Histone variant innovation in a rapidly evolving chordate lineage

Background

Histone variants alter the composition of nucleosomes and play crucial roles in transcription, chromosome segregation, DNA repair, and sperm compaction. Modification of metazoan histone variant lineages occurs on a background of genome architecture that shows global similarities from sponges to vertebrates, but the urochordate, Oikopleura dioica, a member of the sister group to vertebrates, exhibits profound modification of this ancestral architecture.

Results

We show that a histone complement of 47 gene loci encodes 31 histone variants, grouped in distinct sets of developmental expression profiles throughout the life cycle. A particularly diverse array of 15 male-specific histone variants was uncovered, including a testes-specific H4t, the first metazoan H4 sequence variant reported. Universal histone variants H3.3, CenH3, and H2A.Z are present but O. dioica lacks homologs of macroH2A and H2AX. The genome encodes many H2A and H2B variants and the repertoire of H2A.Z isoforms is expanded through alternative splicing, incrementally regulating the number of acetylatable lysine residues in the functionally important N-terminal "charge patch". Mass spectrometry identified 40 acetylation, methylation and ubiquitylation posttranslational modifications (PTMs) and showed that hallmark PTMs of "active" and "repressive" chromatin were present in O. dioica. No obvious reduction in silent heterochromatic marks was observed despite high gene density in this extraordinarily compacted chordate genome.

Conclusions

These results show that histone gene complements and their organization differ considerably even over modest phylogenetic distances. Substantial innovation among all core and linker histone variants has evolved in concert with adaptation of specific life history traits in this rapidly evolving chordate lineage.

Vagal nerve stimulator infection: a lead-salvage protocol

OBJECT

Vagal nerve stimulator (VNS) hardware infections are fraught with difficult management decisions. As with most implanted medical device-related infections, standard practice traditionally involves complete hardware removal, systemic antibiotic therapy, and subsequent reimplantation of the device. To avoid the potential morbidity of 2 repeat left carotid sheath surgical dissections, the authors have implemented a clinical protocol for managing VNS infections that involves generator removal and antibiotic therapy without lead removal.

METHODS

A prospective, single-surgeon database was compared with hospital billing records to identify patients who underwent primary implantation or reimplantation of a VNS lead, generator, or both, from January 2001 to May 2010, at Oregon Health & Science University. From these records, the authors identified patients with VNS hardware infections and characterized their management, using a lead salvage protocol.

RESULTS

In their review, the authors found a matching cohort of 206 children (age 3 months-17 years) who met the inclusion criteria. These children underwent 258 operations (including, in some children, multiple operations for generator end of life and/or lead malfunction). Six children experienced a single postimplantation infection (2.3% of the 258 operative cases), and no child experienced repeated infection. A lead-salvage protocol was used in 4 of 6 infected patients and was successful in 3 (75%), with clinical follow-up ranging from 10 months to 7.5 years. The fourth patient subsequently underwent lead removal and later reimplantation in standard fashion, with no adverse sequelae.

CONCLUSIONS

Vagal nerve stimulator lead salvage is a safe and potentially advantageous strategy in the management of VNS-related infection. Further study is necessary to validate appropriate patient selection, success rates, and risks of this approach.

The paradoxical effect of bevacizumab in the therapy of malignant gliomas

One rationale behind the use of agents that inhibit vascular endothelial growth factor in the therapy of primary CNS malignancies is based upon the concept that normalization of tumor vasculature with a decrease in tumor interstitial pressure will improve access of cytoreductive drugs and improve radiotherapy efficacy due to increased oxygen delivery. However, several studies have raised the concern that these agents may both rapidly restore the low permeability characteristics of the blood-brain barrier and counteract the beneficial effect of pseudoprogression. The result may be decreased therapeutic efficacy while increasing infiltration by co-opting normal vessels. In this discussion, we examine both histologic and radiographic tumor progression in the context of antiangiogenic agents. Issues dealing with the safety of bevacizumab (Avastin®, Genentech, South San Francisco, CA) and its potential to decrease efficacy of standard radiochemotherapy when used to treat patients with newly diagnosed malignant glioma are emphasized.

Neuroform Stent-Assisted Embolization of Incidental Anterior Communicating Artery Aneurysms: Long-term Clinical and Angiographic Follow-up

BACKGROUND:

Anterior communicating artery (A-comm) aneurysm is one of the most common intracranial aneurysms. Treatments include neurosurgical clipping or endovascular embolization.

