Evolution of cyclic sexual reproduction under host-parasite interactions

Alternate changes of sexual and asexual generations was studied theoretically by numerical analyses, from the viewpoint of host-parasite infective interactions. The host species was considered a diploid organism, characterized by two loci determining parasite resistance type and sexual strategy, between which a linkage exists to a certain degree. The sexual reproductive strategy was assumed to be determined by three alleles: asexual, complete sexual and cyclic sexual alleles. The effect of the parasites was represented by decreasing fitness functions of each host-type frequency. Numerical analyses of various linkages between genes and frequency dependence of host fitness revealed that the dynamics varied depending on whether the cyclic sexual allele was dominant or recessive against the complete sexual allele. When the cyclic sexual allele was dominant, the cyclic sexual strain persisted under intermediate frequency dependence of host fitness. On the other hand, when the cyclic sexual allele was recessive, it always tended to spread and to be maintained in the population. In such situations, both complete and cyclic sexual strains coexist, but the asexual strain is lost.     

Structure of a gametocyte protein essential for sexual development in Plasmodium falciparum

Malaria transmission is dependent on the development of sexual forms of Plasmodium falciparum, called gametocytes, in the vertebrate host. Pfg27 is an abundantly expressed sexual stage-specific protein that is essential for gametocytogenesis in P. falciparum. We describe the crystal structure of Pfg27, which reveals a novel fold composed of two pseudo dyad-related repeats of the helix-turn-helix motif. Structurally equivalent helices of each repeat either form a dimer interface or interact with RNA in vitro. One side of the dimer presents an unprecedented juxtaposition of four polyproline (PXXP) motifs. Preliminary binding data indicate that these sites are capable of binding Src homology-3 (SH3) modules. Molecular modeling suggests that the dimer can accommodate two SH3 modules simultaneously, potentially enabling molecular crosstalk between SH3-containing proteins. The structural and initial biochemical evidence suggests that Pfg27 may serve as a platform for RNA and SH3 binding.     

Gamete interruptus; a novel calcium-dependent kinase is essential for malaria sexual reproduction

Malaria parasites undergo sexual fertilization minutes after the bloodmeal enters the mosquito midgut. In this issue of Cell, Billker et al. (2004) describe a new Plasmodium calcium-dependent protein kinase essential for gamete formation, and show that it is required for parasite transmission.     

The requirements for sexual reproduction

Summary Sexual reproduction requires controls for gonadogenesis, genital differentiation, and sexuality. The initiating event is induction of testicular differentiation by an effect of the H-Y locus or a combination of X- and Y-borne genes. This paper reviews the evidence that testosterone, either directly or via regulated conversion to other steroids, controls sexuality as it does male genital differentiation. The point is also stressed that, despite their difference, sexual reproduction and individual homcostasis are intimately linked.     

Arabinogalactan proteins as molecular markers in Arabidopsis thaliana sexual reproduction

Some of the most important changes that occur in plants during sexual reproduction involve the transition from a sporophytic to a gametophytic type of development. In this paper, these changes were evaluated for Arabidopsis thaliana. The results obtained clearly show differences in the pattern of distribution of specific arabinogalactan protein (AGP) sugar epitopes, during anther and ovule development. AGPs are hydroxyproline-rich glycoproteins that are massively glycosylated and ubiquitous in plants. The molecular mechanism of action of AGPs is still unknown, mainly due to the difficulties posed by the complex saccharide chains. However, the complex structure of the sugar fraction of AGPs makes them a potential source of signalling molecules. The selective labelling obtained with AGP mAbs JIM8, JIM13, MAC207, and LM2, during Arabidopsis pollen and pistil development, suggests that some AGPs can work as markers for gametophytic cell differentiation. Specific labelling of the first gametophytic cells in the pistil, the strong labelling of the secretory cells of the embryo sac, the synergid cells, and the labelling of the integument micropylar cells, apparently outlining the pollen tube pathway into its final target, the embryo sac, have all been shown. In the anthers, the specific labelling of gametophytic cells, and of the male gametes that travel along the pollen tube, may indicate AGP epitopes acting as signals for the pollen tube to reach its final destiny. The specific labelling of cells destined to go into programmed cell death is also discussed.     

