Role of genetic control and self-assembly in gametophyte sporoderm ontogeny: Hypotheses and experiment

A review of our own and literature data on the mechanisms of sporoderm (the wall of pollen grains and spores) development is presented in terms of colloidal interactions—the so-called micellar hypothesis (Gabarayeva and Hemsley, 2006; Hemsley and Gabarayeva, 2007), which suggests the participation of self-assembly processes in development. The development of exine (sporopollenin-containing part of the sporoderm) in five plant species from remote taxa has been traced in detail and interpreted as a micellar sequence. An experimental modeling of exine-like structures carried out in vitro, in which physicochemical patterns of colloidal systems (hydrophobic interactions) were the driving force, is strong evidence for the relevance of the micellar hypothesis and the promising nature of these studies. The correlation between the role of genomic control and self-assembly in the development of complex biological walls is discussed.     

Exine and tapetum development in Symphytum officinale (Boraginaceae). Exine substructure and its interpretation

After detailing the exine ontogeny, our purpose was to find out whether the sequence of sporoderm developmental events corresponds to self-assembling micellar mesophases, initiated by genomically determined physicochemical parameters and induced by surfactant glycoproteins at increasing concentrations. Indeed, a scaffolding of the future exine, i.e., the glycocalyx, initiates with scattered clots, which then appear as clusters of spherical and worm-like micelles, derived from surface-active glycoproteins. At the middle tetrad stage, a continuous layer of the glycocalyx emerges, consisting of parallel, tightly packed cylinder-like units, which we interpret as a layer of cylindrical micelles, the so-called middle mesophase. These units bear dark-contrasted particles, arranged in strings or columns. These sites of the glycocalyx units?Cmicelles accumulate initial sporopollenin, hence the term ??sporopollenin acceptor particles?? (SAPs). This process leads to the appearance of procolumellae at the late tetrad stage. The glycocalyx units are rooted into callose and into the microspore cytoplasm. After formation of the tectum and the foot layer, the endexine initiates as a thin layer, and the latter develops into a very thick layer in the post-tetrad period. When callose disintegrates, ??bouquets?? of SAPs become evident on the tectum, which were evidently hidden inside the callose layer; these structures self-assemble into supratectal gemmae. An unusual, ??hybrid?? type of tapetum was observed. What is observed in Symphytum exine development allows us to obtain more evidence for the hypothesis of the participation of micellar self-assembly in sporoderm development and to bring together the concepts of micelles and of SAPs.     

Pollen wall and tapetum development in Plantago major L. (Plantaginaceae): assisting self-assembly

We have attempted to elucidate the underlying mechanisms of sporoderm development and pattern determination in Plantago major through a detailed ontogenetic study, using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). We aim to compare our observations and interpretation with those on other species. Our study of sporoderm development in Plantago from the early tetrad stage to mature pollen grains has shown that pure physical processes, including self-assembly, which are not under direct genetic control, play an important role and represent evidently one of the instruments of evolution. Our observations fit well with the sequence of self-assembling micellar mesophases and show reiteration of some of them, confirming our self-assembly hypothesis. Some attention was also paid to the possible role of rough and smooth endoplasmic reticulum in the cortical cytoplasm of the developing microspores. The tapetum and Ubisch bodies development are also traced. The importance of detailed ontogenetic studies for understanding the establishment of complex pollen walls in any species and for understanding mechanisms underlying sporoderm development was demonstrated. We also present a simulation, obtained in vitro experiments by self-assembly, mimicking pollen grain of Plantago major. It is clear that, in pollen wall development, biological processes and purely physical factors work in tandem.     

I. Primexine development in Passiflora racemosa Brot.: overlooked aspects of development

Developmental stages during the tetrad period were examined in detail by transmission electron microscopy with an emphasis on substructure. Our purpose was to find out whether the sequence of sporoderm developmental events provides additional evidence for our recent hypothesis on the underlying cause of exine ontogeny as a sequence of self-assembling micellar mesophases initiated by genomically given physicochemical parameters. Osmiophilic globules encrusting the surface of postmeiotic microspores and tapetal cells are temporary prepattern units which come first. The second prepattern structures are highly ordered bundles of microfilaments and microtubules which determine the position of microspore surface invaginations and clusters of the glycocalyx inside them. The first glycocalyx units are microgranules which during the middle tetrad stage rearrange into radially oriented rod-like units. The latter form lens-like clusters of the glycocalyx-1, located inside the invaginations. These clusters predestine the position of the future luminae in the exine reticulum. The second glycocalyx layer is laid down as a continuous layer over the whole microspore surface and has similar substructure, that is radial rods. Glycocalyx-2 is a framework for procolumellae which appear at the late tetrad stage. Therefore, the sequence of substructural units in the primexine is: globules, microgranules, rod-like units, and layers of radially oriented rods tightly packed in the periplasmic space. This sequence corresponds to the first three mesophases of self-assembling micelles: spherical micelles, cylindrical micelles, and layers of hexagonally packed cylindrical micelles (middle mesophase). We observed the same sequence in other species during primexine development, and the findings of this study provide new evidence for our hypothesis.     