OBJECTIVE:

To retrospectively examine the long-term results of Neuroform stent-assisted coil embolization of incidental A-comms, with a focus on stent-associated stenosis, long-term angiographic aneurysm occlusion outcome, delayed stent-related thromboembolus, subsequent subarachnoid hemorrhage from the treated aneurysm, and procedural complications.

METHODS:

Between January 7, 2003 and June 16, 2009, 44 Neuroform stents were placed as an adjunct to embolization of A-comms. Patient charts were reviewed retrospectively. Angiographic follow-up of at least 3 months (up to 6.5 years, mean 65 weeks) was available for 33 patients. Aneurysm occlusion success was determined using the Raymond classification for aneurysm remnants.

RESULTS:

Referencing the last angiogram in the follow-up course, complete occlusion, dog-ear residual, residual neck, and residual aneurysm were found in 24, 2, 3, and 4 patients, respectively. Stenosis (45% and asymptomatic) of the artery where the stent had been placed was found in 1 patient. One patient had delayed transient ischemic attack after dual antiplatelet therapy was stopped prematurely. Retreatment based on the presence of residual aneurysm was performed or recommended in 2 patients. In 2 patients with residual or recurrent aneurysm filling, their age or clinical condition did not warrant retreatment.

CONCLUSION:

Neuroform stent-assisted embolization provides long-term control of A-comms with a low incidence of aneurysm growth after treatment. The need for retreatment is uncommon, and retreatment is safe if performed. Subsequent bleeding from treated aneurysms was not observed in this study.

Skull-fixated fiducial markers improve accuracy in staged frameless stereotactic epilepsy surgery in children

OBJECT

Surgery to monitor and resect epileptogenic foci may be undertaken in 2 stages, providing an opportunity to use skull-fixated fiducials implanted during the first stage to improve the accuracy of cortical resection during the second stage. This study compared the intrinsic accuracy of skin-based and skull-fixated fiducial markers in registering frameless stereotaxy during pediatric epilepsy surgery. To the authors' knowledge, these modalities of registration have not previously been directly compared in this population.

METHODS

The authors undertook a retrospective review of pediatric patients who underwent resection of epileptogenic foci in 2 stages with frameless stereotactic assistance, performed by a single surgeon at Oregon Health & Science University. For the first stage (subdural grid implantation), 9 skin fiducial markers were used to register anatomical data in a frameless stereotactic station. Intraoperatively, four 3-mm screws were placed circumferentially around the craniotomy. Postoperatively, thin-slice brain MR and CT images were obtained and fused. For the second stage, the 4 screws were used as fiducial markers to register the stereotactic anatomical data. For both stages, accuracy (difference in millimeters from zero of the manual fiducial registration compared with the computer model) was determined using navigation software. The intrinsic accuracy of these 2 methods of fiducial registration was compared using a paired Student t-test.

RESULTS

Between 2004 and 2009, 40 pediatric patients with epilepsy underwent frameless stereotactic surgical procedures. Fourteen patients who had 2-stage procedures using skin-based and skull-fixated registration with complete accuracy data were included in this retrospective review. Mean registration error was significantly lower using skull-fixated fiducials (1.35 mm, 95% CI 1.09-1.60 mm) than using skin-based fiducials (1.85 mm, 95% CI 1.56-2.13 mm; p = 0.0016).

CONCLUSIONS

A significantly higher degree of accuracy was achieved using 4 skull-fixated fiducials compared with using 9 skin-based fiducials. This simple and accurate method for registering frameless stereotactic anatomical data does not involve the potential time, expense, discomfort, and morbidity of extraoperative skull-fixated fiducial placement. The method described in this paper could also be extrapolated to other planned 2-stage cranial surgical procedures such as combined skull base approaches.

Transendoscopic intraoperative recording of gelastic seizures from a hypothalamic hamartoma

OBJECT

The differential diagnosis of hypothalamic masses in children includes hamartomas, which are associated with gelastic seizures and endocrine dysfunction. The purpose of this study was to utilize transendoscopic electroencephalography (EEG) recording at the time of tissue biopsy to further assist in diagnosis, determination of prognosis, and treatment planning.

METHODS

We present the case of an infant with gelastic seizures and a large hypothalamic mass lesion. Despite a clinical and radiographic presentation typical of hypothalamic hamartoma (HH), slight growth on serial imaging raised concern for a diagnosis of intrinsic neoplasm. Biopsy of the lesion was recommended.