Potassium flux in the pollen tubes was essential in plant sexual reproduction

Potassium channels are controlling K⁺ transport across plasma membrane and thus playing a central role in all aspects of osmolarity as well as numerous other functions in plants, including in sexual reproduction. We have used whole-cell and single-channel patch-clamp recording techniques investigated the regulation of intracellular free Ca²⁺-activated outward K⁺ channels in Pyrus pyrifolia pollen tube protoplasts. We have also showed the channels could be inhibited by heme and activated carbon monoxide (CO). In the presence of oxygen and NADPH, hemoxygenases catalyzes heme degradation, producing biliverdin, iron and CO. Considered the oxygen concentration approaching zero in the ovary, the heme will inhibit the K⁺ outward flux from the intracellular of pollen tube, increasing the pollen tubes osmolarity, inducing pollen tube burst. Here we discuss the putative role of K⁺ channels in plant sexual reproduction.Key words: pear, pollen, K⁺ channels, heme, carbon monoxideIon channels in the pollen tube play critical roles in mediation pollen germination and pollen tube growth.^1–3 Early studies were focus on the plasma membrane calcium channel regulation and cytosolic free calcium concentration variation in the pollen tube reason by which was one of the most important second messengers in plants.^3–7 However, reports have also showed that the potassium channels in the pollen tubes were also involved in several important steps of plant sexual reproduction.^8–19 Recently, more reports further demonstrated this phenomena.^20–24 In the report by Lu et al. they demonstrated that two cation/proton exchangers (CHX), CHX21 and CHX23, are essential for pollen tube growth guidance in Arabidopsis.²² chx21 chx23 double mutant induces the fertility impaired, but which is unchanged in both single chx21 or chx23 mutants. They have also found that the double mutant pollen grains germination and pollen tube growth in the transmitting tract were not difference with the wild-type, however, the double mutant pollen tubes fail to turn toward ovules.²² Protein localization experiments show CHX23 is expressed in the endoplasmic reticulum of pollen tubes; functional analysis results showed that CHX23 as a K⁺ transporter mediates K⁺ uptake in a pH-dependent manner. So, these protein affect the signal transduction pathway of pollen tube growth toward to the ovule by controlling the cation balance and pH in the pollen tube.²² Amien et al. identified a signaling ligand of defensin-like (DEFL) protein, ZmES4, which expressed in maize synergid. ZmES4 activates the maize pollen tube tip plasma membrane K⁺ Shaker channel KZM1.²⁰ This finding is also very interesting. Pollen tube bursting suggested to be based on the osmotic stress; the influx of K⁺ mediated by ZmES4-activated KZM1 will trigger rapid plasma membrane depolarization, which induced the pollen tube tip burst.²⁰ Furthermore, the osmotic increasing induced by too much K⁺ in the cytosolic of pollen tube was not only resulted by inward K⁺ channel activation, but also resulted by outward K⁺ channel inhibition in the pollen tube plasma membrane. In our report, we find a intracellular Ca²⁺-sensitive outward K⁺ channel in pear pollen tube plasma membrane, which could be inhibited by heme and activated by heme oxidative production, carbon monoxide (CO), may play a functional role in the pollen tube brusting.²³In the presence of oxygen and NADPH, hemoxygenases catalyzes heme degradation, producing biliverdin, iron and CO.²⁵ Early reports showed that oxygen plays an important role in plant sexual reproduction. Pollen tubes grow through the style toward the ovary with high speed, a process that consumes tremendous amounts of energy and requires rapid oxygen uptake by pollen tubes.²⁶ Pollen grains have roughly 20 times the level of mitochondria and respire 10 times faster than vegetative tissue.^12,27–29 Furthermore, oxygen has been proposed as a possible cue for pollen-tube guidance.³⁰ Indeed, the existence of an oxygen gradient in the unpollinated style has been shown in some species such as Hipeastrum hybridum. Oxygen pressure is high in the stigma and style but suddenly decreases at the base of the style, approaching zero in the ovary. Moreover, pollen-tube growth itself creates hypoxic regions within the style.³¹ Therefore, we suggest that the outward K⁺ channel inhibited by heme is dominant compared with which activated by CO when pollen tubes reach the ovary, based on where the hypoxic condition (Fig. 1). However, the gene encode the outward K⁺ channel in the pear pollen tube remains to be determined in the further study.Open in a separate windowFigure 1Reciprocal regulation of heme and carbon monoxide in putative Ca²⁺-activated outward K⁺ channel. Under normal condition, in the presence of NADPH, heme is metabolized by hemeoxygenase to generate carbon monoxide (CO), which activates outward K⁺ channel. However, without the oxygen, heme cannot be metabolized. The accumulated heme acts as an inhibitor of outward K⁺ channel, even in the presence of NADPH. The accumulated K⁺ in the cytosolic of pollen will induced the pollen tube depolarized, then burst.     

