Self-incompatibilty in gamete recognition: single self-recognizing determinants and multiple, non-self-recognizing ones function in the same individual.

The frameworks (key mechanisms) of the self/non-self-discrimination systems that are found in various organisms have not been actively selected for, but have evolved by genetic drift such that the genetic frequency of random, advantageous mutations has increased within the genomes of these species by natural selection. The passive nature of this process leads to an important conclusion: in the self/non-self-discrimination system, the number of self-recognizing determinants becomes one compared to multiple non-self-recognizing determinants. Thus, the number of determinants is defined not by the character of the determinant, but by the system framework.

Allorecognition between compound ascidian colonies.

The compound ascidian Botryllus schlosseri reproduces asexually, and forms colonies to adhere to matrices such as rocks. This species has developed a mechanism to distinguish between self stem cells and invasive parasitizing nonself cells from other individuals of the same species, probably as a defense against parasitism. It is highly likely that such adult colony histocompatibility is controlled differently from its gametic allorecognition during fertilization. Allorecognition in adults is controlled by a single fusion/histocompatibility (FuHC) locus. In 2005, a candidate gene responsible for the phenotype associated with this genetic locus, named cFuHC, was reported; however, this proposal was subsequently refuted, and the actual determinant may exist elsewhere within the FuHC locus. Given that its is unlikely that a single gene could produce the diversity of FuHC alleles needed to determine individual identity and to distinguish self from nonself colonies, it is possible that the FuHC locus consists of a cluster of multiple determinants aligned in tandem.

Browning Behavior of Calcium Maltobionate-water System.

The purpose of the present study was to clarify the browning behavior of calcium maltobionate (CaMb). CaMb samples with varying water content (0-50 %) and water activity (0-0.98) were prepared. Absorbance due to the browning of CaMb-water samples increased linearly with an increase in holding time, and the browning rate was evaluated as a pseudo-zero-order reaction at each temperature (353-413 K). The effect of temperature on the browning rate of CaMb samples was analyzed using the Arrhenius formula, and the pre-exponential factor and activation energy were determined. In addition, the browning rate at 298 K (typical ambient condition) was determined according to the Arrhenius behavior. The browning rate at 298 K slightly decreased, markedly increased, and then decreased with an increase in water content and water activity. This behavior can be explained by the monolayer effect, plasticizing effect, and dilution effect of water molecules. There was a linear relationship between the natural logarithm of the pre-exponential factor and activation energy according to the thermodynamic compensation rule. The effect of water content on the activation energy was fitted using the cubic function. From these results, an approach for prediction of the browning of CaMB during thermal condensation was established.

Identification and localization of the sperm CRISP family protein CiUrabin involved in gamete interaction in the ascidian Ciona intestinalis.

Ascidians are hermaphrodites, and most release sperm and eggs nearly simultaneously. Many species, including Halocynthia roretzi and Ciona intestinalis, are self-sterile. We previously reported that the interaction between a 12 EGF-like repeat-containing vitelline-coat (VC) protein, HrVC70, and a sperm GPI-anchored CRISP, HrUrabin, in lipid rafts plays a key role in self-/nonself-recognizable gamete interaction in H. roretzi. On the other hand, we recently identified two pairs of polymorphic genes responsible for self-incompatibility in C. intestinalis by positional cloning: The sperm polycystin 1-like receptors s-Themis-A/B and its fibrinogen-like ligand v-Themis-A/B on the VC. However, it is not known if the orthologs of HrVC70 and HrUrabin also participate in gamete interaction in C. intestinalis since they are from different orders. Here, we tested for a C. intestinalis ortholog (CiUrabin) of HrUrabin by searching the genome database and proteomes of sperm lipid rafts. The identified CiUrabin belongs to the CRISP family, with a PR domain and a GPI-anchor-attachment site. CiUrabin appears to be specifically expressed in the testis and localized at the surface of the sperm head, as revealed by Northern blotting and immunocytochemistry, respectively. The specific interaction between CiVC57, a C. intestinalis ortholog of HrVC70, and CiUrabin was confirmed by Far Western analysis, similarly to the interaction between HrVC70 and HrUrabin. The molecular interaction between CiVC57 and CiUrabin may be involved in the primary binding of sperm to the VC prior to the allorecognition process, mediated by v-Themis-A/B and s-Themis-A/B, during fertilization of C. intestinalis.

Vitellogenin C-terminal fragments participate in fertilization as egg-coat binding partners of sperm trypsin-like proteases in the ascidian Halocynthia roretzi.