RESULTS

Transventricular, endoscopic biopsy, was undertaken, with concurrent intraoperative, transendoscopic EEG recording using a standard epilepsy depth recording macroelectrode. Numerous electrographic seizures were recorded. Histopathology revealed a HH.

CONCLUSION

This is the first report of intraoperative macroelectrode recording of electrographic seizures transendoscopically from a HH. This technique may prove useful for diagnosis, prognosis and treatment planning, as well as to guide transendoscopic therapeutic interventions for HH.

Plasticity of animal genome architecture unmasked by rapid evolution of a pelagic tunicate

Genomes of animals as different as sponges and humans show conservation of global architecture. Here we show that multiple genomic features including transposon diversity, developmental gene repertoire, physical gene order, and intron-exon organization are shattered in the tunicate Oikopleura, belonging to the sister group of vertebrates and retaining chordate morphology. Ancestral architecture of animal genomes can be deeply modified and may therefore be largely nonadaptive. This rapidly evolving animal lineage thus offers unique perspectives on the level of genome plasticity. It also illuminates issues as fundamental as the mechanisms of intron gain.

Local adaptation for body color in Drosophila americana

Pigmentation is one of the most variable traits within and between Drosophila species. Much of this diversity appears to be adaptive, with environmental factors often invoked as selective forces. Here, we describe the geographic structure of pigmentation in Drosophila americana and evaluate the hypothesis that it is a locally adapted trait. Body pigmentation was quantified using digital images and spectrometry in up to 10 flies from each of 93 isofemale lines collected from 17 locations across the United States and found to correlate most strongly with longitude. Sequence variation at putatively neutral loci showed no evidence of population structure and was inconsistent with an isolation-by-distance model, suggesting that the pigmentation cline exists despite extensive gene flow throughout the species range, and is most likely the product of natural selection. In all other Drosophila species examined to date, dark pigmentation is associated with arid habitats; however, in D. americana, the darkest flies were collected from the most humid regions. To investigate this relationship further, we examined desiccation resistance attributable to an allele that darkens pigmentation in D. americana. We found no significant effect of pigmentation on desiccation resistance in this experiment, suggesting that pigmentation and desiccation resistance are not unequivocally linked in all Drosophila species.

Lumbar lipomyelomeningocele and sacrococcygeal teratoma in siblings: support for an alternative theory of spinal teratoma formation

Sacrococcygeal teratomas may arise in association with regional developmental errors affecting the caudal embryonic segments and may originate within lumbosacral lipomas. It is therefore possible that sacrococcygeal teratomas and lumbosacral lipomas represent related disorders of embryogenesis. Accordingly, the authors report the cases of 2 siblings. The first child (female) was born with a mature Altman Type III sacrococcygeal teratoma that was resected when she was a neonate. Subsequently, a younger brother was found soon after birth to have an L-4-level lipomyelomeningocele and underwent partial resection and spinal cord untethering at 4 months of age. Although familial forms of each of these conditions have been reported, this is, to the authors' knowledge, the first reported occurrence of lipomyelomeningocele and sacrococcygeal teratoma in siblings. They propose that an inherited regional tendency to developmental error affecting the caudal embryonic segments was shared by these siblings and resulted in spinal teratoma formation in one of them.

Functional specialization of cellulose synthase genes of prokaryotic origin in chordate larvaceans

Extracellular matrices play important, but poorly investigated, roles in morphogenesis. Extracellular cellulose is central to regulation of pattern formation in plants, but among metazoans only tunicates are capable of cellulose biosynthesis. Cellulose synthase (CesA) gene products are present in filter-feeding structures of all tunicates and also regulate metamorphosis in the ascidian Ciona. Ciona CesA is proposed to have been acquired by lateral gene transfer from a prokaryote. We identified two CesA genes in the sister-class larvacean Oikopleura dioica. Each has a mosaic structure of a glycoslyltransferase 2 domain upstream of a glycosyl hydrolase family 6 cellulase-like domain, a signature thus far unique to tunicates. Spatial-temporal expression analysis revealed that Od-CesA1 produces long cellulose fibrils along the larval tail, whereas Od-CesA2 is responsible for the cellulose scaffold of the post-metamorphic filter-feeding house. Knockdown of Od-CesA1 inhibited cellulose production in the extracellular matrix of the larval tail. Notochord cells either failed to align or were misaligned, the tail did not elongate properly and tailbud embryos also exhibited a failure to hatch. Knockdown of Od-CesA2 did not elicit any of these phenotypes and instead caused a mild delay in pre-house formation. Phylogenetic analyses including Od-CesAs indicate that a single lateral gene transfer event from a prokaryote at the base of the lineage conferred biosynthetic capacity in all tunicates. Ascidians possess one CesA gene, whereas duplicated larvacean genes have evolved distinct temporal and functional specializations. Extracellular cellulose microfibrils produced by the pre-metamorphic Od-CesA1 duplicate have a role in notochord and tail morphogenesis.