Character compatibility of molecular markers to distinguish asexual and sexual reproduction

Particularly in polyploids, the potential of the high variability of dominant markers such as random amplified polymorphic DNA fragments (RAPDs) and amplified fragment length polymorphisms (AFLPs) in population genetic studies and analysis of breeding systems is reduced due to their dominant nature. In contrast, the criterion of character compatibility is hindered neither by dominance nor by polyploidy as allelic interpretation is not necessary. Character compatibility, which can be used to detect events of genetic exchange (or recombination), is particularly informative if these events are expected to be rare such as in taxa with extensive vegetative reproduction or apomixis. Binary unordered characters such as presence and absence of anonymous DNA markers are incompatible if all four pairwise combinations of character states are present among the individuals studied. Because incompatible character state distributions defy any progenitor–derivative relationship among individuals, they provide strong evidence for genetic exchange. Both the absolute number of incompatible character combinations and the probability of compatibility can be used as a measure of incompatibility. Although these measures may not directly relate to the frequency of genetic exchange, they provide a useful tool to heuristically explore data sets. The most commonly used input for multivariate analyses and analysis of molecular variance in population genetic studies of (dis)similarity of marker distributions are amalgamates of mutation and recombination. Character compatibility can be used to complement these traditional methods of analysis. Advantages and disadvantages of character incompatibility relative to multilocus analysis of modes of reproduction and population genetics are demonstrated with data from RAPDs, isozymes, and restriction fragment length polymorphisms (RFLPs) of the nuclear ribosomal and chloroplast genome.     

The Tangled Bank: The maintenance of sexual reproduction through competitive interactions

The maintenance of sexual reproduction is discussed using a model based on the familiar Lotka-Volterra competition equations. Both the equilibrium and the stability conditions that allow a sexual population to resist invasion by a single asexual clone are considered. The equilibrium conditions give results similar to previous models: When the cost of sex, within phenotype niche width, and environmental variance are low, the sexual population coexists with the asexual clone and remains at a high density. However, the asexual clone is never completely excluded. Analysis of the stability conditions shows a different picture: The introduction of an asexual clone considerably reduces the stability of the community. However, owing to its larger total niche width, the sexual population exists partly in a “competitor-free space” where the asexual clone has almost no influence on the outcome of the interactions. Therefore the asexual clone is less stable than the sexual population and has a higher probability of extinction. In contrast, the sexual population does not become extinct, since the extreme phenotypes remain at a stable, though low, density, and the central phenotypes, where stability is low, are recreated every generation through recombination. I therefore conclude that the ecological conditions under which sexual reproduction is favored over asexual reproduction are fairly easily attained and are more general than previous analyses had suggested.     