Sperm trypsin-like proteases are known to play important roles in fertilization, but their detailed functions are still unknown. We previously explored the binding partners of sperm trypsin-like proteases, HrProacrosin and HrSpermosin, in the ascidian Halocynthia roretzi, and we isolated several candidate proteins on the vitelline coat. We found that some of these proteins are identical to the C-terminal coding region (CT) and von Willebrand factor type D (vWF-D) domain of vitellogenin. We also found that CT on the vitelline coat disappears after fertilization. Vitellogenin is a large lipid transfer protein that is enzymatically processed during vitellogenesis. Although the processed domains including phosvitin and lipovitellin are known to function as yolk nutrient proteins, the roles of the CT and vWF-D domain remain elusive. Our results showed that the CT and vWF-D domain of vitellogenin are processed and attached to the vitelline coat, which in turn participate in fertilization as the binding partners of sperm proteases.

Identification of testis-specific ubiquitin-conjugating enzyme in the ascidian Ciona intestinalis.

The ubiquitin-proteasome system is known to play a key role in fertilization in ascidians, sea urchins, and mammals. To obtain insights into the ubiquitin-conjugating enzymes (Ube2) involved in reproductive systems, we systematically explored Ube2 enzymes expressed in the testis of the ascidian Ciona intestinalis. Here, we report cDNA cloning and characterization of a novel type of Ube2r (Ci0100152677) that is capable of making a thiolester bond with ubiquitin. Northern analysis, whole-mount in situ hybridization and immunocytochemistry indicate that this enzyme is exclusively expressed in the testis, mainly in the germ cells during the late stage of spermatogenesis, and is localized in the sperm head and tail, suggesting possible participation in fertilization or spermatogenesis/spermiogenesis.

Allorecognition mechanisms during ascidian fertilization.

Ascidians (primitive chordates) are hermaphroditic animals, releasing sperm and eggs nearly simultaneously. But, many ascidians, including Ciona intestinalis and Halocynthia roretzi, show self-sterility or preference for cross-fertilization rather than self-fertilization. The molecular mechanisms underlying this allorecognition process are only poorly understood. We recently identified the genes responsible for self-incompatibility in C. intestinalis by a positional cloning: sperm-borne polycystin 1-like receptor, referred to as s-Themis, and its fibrinogen-like ligand called v-Themis on the vitelline coat (VC) are highly polymorphic and appear to be responsible for allorecognition in the fertilization of C. intestinalis. In H. roretzi, on the other hand, we revealed that HrVC70, a 70-kDa main component of the VC consisting of 12 epidermal-growth-factor (EGF)-like repeats, is a candidate allorecognition protein, since the attachment of this protein to the VC during oocyte maturation and its detachment by weak acid are closely linked to the gain and the loss of self-sterility, respectively, and also since nonself-sperm rather than self-sperm efficiently bound to HrVC70-agarose. As a binding partner of HrVC70, a 35-kDa GPI-anchored glycoprotein in sperm lipid rafts, referred to as HrUrabin, was identified: HrUrabin appears to play a key role in allorecognizable sperm binding to HrVC70 during fertilization. In the present review, we describe the current progress on the molecular bases of allorecognition, or self-incompatibility, during ascidian fertilization, by considering the SI systems in another organisms including fungies and flowering plants.

Conserved expression pattern of BMP-2/4 in hemichordate acorn worm and echinoderm sea cucumber embryos.

The auricularia larva of sea cucumbers and tornaria larva of acorn worms share striking developmental and morphological similarities. They are regarded as not only an archetype of the nonchordate deuterostome larva, but also an archetype of the origin of chordates. Here we report the characterization and spatial expression patterns of the BMP-2/4 genes of a hemichordate acorn worm (Pf-bmp2/4) and an echinoderm sea cucumber (Sj-bmp2/4). Both the Pf-bmp2/4 and Sj-bmp2/4 genes exhibited apparently conserved expression in the region of the coelomopore complex. This is in agreement with the homology between their basic larval body plans with respect to coelomogenesis and allows us to discuss the evolutionary counterparts of the coelomopore complex in chordates.

Comprehensive egg coat proteome of the ascidian Ciona intestinalis reveals gamete recognition molecules involved in self-sterility.

Despite central roles of egg coat proteins in gamete recognition, their functions and composition are poorly understood. Here, we report that the proteome of the egg coat in the solitary ascidian Ciona intestinalis, called vitelline coat (VC) fraction, contains more than 800 proteins identified by mass spectrometry-based analyses. Over 100 proteins were enriched in the VC fraction compared with the VC-free egg proteome. The most abundant component in the VC was an apolipoprotein-like protein. The VC contained multiple homologs of mammalian zona pellucida (ZP) proteins, the number of which was unexpectedly large and most of which possessed epidermal growth factor-like repeats. Furthermore, the present study revealed that two fibrinogen-like proteins, v-Themis-A and -B, both of which are expressed in the VC, are the molecules responsible for the two self-sterility loci that were identified by our previous genetic study in this species.