Successful ventricle to direct heart shunt placement as a salvage cerebrospinal fluid diversion technique. Case report

The authors report a complex case in a 35-year-old woman who underwent shunt placement at birth for myelomeningocele. She had previously undergone more than 30 shunt revisions, with placement of the distal catheter in the peritoneum multiple times, and also in the pleura, the gall bladder, and the upper venous system. All shunts had failed and the possible placement sites were now anatomically hostile. A median sternotomy was performed as the next option. The catheter was placed directly into the appendage of the right atrium and secured with a pursestring suture. One month postoperatively, the patient presented with a large pericardial effusion after the distal catheter migrated out of the atrium and into the pericardial space. A repeat sternotomy was performed to drain the pericardial CSF collection. The catheter was reinserted into the atrial appendage, and a tunnel was created in the atrial wall to fix the device more securely. At 1 year postoperatively, the patient had no further symptoms of shunt obstruction or cardiac tamponade, and imaging studies suggested that the shunt system was functional. The authors report the first successful ventricle to direct heart shunt in an adult.

Culture optimization for the emergent zooplanktonic model organism Oikopleura dioica

The pan-global marine appendicularian, Oikopleura dioica, shows considerable promise as a candidate model organism for cross-disciplinary research ranging from chordate genetics and evolution to molecular ecology research. This urochordate, has a simplified anatomical organization, remains transparent throughout an exceptionally short life cycle of less than 1 week and exhibits high fecundity. At 70 Mb, the compact, sequenced genome ranks among the smallest known metazoan genomes, with both gene regulatory and intronic regions highly reduced in size. The organism occupies an important trophic role in marine ecosystems and is a significant contributor to global vertical carbon flux. Among the short list of bona fide biological model organisms, all share the property that they are amenable to long-term maintenance in laboratory cultures. Here, we tested diet regimes, spawn densities and dilutions and seawater treatment, leading to optimization of a detailed culture protocol that permits sustainable long-term maintenance of O. dioica, allowing continuous, uninterrupted production of source material for experimentation. The culture protocol can be quickly adapted in both coastal and inland laboratories and should promote rapid development of the many original research perspectives the animal offers.

Risk and protective factors in the development of post-traumatic stress disorder symptoms among a cohort of students in two tertiary institutions post Hurricane Ivan

OBJECTIVE

This study seeks to identify factors associated with post-traumatic stress disorder (PTSD) following Hurricane Ivan among a cohort of tertiary-level students in Jamaica.

METHODS

A 22-item, self-administered questionnaire was administered to a non-random sample. Using parametric and non-parametric analyses, the relationship between PTSD symptoms and several variables was examined.

RESULTS

Feeling anxious, worried or fearful and/or experiencing loss and injury during the hurricane were significant (p < 0.01) predictors for PTSD symptoms.

CONCLUSIONS

Factors predicting the development of PTSD symptoms were feeling anxious, worried or fearful during the hurricane and experiencing loss of property and/or death or injury of a family member during the hurricane.

Altered miRNA repertoire in the simplified chordate, Oikopleura dioica

Recent studies reveal correlation between microRNA (miRNA) innovation and increased developmental complexity. This is exemplified by dramatic expansion of the miRNA inventory in vertebrates, a lineage where genome duplication has played a significant evolutionary role. Urochordates, the closest extant group to the vertebrates, exhibit an opposite trend to genome and morphological simplification. We show that the urochordate, larvacean, Oikopleura dioica, possesses the requisite miRNA biogenic machinery. The miRNAs isolated by small RNA cloning were expressed throughout the short life cycle, a number of which were stocked as maternal determinants prior to rapid embryonic development. We identify sex-specific miRNAs that appeared as male/female gonad differentiation became apparent and were maintained throughout spermatogenesis. Whereas 80% of mammalian miRNAs are hosted in introns of protein-coding genes, the majority of O. dioica miRNA loci were located in antisense orientations to such genes. Including sister group ascidians in analysis of the urochordate miRNA repertoire, we find that 11 highly conserved bilaterian miRNA families have been lost or derived to the point they are not recognizable in urochordates and a further 4 of these families are absent in larvaceans. Subsequent to this loss/derivation, at least 29 novel miRNA families have been acquired in larvaceans. This suggests a profound reorganization of the miRNA repertoire integral to evolution in the urochordate lineage.

