The Behavior of Chromosomes During Parthenogenetic Oogenesis in Marmorkrebs Procambarus fallax f. virginalis.

Parthenogenetic oogenesis varies among and even within species. Based on cytological mechanisms, it can largely be divided into apomixis (ameiotic parthenogenesis) producing genetically identical progeny, and automixis (meiotic parthenogenesis) producing genetically non-identical progeny. Polyploidy is common in parthenogenetic species, although the association between parthenogenesis and polyploidy throughout evolution is poorly understood. Marmorkrebs, or the marbled crayfish, was first identified as a parthenogenetic decapod and was tentatively named as Procambarus fallax f. virginalis. Previous studies revealed that Marmorkrebs is triploid and produces genetically identical offspring, suggesting that apomixis occurs during parthenogenetic oogenesis. However, the behavior of chromosomes during the process of oogenesis is still not well characterized. In this study, we observed parthenogenetic oogenesis around the time of ovulation in P. fallax f. virginalis by histology and immunohistochemistry. During oogenesis, the chromosomes were separated into two groups and behaved independently from each other, and one complete division corresponding to mitosis (the second meiosis-like division) was observed. This suggests that parthenogenetic oogenesis in Marmorkrebs exhibits gonomery, a phenomenon commonly found in apomictic parthenogenesis in polyploid animals.

MiR-124 is involved in post-transcriptional regulation of Polypyrimidine tract binding protein 1 (PTBP1) during neural development in the medaka, Oryzias latipes.

MicroRNAs (miRNAs) comprise a group of small noncoding RNA molecules thought to have contributed to the evolution of vertebrate brain homogeneity and diversity. The miRNA miR-124 is well conserved between invertebrates and vertebrates and is expressed abundantly in the central nervous system (CNS). We identified miR-124 in the medaka, Oryzias latipes, and investigated its role in neural development. The five candidate genes for medaka precursor miR-124 are unlinked on four different chromosomes and differ in nucleotide length. Their sequences suggest that they can generate functional miRNAs through conventional miRNA biogenesis by folding into stem-loop structures. Whole-mount in situ hybridization and northern blotting revealed that mature miR-124 is specifically expressed in the CNS and the eyes starting at two days post-fertilization. We also examined the sequences and expression of medaka Polypyrimidine tract binding protein 1 (Ptbp1), a possible direct target of miR-124. The 3'UTR of medaka Ptbp1 contains predicted binding motifs (target sites) for miR-124. A GFP reporter assay for the target sites or the entire 3'UTR showed that exogenous miR-124 silences PTBP1 expression in vivo. Our study suggests that medaka miR-124 is involved in post-transcriptional regulation of target genes in neural development and that medaka miR-124 homologs may have spatiotemporal roles different from those in other vertebrates.

Identification of the precise kairomone-sensitive period and histological characterization of necktooth formation in predator-induced polyphenism in Daphnia pulex.

Many organisms have the ability to alter their development in the presence of predators, leading to predator-induced defenses that reduce vulnerability to predation. In the water flea Daphnia pulex, small protuberances called 'neckteeth' form in the dorsal neck region in response to kairomone(s) released by predatory phantom midges (Chaoborus larvae). Although previous studies suggested that kairomone sensitivity begins when chemoreceptors begin to function during embryogenesis, the exact critical period was unknown to date. In this study, we investigated the period of kairomone sensitivity and the process of necktooth formation in D. pulex through extensive treatments with pulses of kairomone(s). First, we described the time course of embryogenesis, which we suggest should be used as the standard in future studies. We found the kairomone-sensitive period to be relatively short, extending from embryonic stage 4 to postembryonic first instar. We observed cell proliferation and changes in cell structure in response to the kairomone treatment, and propose a model for necktooth formation. Preliminary LiCl treatment suggests the Wnt signaling pathway involved in crest formation as a candidate for the molecular mechanism underlying this process. Our study provides basic insight toward understanding the mechanisms underlying adaptive polyphenism in D. pulex.

Elaborate regulations of the predator-induced polyphenism in the water flea Daphnia pulex: kairomone-sensitive periods and life-history tradeoffs.

Adaptive polyphenism produces alternative phenotypes depending on environmental stimuli. The water flea Daphnia pulex shows predator-induced polyphenism, facultatively forming neckteeth in response to kairomones released by Chaoborus larvae. This study was designed to reveal the regulatory systems producing the defensive morph during embryonic and postembryonic development. As noted previously, the crest epithelium at the site of neckteeth is shown to thicken earlier the neckteeth formation, and the neckteeth number increased until the third instar, and later disappeared. Exposure to kairomone at various time points and intervals during development showed that the signal was required even at early postembryonic stages to maintain neckteeth. Moreover, two different induction methods, i.e. embryonic and maternal exposures, enabled us to discriminate maternal and zygotic effects in response to kairomone. Direct embryonic exposure is shown to be sufficient to form neckteeth without maternal effect although their growth was diminished; namely, there is a trade-off for neckteeth production. However, maternal exposures resulted in larger progenies in smaller numbers, suggesting that the mother daphnids change their reproductive strategy depending on kairomone signals. Taken together, the developmental responses to the presence of predators are regulated elaborately at various levels.

Xenopus laevis macrophage migration inhibitory factor is essential for axis formation and neural development.

Macrophage migration inhibitory factor (MIF) is an immunoregulatory cytokine involved in both acquired and innate immunity. MIF also has many functions outside the immune system, such as isomerase and oxidoreductase activities and control of cell proliferation. Considering the involvement of MIF in various intra- and extracellular events, we expected that MIF might also be important in vertebrate development. To elucidate the possible role of MIF in developmental processes, we knocked down MIF in embryos of the African clawed frog Xenopus laevis, using MIF-specific morpholino oligomers (MOs). For the synthesis of the MOs, we cloned a cDNA for a Xenopus homolog of MIF. Sequence analysis, determination of the isomerase activity, and x-ray crystallographic analysis revealed that the protein encoded by the cDNA was the ortholog of mammalian MIF. We carried out whole mount in situ hybridization of MIF mRNA and found that MIF was expressed at high levels in the neural tissues of normal embryos. Although early embryogenesis of MO-injected embryos proceeded normally until the gastrula stage, their neurulation was completely inhibited. At the tailbud stage, the MO-injected embryos lacked neural and mesodermal tissues, and also showed severe defects in their head and tail structures. Thus, MIF was found to be essential for axis formation and neural development of Xenopus embryos.