Advantages of sexual reproduction

Despite the obvious efficiencies of many forms of asexual reproduction, sexual reproduction abounds. Asexual species, for the most part, are relatively short-lived offshoots of sexual ancestors. From the nineteenth century, it has been recognized that, since there is no obvious advantage to the individuals involved, the advantages of sexual reproduction must be evolutionary. Furthermore, the advantage must be substantial; for example, producing males entails a two-fold cost, compared to dispensing with them and reproducing by parthenogenetic females. There are a large number of plausible hypotheses. To me the most convincing of these are two. The first hypothesis, and the oldest, is that sexual reproduction offers the opportunity to produce recombinant types that can make the population better able to keep up with changes in the environment. Although the subject of a great deal of work, and despite its great plausibility, the hypothesis has been very difficult to test by critical observations or experiments. Second, species with recombination can bunch harmful mutations together and eliminate several in a single “genetic death.” Asexual species, can eliminate them only in the same genotype in which they occurred. If the rate of occurrence of deleterious mutations is one or more per zygote, some mechanism for eliminating them efficiently must exist. A test of this mutation load hypothesis for sexual reproduction, then, is to find whether deleterious mutation rates in general are this high-as Drosophila data argue. Unfortunately, although molecular and evolutionary studies can give information on the total mutation rate, they cannot determine what fraction are deleterious. In addition, there are short discussions of the advantages of diploidy, anisogamy, and separate sexes. © 1994 Wiley-Liss, Inc.     

Plasmodium sexual stage antigens

The sexual stages of the malaria parasite are highly specialized cells adapted to withstand major environmental changes during their development. They also induce immune responses that may affect the outcome of the infection in the mosquito. In this review Pietro Alano considers the nature and the role of the antigens expressed by Plasmodium sexual stages.     

Targeting protein-protein interactions for parasite control

Finding new drug targets for pathogenic infections would be of great utility for humanity, as there is a large need to develop new drugs to fight infections due to the developing resistance and side effects of current treatments. Current drug targets for pathogen infections involve only a single protein. However, proteins rarely act in isolation, and the majority of biological processes occur via interactions with other proteins, so protein-protein interactions (PPIs) offer a realm of unexplored potential drug targets and are thought to be the next-generation of drug targets. Parasitic worms were chosen for this study because they have deleterious effects on human health, livestock, and plants, costing society billions of dollars annually and many sequenced genomes are available. In this study, we present a computational approach that utilizes whole genomes of 6 parasitic and 1 free-living worm species and 2 hosts. The species were placed in orthologous groups, then binned in species-specific orthologous groups. Proteins that are essential and conserved among species that span a phyla are of greatest value, as they provide foundations for developing broad-control strategies. Two PPI databases were used to find PPIs within the species specific bins. PPIs with unique helminth proteins and helminth proteins with unique features relative to the host, such as indels, were prioritized as drug targets. The PPIs were scored based on RNAi phenotype and homology to the PDB (Protein DataBank). EST data for the various life stages, GO annotation, and druggability were also taken into consideration. Several PPIs emerged from this study as potential drug targets. A few interactions were supported by co-localization of expression in M. incognita (plant parasite) and B. malayi (H. sapiens parasite), which have extremely different modes of parasitism. As more genomes of pathogens are sequenced and PPI databases expanded, this methodology will become increasingly applicable.     

The analysis of desensitizing CNGA1 channels reveals molecular interactions essential for normal gating

The pore region of cyclic nucleotide–gated (CNG) channels acts as the channel gate. Therefore, events occurring in the cyclic nucleotide–binding (CNB) domain must be coupled to the movements of the pore walls. When Glu363 in the pore region, Leu356 and Thr355 in the P helix, and Phe380 in the upper portion of the S6 helix are mutated into an alanine, gating is impaired: mutant channels E363A, L356A, T355A, and F380A desensitize in the presence of a constant cGMP concentration, contrary to what can be observed in wild-type (WT) CNGA1 channels. Similarly to C-type inactivation of K⁺ channels, desensitization in these mutant channels is associated with rearrangements of residues in the outer vestibule. In the desensitized state, Thr364 residues in different subunits become closer and Pro366 becomes more accessible to extracellular reagents. Desensitization is also observed in the mutant channel L356C, but not in the double-mutant channel L356C+F380C. Mutant channels L356F and F380K did not express, but cGMP-gated currents with a normal gating were observed in the double-mutant channels L356F+F380L and L356D+F380K. Experiments with tandem constructs with L356C, F380C, and L356C+F380C and WT channels indicate that the interaction between Leu356 and Phe380 is within the same subunit. These results show that Leu356 forms a hydrophobic interaction with Phe380, coupling the P helix with S6, whereas Glu363 could interact with Thr355, coupling the pore wall to the P helix. These interactions are essential for normal gating and underlie the transduction between the CNB domain and the pore.     