Identification of the novel HLA-C allele Cw*0751

We report the novel HLA-Cw allele HLA-Cw*0751. The allele was identified during routine sequence-based typing in our laboratory. The novel allele is identical to Cw*07020101 except for a single nucleotide change in codon 90.2 in position 268. HLA-Cw*0751 allele possesses an adenine at position 268 in exon 2, while HLA-Cw*07020101 has a cytosine at this position. Although this substitution does not change serologic reactivity of HLA-Cw7 molecule, it changes the amino acid at codon 90 from an aspartic acid to an alanine. Aspartic acid is polar and acidic, while alanine is non-polar and neutral.

Endostyle cell recruitment as a frame of reference for development and growth in the Urochordate Oikopleura dioica

In models of growth and life history, and in molecular and cell biology, there is a need for more accurate frames of reference to characterize developmental progression. In Caenorhabditis elegans, complete fate maps of cell lineage provide such a standard of reference. To be more widely applicable, reference frames should be easier to measure while still providing strong predictive capacity. Towards this aim, we have analyzed growth of the endostyle in the appendicularian Oikopleura dioica at the cellular level, and measured its response to temperature and food availability. Specifically, we test the hypothesis that age of a specific developmental stage in O. dioica can be predicted from the number of endostyle cells and temperature. We show that the endostyle grows by recruiting cells from the posterior tip into the lateral arms of the organ in an anterior-posterior orientation and that the rate of increase in lateral arm endostyle cells is temperature-dependent but unresponsive to nutritional intake. Endostyle cells therefore serve as an accurate and easily measured marker to describe developmental progression. Conceptually, such a method of characterizing developmental progression should help bridge life-history events and molecular mechanisms throughout organismal aging, facilitating cross-disciplinary understanding by providing a common experimental framework.

Rapidly evolving lamins in a chordate, Oikopleura dioica, with unusual nuclear architecture

Metazoan lamins are implicated in the organization of numerous critical nuclear processes. Among chordates, the appendicularian, Oikopleura dioica, has an unusually short life cycle involving rapid growth through extensive recourse to endoreduplication, a characteristic more associated with some invertebrates. In some tissues, this is accompanied by the formation of elaborate, bilaterally symmetric nuclear morphologies associated with specific gene expression patterns. Lamin composition can mediate nuclear shape in spermiogenesis as well as during pathological and normal aging and we have analyzed the O. dioica lamin and intermediate filament (IF) complement, comparing it to that present in other deuterostomes. O. dioica has one lamin gene coding two splice variants. Both variants share with the sister class ascidians a highly reduced C-terminal tail region lacking the immunoglobulin fold, indicating this derivation occurred at the base of the urochordate lineage. The OdLamin2 variant has a unique insertion of 63 amino acids in the normally short N-terminal region and has a developmental expression profile corresponding to the occurrence of endocycling. O. dioica has 4 cytoplasmic IF proteins, IF-A, C, Dalpha, and Dbeta. No homologues to the ascidian IF-B or F proteins were identified, reinforcing the suggestion that these proteins are unique to ascidians. The degree of sequence evolution in the rod domains of O. dioica cytoplasmic IF proteins and their closest ascidian and vertebrate homologues was similar. In contrast, whereas the rate of lamin B rod domain sequence evolution has also been similar in vertebrates, cephalochordates and the sea urchin, faster rates have occurred among the urochordates, with the O. dioica lamin displaying a far greater rate than any other lamin.

Conserved patterns of nuclear compartmentalization are not observed in the chordate Oikopleura

BACKGROUND INFORMATION

Recent results from a limited number of eukaryotic model organisms suggest that major principles governing spatial organization of the genome in functionally distinct nuclear compartments are conserved through evolution.

