Comparative electron microscopy study of spermatozoa in snakes (Lepidosauria, Squamata)

The ultrastructure of snake sperm has received substantial attention primarily because snakes exhibit considerable variability in reproductive characteristics between species, with a wide range of mating systems and reproductive behaviors. Variability of sperm morphology among snake species may be associated with the reproductive strategies of each taxon, such as competition or sperm storage. We provide a detailed description of the sperm ultrastructure of nine snake species (Anilius scytale, Tropidophis paucisquamis, Bothrops jararaca, Oxyrhopus guibei, Dipsas mikanii, Micrurus corallinus, Xenopholis scalaris, Acrochordus javanicus, and Cylindrophis ruffus) and compared this with sperm data from the literature for the following taxa: Liotyphlops beui, Amerotyphlops reticulatus, Trilepida koppesi, Anilios waitii, Anilios endoterus, Aspidites melanochephalus, Boa constrictor amarali, Corallus hortulana, Epicrates cenchria, Boa constrictor occidentalis, Eryx jayakari, Micrurus corallinus, Micrurus surinamensis, Micrurus frontalis, Micrurus altirostris, Oxyuranus microlepidotus, Bothrops alternatus, Bothrops diporus, Crotalus durissus, Agkistrodon contortrix, Vipera aspis, Boiga irregularis, Zamenis schrenckii, Zamenis scalaris, Stegonotus cuculatus, Nerodia sipedon, Liodytes pygaea, and Myrrophis chinensis. We found twelve polymorphic characters in the ultrastructure of sperm among the described snakes. Our work supports the importance of ultrastructural analysis of sperm morphology to understand snake reproduction, and provides sperm-derived morphological characters for phylogenetic analysis.

Separable fragments and membrane tethering of Arabidopsis RIN4 regulate its suppression of PAMP-triggered immunity

RPM1-interacting protein 4 (RIN4) is a multifunctional Arabidopsis thaliana protein that regulates plant immune responses to pathogen-associated molecular patterns (PAMPs) and bacterial type III effector proteins (T3Es). RIN4, which is targeted by multiple defense-suppressing T3Es, provides a mechanistic link between PAMP-triggered immunity (PTI) and effector-triggered immunity and effector suppression of plant defense. Here we report on a structure-function analysis of RIN4-mediated suppression of PTI. Separable fragments of RIN4, including those produced when the T3E AvrRpt2 cleaves RIN4 and each containing a plant-specific nitrate-induced (NOI) domain, suppress PTI. The N-terminal and C-terminal NOIs each contribute to PTI suppression and are evolutionarily conserved. Native RIN4 is anchored to the plasma membrane by C-terminal acylation. Nonmembrane-tethered derivatives of RIN4 activate a cell death response in wild-type Arabidopsis and are hyperactive PTI suppressors in a mutant background that lacks the cell death response. Our results indicate that RIN4 is a multifunctional suppressor of PTI and that a virulence function of AvrRpt2 may include cleaving RIN4 into active defense-suppressing fragments.

Specific threonine phosphorylation of a host target by two unrelated type III effectors activates a host innate immune receptor in plants

The Arabidopsis NB-LRR immune receptor RPM1 recognizes the Pseudomonas syringae type III effectors AvrB or AvrRpm1 to mount an immune response. Although neither effector is itself a kinase, AvrRpm1 and AvrB are known to target Arabidopsis RIN4, a negative regulator of basal plant defense, for phosphorylation. We show that RIN4 phosphorylation activates RPM1. RIN4(142-176) is necessary and, with appropriate localization sequences, sufficient to support effector-triggered RPM1 activation, with the threonine residue at position 166 being critical. Phosphomimic substitutions at T166 cause effector-independent RPM1 activation. RIN4 T166 is phosphorylated in vivo in the presence of AvrB or AvrRpm1. RIN4 mutants that lose interaction with AvrB cannot be coimmunoprecipitated with RPM1. This defines a common interaction platform required for RPM1 activation by phosphorylated RIN4 in response to pathogenic effectors. Conservation of an analogous threonine across all RIN4-like proteins suggests a key function for this residue beyond the regulation of RPM1.

Interaction between estradiol and follicle-stimulating hormone promotes in vitro survival and development of caprine preantral follicles

The aim of this study was to investigate the effects of estradiol and follicle-stimulating hormone (FSH) on survival and growth of caprine preantral follicles. Pieces of ovarian tissue were cultured for 1 or 7 days in minimum essential medium (MEM) containing estradiol (1, 5, 10, 20 or 40 pg/ml), FSH (50 ng/ml), or a combination of the two hormones. Cultured and noncultured control ovarian tissues were processed for histological and ultrastructural studies. The results showed that after 7 days of culture, the treatments that yielded the highest percentage of normal follicles relative to MEM alone were those that combined FSH with estradiol at 1, 5 or 20 pg/ml. The addition of FSH to 1-day cultures containing 1 pg/ml estradiol or to 7-day cultures with 1 or 5 pg/ml estradiol increased the percentage of normal follicles compared to estradiol alone at the same concentrations. After 7 days of culture, all treatments generated higher percentages of developing follicles as compared to control and MEM alone. The addition of either FSH or 10 pg/ml of estradiol to the culture media or estradiol (1, 5, 10 or 20 pg/ml) and FSH in combination significantly increased follicular diameter as compared with MEM alone following 7 days of culture. Ultrastructural studies confirmed follicular integrity after 7 days of culture in the presence of 1 pg/ml estradiol plus FSH. In conclusion, this study demonstrated that the interaction between estradiol and FSH maintains ultrastructural integrity and stimulates activation and further growth of cultured caprine preantral follicles.