Targeting the Plasmodium vivax Duffy-binding protein

Plasmodium vivax requires interaction with the Duffy antigen receptor for chemokines (DARC) to enable its invasion of human erythrocytes. Interaction with DARC is mediated by the P. vivax Duffy-binding protein (PvDBP) and is essential for junction formation, which is a key step in the invasion process. The receptor-binding domain of PvDBP maps to a conserved cysteine-rich region, referred to as region II (PvRII). Here, we review data on the interaction of PvRII with DARC and explore the potential of targeting this crucial receptor-ligand interaction to develop new intervention strategies against P. vivax.     

Selective advantage for sexual reproduction with random haploid fusion

This article develops a simplified set of models describing asexual and sexual replication in unicellular diploid organisms. The models assume organisms whose genomes consist of two chromosomes, where each chromosome is assumed to be functional if it is equal to some master sequence σ₀, and non-functional otherwise. We review the previously studied case of selective mating, where it is assumed that only haploids with functional chromosomes can fuse, and also consider the case of random haploid fusion. When the cost for sex is small, as measured by the ratio of the characteristic haploid fusion time to the characteristic growth time, we find that sexual replication with random haploid fusion leads to a greater mean fitness for the population than a purely asexual strategy. However, independently of the cost for sex, we find that sexual replication with a selective mating strategy leads to a higher mean fitness than the random mating strategy. The results of this article are consistent with previous studies suggesting that sex is favored at intermediate mutation rates, for slowly replicating organisms, and at high population densities. Furthermore, the results of this article provide a basis for understanding sex as a stress response in unicellular organisms such as Saccharomyces cerevisiae (Baker’s yeast).     

Experimental evidence for the adaptive value of sexual reproduction

It is generally believed that recombination by sexual reproduction is unfavourable in constant environments but is of adaptive value under changing environmental conditions. To test this theory, experimental populations of yeast (Saccharomyces cerevisiae) were set up and maintained at different levels of environmental heterogeneity. Recombination was estimated by determining sporulation rates. Sporulation rates first increased in populations living in highly variable environments, but after some time began to decrease. The decrease started last and was slowest in populations which were maintained under the same conditions for a sufficiently long time, to allow some adaptation of the gene pool to the respective environment. Patterns of genotypic variability could not be interpreted in such simple terms, but there was a statistically significant correlation between sporulation rate and genotypic variability. This correlation is to be expected because recombination generates genotypic variability. Summing up, recombination by sexual reproduction is advantageous in changing environments if the population can track the changes in the environment by changing its genotypic structure.     

Small Molecule Screen for Candidate Antimalarials Targeting Plasmodium Kinesin-5

Plasmodium falciparum and vivax are responsible for the majority of malaria infections worldwide, resulting in over a million deaths annually. Malaria parasites now show measured resistance to all currently utilized drugs. Novel antimalarial drugs are urgently needed. The Plasmodium Kinesin-5 mechanoenzyme is a suitable “next generation” target. Discovered via small molecule screen experiments, the human Kinesin-5 has multiple allosteric sites that are “druggable.” One site in particular, unique in its sequence divergence across all homologs in the superfamily and even within the same family, exhibits exquisite drug specificity. We propose that Plasmodium Kinesin-5 shares this allosteric site and likewise can be targeted to uncover inhibitors with high specificity. To test this idea, we performed a screen for inhibitors selective for Plasmodium Kinesin-5 ATPase activity in parallel with human Kinesin-5. Our screen of nearly 2000 compounds successfully identified compounds that selectively inhibit both P. vivax and falciparum Kinesin-5 motor domains but, as anticipated, do not impact human Kinesin-5 activity. Of note is a candidate drug that did not biochemically compete with the ATP substrate for the conserved active site or disrupt the microtubule-binding site. Together, our experiments identified MMV666693 as a selective allosteric inhibitor of Plasmodium Kinesin-5; this is the first identified protein target for the Medicines of Malaria Venture validated collection of parasite proliferation inhibitors. This work demonstrates that chemical screens against human kinesins are adaptable to homologs in disease organisms and, as such, extendable to strategies to combat infectious disease.