RESULTS

We examined the in situ spatial organization of major nuclear components and nuclear patterns of gene loci with strictly defined expression patterns in endocycling cells of the transparent urochordate Oikopleura dioica, a complex metazoan with a very compact genome. Endocycling cells with different functions and similar DNA content displayed distinct topologies of nuclear components. However, the generation of the diverse nuclear architectures did not involve specific local organization of active genes or their preferential amplification. Interestingly, endocycling cells lacked nuclear-envelope-associated heterochromatin and prominent splicing-factor domains, which in mammalian cells associate with transcriptionally silent and active loci respectively. In addition, no correlation was found between transcriptional activity of a locus and its association with chromatin domains rich in specific histone modifications.

CONCLUSIONS

Together, these findings and the absence of typical eukaryotic replication patterns reveal a surprisingly limited functional compartmentalization of O. dioica endocycling nuclei. This indicates that robust cell-type-specific gene expression does not necessarily require high levels of spatial genome organization.

Intracranial leptomeningeal metastasis from thymic carcinoma: case report and review

BACKGROUND

Thymic carcinoma is an uncommon malignant tumor of the anterior mediastinum. Meningeal metastasis from this type of neoplasm is extraordinarily rare and the prognosis is abysmal.

CASE DESCRIPTION

This article presents the case of a 45-year-old man with known metastatic thymic carcinoma who presented with intractable headaches. An MRI scan was highly suggestive of a meningioma, and it was initially suspected that this patient had 2 primary tumors. Surgical resection of the mass both demonstrated a metastatic thymic lesion and ameliorated the patient's quality of life.

CONCLUSION

The authors report a case of intracranial meningeal metastasis from a lymphoepithelioma-like poorly differentiated metastatic thymic carcinoma, which was treated by resection and WBRT. A review of the current literature revealed no other cases of this uncommon alhistologic subtype of thymic carcinoma metastatic to the cranium. The incidence, histologic classification of subtypes, and treatment are discussed. This case also illustrates the importance of maintaining a high degree of suspicion for a metastasis in patients with known primary malignancy who present with an MRI highly suspicious for meningioma.

Phosphorylation of the histone H3.3 variant in mitosis and meiosis of the urochordate Oikopleura dioica

Mammalian histone variant H3.3 differs from replication-dependent histone H3.1 by five amino acids, including replacement of alanine 31 by serine. H3.3 is expressed throughout the cell cycle, primarily deposited at transcriptionally active loci independent of S-phase. Data from mammalian cells suggest that phosphorylation of serine 31 (H3.3S31P) plays a role in mitosis. Here we show that H3.3S31P also occurs during mitosis of the urochordate Oikopleura dioica, suggesting this histone modification and its function in mitosis is already present at the invertebrate-vertebrate transition. The spatial pattern differed from that of H3 phosphorylation at serine 28 (H3S28P). H3S28P was enriched near telomeric regions, but H3.3S31P differed both temporally and spatially from the mammalian pattern, being more widely distributed throughout prophase, prometaphase and metaphase chromosomes. We also identified an important role for H3.3S31P during oogenic meiosis in the semelparous O. dioica. H3.3S31P initiated together with H3S28P in all meiotic nuclei in late diplotene, after H3S10P. However, H3.3S31P was retained only on the subset of meiotic nuclei that seeded maturing oocytes and proceeded through meiosis to arrest in metaphase I. Thus, this epigenetic mark is part of a regulatory circuitry that enables O. dioica to numerically adjust oocyte production over two orders of magnitude.

The Oikopleura coenocyst, a unique chordate germ cell permitting rapid, extensive modulation of oocyte production

The ability to adjust reproductive output to environmental conditions is important to the fitness of a species. The semelparous, chordate, Oikopleura dioica, is particularly adept in producing a highly variable number of oocytes in its short life cycle. Here we show that this entails an original reproductive strategy in which the entire female germline is contained in a single multinucleate cell, the "coenocyst". After an initial phase of syncytial nuclear proliferation half of the nuclei entered meiosis whereas the other half became highly polyploid. The inner F-actin network, with associated plasma membranes, formed a highly ramified infrastructure in which each meiotic nucleus was contained in a pseudo-compartmentalized pro-oocyte linked to the common cytoplasm via ring canals. At a set developmental time, a subset of the pro-oocytes was selected for synchronous growth and the common coenocyst cytoplasm was equally partitioned by transfer through the ring canals. Examination of related species indicated that the coenocyst arrangement is a conserved feature of Appendicularian oogenesis allowing efficient numerical adjustment of oocyte production. As Appendicularia are the second most abundant class of zooplankton, with a world-wide distribution, the coenocyst is clearly a common and successful reproductive strategy on a global scale.