Ascidian sperm glycosylphosphatidylinositol-anchored CRISP-like protein as a binding partner for an allorecognizable sperm receptor on the vitelline coat.

Although ascidians are hermaphroditic, many species including Halocynthia roretzi are self-sterile. We previously reported that a vitelline coat polymorphic protein HrVC70, consisting of 12 EGF (epidermal growth factor)-like repeats, is a candidate allorecognition protein in H. roretzi, because the isolated HrVC70 shows higher affinity to nonself-sperm than to self-sperm. Here, we show that a sperm 35-kDa glycosylphosphatidylinositol-anchored CRISP (cysteine-rich secretory protein)-like protein HrUrabin in a low density detergent-insoluble membrane fraction is a physiological binding partner for HrVC70. We found that HrVC70 specifically interacts with HrUrabin, which had been separated by SDS-PAGE and transferred onto a nitrocellulose membrane. HrUrabin has an N-linked sugar chain, essential for binding to HrVC70. HrUrabin mRNA is expressed in the testis but not in the ovary, and the protein appears to be localized on the surface of sperm head and tail. Anti-HrUrabin antibody, which neutralizes the interaction between HrUrabin and HrVC70, potently inhibited fertilization and allorecognizable sperm-binding to HrVC70-agarose. However, no significant difference in the binding ability of HrUrabin to HrVC70 was observed in autologous and allogeneic combinations by Far Western analyses. These results indicate that sperm-egg binding in H. roretzi is mediated by the molecular interaction between HrUrabin on the sperm surface and HrVC70 on the vitelline coat, but that HrUrabin per se is unlikely to be a direct allorecognition protein.

Mechanism of self-sterility in a hermaphroditic chordate.

Hermaphroditic organisms avoid inbreeding by a system of self-incompatibility (SI). A primitive chordate (ascidian) Ciona intestinalis is an example of such an organism, but the molecular mechanism underlying its SI system is not known. Here, we show that the SI system is governed by two gene loci that act cooperatively. Each locus contains a tightly linked pair of polycystin 1-related receptor (s-Themis) and fibrinogen-like ligand (v-Themis) genes, the latter of which is located in the first intron of s-Themis but transcribed in the opposite direction. These genes may encode male- and female-side self-recognition molecules. The SI system of C. intestinalis has a similar framework to that of flowering plants but utilizing different molecules.

Proteins interacting with the ascidian vitelline-coat sperm receptor HrVC70 as revealed by yeast two-hybrid screening.

Ascidians are hermaphrodites releasing sperm and eggs nearly simultaneously, but many species are self sterile. We have previously reported that HrVC70 consisting of 12 EGF-like repeats is a major component of the vitelline coat, functioning as a self/nonself-recognizable sperm receptor during fertilization of the ascidian Halocynthia roretzi. Here, in order to identify the binding partner of HrVC70, we explored HrVC70-interacting proteins by yeast two-hybrid screening. HrVC70 is capable of interacting with HrVC70 precursor HrVC120 itself and also with three additional extracellular and/or transmembrane proteins, HrVLP-1, -2, and HrTTSP-1. Specific interaction of HrVC120, HrVLP-1, -2, and HrTTSP-1 with HrVC70 was confirmed by exchanging prey and bait, and also by a pulldown assay using the GST-fusion proteins. HrVLP-1 and -2 are proteins structurally related to HrVC120; both are expressed in the oocytes and may be novel components of the ascidian vitelline coat. HrTTSP-1 appears to be a member of the serine protease family with type II transmembrane topology. HrTTSP-1 is expressed in the testis and its gene product contains multiple conserved motifs known to be involved in protein-protein or protein-carbohydrate interactions. Close inspection revealed that the protease domain of HrTTSP-1 is considerably divergent, in particular around the region of the catalytic center Ser residue. Possible roles of these proteins in ascidian fertilization are also discussed.

Highly polymorphic vitelline-coat protein HaVC80 from the ascidian, Halocynthia aurantium: structural analysis and involvement in self/nonself recognition during fertilization.

Ascidians release sperm and eggs simultaneously, but self-fertilization is effectively blocked by unknown mechanisms. We previously reported that a 70-kDa sperm receptor HrVC70 on the egg vitelline coat (VC) consisting of 12 EGF-like repeats is a candidate self/nonself recognition molecule during fertilization of the ascidian, Halocynthia roretzi. Here, we report that Halocynthia aurantium also utilizes a homolog (HaVC80) of HrVC70 as an allorecognizable sperm receptor. HaVC80 is attached to the VC during the acquisition of self-sterility and is detached from the VC by acid treatment, allowing self-fertilization. A cDNA clone of the HaVC80 precursor, HaVC130, consists of 3726 nucleotides and encodes an open reading frame of 1208 amino acids. The structure of HaVC130 is very similar to the HrVC70 precursor HrVC120, but the number of EGF-like repeats of HaVC130/VC80 is one repeat larger than that of HrVC120/VC70. There are several amino acid substitutions between different individuals, and two alleles of the HaVC80 sequence were detected in each individual. Genomic DNA sequence analysis reveals that each EGF-like domain corresponds to a specific exon, and HaVC130 may have been evolutionarily generated from HrVC120 by duplication of the 8th EGF-like repeat. The data support the hypothesis that HaVC80 is a highly polymorphic protein responsible for self-sterility in H. aurantium.

