Therefore, it has been suggested that L-LDH and DLDH are mutually exclusive. However, recent characterization of putative mammalian D-LDH with significant similarity to yeast proteins showing D-LDH activity suggests that at least mammals have the two naturally occurring forms of LDH specific to L- and D-lactate. This study describes the phylogenetic relationships of invertebrate L-LDH and D-LDH with special emphasis on crustaceans, and discusses gene duplication events during the evolution of L-Ldh. Results: Our phylogenetic analyses of L-LDH in vertebrates are consistent with the general view that the main isozymes (LDH-A, LDH-B and LDH-C) evolved through a series of gene duplications after the vertebrates diverged from tunicates. We report several gene duplication events in the crustacean, Daphnia pulex, and the leech, Helobdella robusta. Several amino acid sequences with strong similarity to putative mammalian D-LDH and to yeast DLD1 with D-LDH activity were found in both vertebrates and invertebrates. Conclusion: The presence of both L-Ldh and D-Ldh genes in several chordates and invertebrates suggests that the two enzymatic forms are not necessarily mutually exclusive. Although, the evolution of L-Ldh has been punctuated by multiple events of gene duplication in both vertebrates and invertebrates, a shared evolutionary history of this gene in the two groups is apparent. Moreover, the high degree of sequence similarity among D-LDH amino acid sequences suggests that they share a common evolutionary history

Mitochondrial genomes of Clymenella torquata (Maldanidae) and Riftia pachyptila (Siboglinidae) : evidence for conserved gene order in Annelida

Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of Society for Molecular Biology and Evolution for personal use, not for redistribution. The definitive version was published in Molecular Biology and Evolution 22 (2005): 210-222, doi:10.1093/molbev/msi008.Mitochondrial genomes are useful tools for inferring evolutionary history. However, many taxa are poorly represented by available data. Thus, to further understand the phylogenetic potential of complete mitochondrial genome sequence data in Annelida (segmented worms), we examined the complete mitochondrial sequence for Clymenella torquata (Maldanidae) and an estimated 80% of the sequence of Riftia pachyptila (Siboglinidae). These genomes have remarkably similar gene orders to previously published annelid genomes, suggesting that gene order is conserved across annelids. This result is interesting given the high variation seen in the closely related Mollusca and Brachiopoda. Phylogenetic analyses of DNA sequence, amino acid sequence and gene order all support the recent hypothesis that Sipuncula and Annelida are closely related. Our findings suggest that gene order data is of limited utility in annelids but that sequence data holds promise. Additionally, these genomes show AT bias (~66%) and codon usage biases, but have a typical gene complement for bilaterian mitochondrial genomes.Support by CICOR to RJM is gratefully acknowledged. This work was support by the National Science Foundation grants (DEB-0075618 and EAR-0120646) to KMH

動物の高い再生能力に関連する進化的に保存された遺伝子の同定と機能解析

Tohoku University牧野能士課

Unravelling spiral cleavage

Snails, earthworms and flatworms are remarkably different animals, but they all exhibit a very similar mode of early embryogenesis: spiral cleavage. This is one of the most widespread developmental programs in animals, probably ancestral to almost half of the animal phyla, and therefore its study is essential for understanding animal development and evolution. However, our knowledge of spiral cleavage is still in its infancy. Recent technical and conceptual advances, such as the establishment of genome editing and improved phylogenetic resolution, are paving the way for a fresher and deeper look into this fascinating early cleavage mode