Gene expression profiling during asexual development of the late blight pathogen Phytophthora infestans reveals a highly dynamic transcriptome

Much of the pathogenic success of Phytophthora infestans, the potato and tomato late blight agent, relies on its ability to generate from mycelia large amounts of sporangia, which release zoospores that encyst and form infection structures. To better understand these stages, Affymetrix GeneChips based on 15,650 unigenes were designed and used to profile the life cycle. Approximately half of P. infestans genes were found to exhibit significant differential expression between developmental transitions, with approximately (1)/(10) being stage-specific and most changes occurring during zoosporogenesis. Quantitative reverse-transcription polymerase chain reaction assays confirmed the robustness of the array results and showed that similar patterns of differential expression were obtained regardless of whether hyphae were from laboratory media or infected tomato. Differentially expressed genes encode potential cellular regulators, especially protein kinases; metabolic enzymes such as those involved in glycolysis, gluconeogenesis, or the biosynthesis of amino acids or lipids; regulators of DNA synthesis; structural proteins, including predicted flagellar proteins; and pathogenicity factors, including cell-wall-degrading enzymes, RXLR effector proteins, and enzymes protecting against plant defense responses. Curiously, some stage-specific transcripts do not appear to encode functional proteins. These findings reveal many new aspects of oomycete biology, as well as potential targets for crop protection chemicals.

Defense suppression by virulence effectors of bacterial phytopathogens

Phytopathogenic bacteria and plants are locked in molecular struggles that determine the outcome of an infection. Bacteria make effector molecules that can induce defenses if recognized by specific host resistance (R) proteins. In susceptible hosts, however, effectors frequently promote virulence by suppressing host defenses. Defense-inducing and defense-suppressing activities are often related, as virulence-associated host modifications can elicit R protein activation. Thus, understanding of how an effector elicits defenses can translate into understanding of how it promotes virulence and vice versa. To control host cell functions, such as defense gene expression and vesicle trafficking, effectors use various biochemical activities, including protein modification, transcriptional regulation, and hormone mimicry. Progress with individual effectors will lead to an integrated view of how the activities of a collection of effectors intersect with genetically variable host plants to regulate susceptibility and resistance.

Innate immunity in plants: a continuum of layered defenses

Plant responses to pathogenic invaders result from recognition of nonself elicitors. Host surveillance proteins activate distinct signaling pathways that induce partially overlapping defensive responses. Pathogen virulence is promoted by inhibition of these pathways. This evolutionary struggle has produced plant immune systems that rely on a continuum of layered defenses.

Two Pseudomonas syringae Type III Effectors Inhibit RIN4-Regulated Basal Defense in Arabidopsis

Plant cells have two defense systems that detect bacterial pathogens. One is a basal defense system that recognizes complex pathogen-associated molecular patterns (PAMPs). A second system uses disease-resistance (R) proteins to recognize type lll effector proteins that are delivered into the plant cell by the pathogen's type III secretion system. Here we show that these two pathways are linked. We find that two Pseudomonas syringae type III effectors, AvrRpt2 and AvrRpm1, inhibit PAMP-induced signaling and thus compromise the host's basal defense system. RIN4 is an Arabidopsis protein targeted by AvrRpt2 and AvrRpm1 for degradation and phosphorylation, respectively. We find that RIN4 is itself a regulator of PAMP signaling. The R proteins, RPS2 and RPM1, sense type III effector-induced perturbations of RIN4. Thus, R proteins guard the plant against type III effectors that inhibit PAMP signaling and provide a mechanistic link between the two plant defense systems.

Patterns of diversifying selection in the phytotoxin-like scr74 gene family of Phytophthora infestans

Phytophthora infestans, the organism responsible for the Irish famine, causes late blight, a re-emerging disease of potato and tomato. Little is known about the molecular evolution of P. infestans genes. To identify candidate effector genes (virulence or avirulence genes) that may have co-evolved with the host, we mined expressed sequence tag (EST) data from infection stages of P. infestans for secreted and potentially polymorphic genes. This led to the identification of scr74, a gene that encodes a predicted 74-amino acid secreted cysteine-rich protein with similarity to the Phytophthora cactorum phytotoxin PcF. The expression of scr74 was upregulated approximately 60-fold 2 to 4 days after inoculation of tomato and was also significantly induced during early stages of colonization of potato. The scr74 gene was found to belong to a highly polymorphic gene family within P. infestans with 21 different sequences identified. Using the approximate and maximum likelihood (ML) methods, we found that diversifying selection likely caused the extensive polymorphism observed within the scr74 gene family. Pairwise comparisons of 17 scr74 sequences revealed elevated ratios of nonsynonymous to synonymous nucleotide-substitution rates, particularly in the mature region of the proteins. Using ML, all 21 polymorphic amino acid sites were identified to be under diversifying selection. Of these 21 amino acids, 19 are located in the mature protein region, suggesting that selection may have acted on the functional portions of the proteins. Further investigation of gene copy number and organization revealed that the scr74 gene family comprises at least three copies located in a region of no more than 300 kb of the P. infestans genome. We found evidence that recombination contributed to sequence divergence within at least one gene locus. These results led us to propose an evolutionary model that involves gene duplication and recombination, followed by functional divergence of scr74 genes. This study provides support for using diversifying selection as a criterion for identifying candidate effector genes from sequence databases.