Construction of a backcross progeny collection of dextral and sinistral individuals of a freshwater gastropod, Lymnaea stagnalis.

The handedness of gastropods is genetically determined, but the molecular nature of the gene responsible remains unknown. In order to identify the gene by a forward genetic approach, we carried out backcross breeding between dextral and sinistral inbred strains of a freshwater gastropod, Lymnaea stagnalis, cultivated in the laboratory. We used the dextral animals as donor "fathers" and the sinistral animals as recurrent "mothers". Each of the backcross progeny obtained was typed for the chirality of the next generation offspring oviposited by it, because the genotype for the handedness locus emerges as filial chirality. We constructed a collection of DNA and RNA specimens that included about 200 of the backcross F(2) progeny. Success in breeding was confirmed by the expected inheritance behavior of strain-specific DNA markers in the progeny.

Isolation and evaluation of dextral-specific and dextral-enriched cDNA clones as candidates for the handedness-determining gene in a freshwater gastropod, Lymnaea stagnalis.

The handedness of gastropods is genetically determined. The freshwater gastropod Lymnaea stagnalis is a normally dextral species, but contains minor sinistral populations. The gene responsible for handedness determination in this species is predicted to function maternally and specifically in the dextral-ovipositing snail. In this study, we used differential screening and cDNA subtraction to isolate eight "dextral genes" that are specific to, or enriched in, the dextral-ovipositing strains of L. stagnalis. These genes were promising candidates for the handedness-determining gene. In order to determine whether the true handedness-determining gene was among them, we tested for genetic correlations between the level of expression of each dextral gene and the handedness phenotype, i.e., the chirality of the next generation offspring, by using a collection of backcross F2 progeny of F1 offspring from crosses between dextral and sinistral strains. Although the present study could not identify the handedness-determining molecules, this approach appears to be promising for the isolation of such developmentally important genes.

Molecular heterotopy in the expression of Brachyury orthologs in order Clypeasteroida (irregular sea urchins) and order Echinoida (regular sea urchins).

The expression patterns of Brachyury (Bra) orthologs in the development of four species of sand dollars (order: Clypeasteroida), including a direct-developing species, and of a sea urchin species (order: Echinoida) were investigated during the period from blastula to the pluteus stage, with special attention paid to the relationship between the expression pattern and the mode of gastrulation. The sand dollar species shared two expression domains of the Bra orthologs with the Echinoida species, in the vegetal ring (the first domain) and the oral ectoderm (the second domain). The following heterotopic changes in the expression of the Bra genes were found among the sand dollar species and between the sand dollars and the Echinoida species. (1) The vegetal ring expressing Bra in the sand dollars was much wider and was located at a higher position along the AV axis, compared with that in the Echinoida species. The characteristic Bra expression in the vegetal ring of the sand dollar embryos was thought to be involved in the mode of gastrulation, in which involution continues from the beginning of invagination until the end of gastrulation. (2) Two of the three indirect-developing sand dollar species that were examined exhibited a third domain, in which Bra was expressed on the oral side of the archenteron. (3) In the direct-developing sand dollar embryos, Bra was expressed with an oral-aboral asymmetry in the vegetal ring and with a left-right asymmetry in the oral ectoderm. In the Echinoida species, Bra was expressed in the vestibule at the six-armed pluteus stage.

T-brain homologue (HpTb) is involved in the archenteron induction signals of micromere descendant cells in the sea urchin embryo.

Signals from micromere descendants play a crucial role in sea urchin development. In this study, we demonstrate that these micromere descendants express HpTb, a T-brain homolog of Hemicentrotus pulcherrimus. HpTb is expressed transiently from the hatched blastula stage through the mesenchyme blastula stage to the gastrula stage. By a combination of embryo microsurgery and antisense morpholino experiments, we show that HpTb is involved in the production of archenteron induction signals. However, HpTb is not involved in the production of signals responsible for the specification of secondary mesenchyme cells, the initial specification of primary mesenchyme cells, or the specification of endoderm. HpTb expression is controlled by nuclear localization of beta-catenin, suggesting that HpTb is in a downstream component of the Wnt signaling cascade. We also propose the possibility that HpTb is involved in the cascade responsible for the production of signals required for the spicule formation as well as signals from the vegetal hemisphere required for the differentiation of aboral ectoderm.