An integral insight into pollen wall development: involvement of physical processes in exine ontogeny in Calycanthus floridus L., with an experimental approach

We aimed to understand the underlying mechanisms of development in the sporopollenin-containing part of the pollen wall, the exine, one of the most complex cell walls in plants. Our hypothesis is that distinct physical processes, phase separation and micellar self-assembly, underpinexine development by taking the molecular building blocks, determined and synthesised by the genome, through several phase transitions. To test this hypothesis, we traced each stage of microspore development in Calycanthus floridus with transmission electron microscopy and then generated in vitro experimental simulations corresponding to every developmental stage. The sequence of structures observed within the periplasmic space around developing microspores starts with spherical units, which are rearranged into columns to then form rod-like units (the young columellae) and, finally, white line centred endexine lamellae. Phase separation precedes each developmental stage. The set of experimental simulations, obtained as self-assembled micellar mesophases formed at the interface between lipid and water compartments, was the same: spherical micelles; columns of spherical micelles; cylindrical micelles; and laminate micelles, separated by gaps, resembling white-lined lamellae. Thus, patterns simulating structures observed at the main stages of exine development in C. floridus were obtained from in vitro experiments, and hence purely physicochemical processes can construct exine-like patterns. This highlights the important part played by physical processes that are not under direct genomic control and share influence on the emerging ultrastructure with the genome during exine development. These findings suggest that a new approach to ontogenetic studies, including a consideration of physical factors, is required for a better understanding of developmental processes.     

II. Exine development in Passiflora racemosa Brot.: post-tetrad period. Overlooked aspects of development

Developmental stages during the post-tetrad period are examined in detail with TEM and SEM, with emphasis upon substructure. Our purpose was to find out whether the sequence of sporoderm developmental events gives additional evidence for our recent hypothesis on the underlying cause of exine ontogeny as a sequence of self-assembling micellar mesophases, initiated by genomically given physico-chemical parameters. Four different layers of the endexine are developed in the post-tetrad period. The first one is a layer of white line centered lamellae which appear as a demarcation line between ect- and endexine. The second layer is sponge-like and consists of “roots” of columellae and a layer between them. The third layer consists of basally disposed radially elongated granules which appear in the aperture sites only. The fourth layer emerges in interapertural sites only and is formed as stacks of uneven lamellae. Therefore, the sequence of substructural units in primexine is the next: white-lined lamellae, a layer of honeycombed substructure, globule-to-rod-like granules, stacks of wavy lamellae. These sequences correspond to the next four mesophases of self-assembling micelles: neat (=laminate) micelles, high-concentrated emulsion of sponge-like (=foam-like) substructure, spherical-to-cylindrical micelles, and laminate micelles with fenestrated laminae. Reiteration of the micellar mesophases, participating in endexine development, is observed during the post-tetrad period.     

Pollen wall ontogeny in <Emphasis Type="Italic">Polemonium caeruleum</Emphasis> (Polemoniaceae) and suggested underlying mechanisms of development

By a detailed ontogenetic study of Polemonium caeruleum pollen, tracing each stage of development at high TEM resolution, we aim to understand the establishment of the pollen wall and to unravel the mechanisms underlying sporoderm development. The main steps of exine ontogeny in Polemonium caeruleum, observed in the microspore periplasmic space, are spherical units, gradually transforming into columns, then to rod-like units (procolumellae), the appearance of the initial tectum, growth of columellae in height and tectum in thickness and initial sporopollenin accumulation on them, the appearance of the endexine lamellae and of dark-contrasted particles on the tectum, the appearance of a sponge-like layer and of the intine in aperture sites, the appearance of the foot layer on the base of the sponge-like layer and of spinules on the tectum, and massive sporopollenin accumulation. This sequence of developmental events fits well to the sequence of self-assembling micellar mesophases. This gives (together with earlier findings and experimental exine simulations) strong evidence that genome and self-assembly probably share control of exine formation. It is highly probable that self-assembly is an intrinsic instrument of evolution.     

Sporoderm development in Acer tataricum (Aceraceae): an interpretation

For the first time, the developmental events in the course of complicated exine structure establishment have been traced in detail with transmission electron microscope in the representative of Acer. A new look at unfolding events is suggested using the knowledge of a boundary field, colloid science. Our purpose was to find out whether the sequence of sporoderm developmental events represents, in essence, the sequence of self-assembling micellar mesophases, initiated by genomically given physicochemical parameters and induced by surfactant glycoproteins at increasing concentration. Indeed, the first units observed in the periplasmic space are globular ones (=spherical micelles) which become arranged into rod-like units (=cylindrical micelles). Then, twisted clusters of rodlets form a layer of procolumellae (middle micellar mesophase). The tectum emerges as an untwisting and merging of distal ends of procolumellae (distal untwist of micelle clusters). In the end of tetrad period, when a hydrophilic–hydrophobic switch occurs in the periplasmic space, the contrast reversal of the columellae corresponds to the change of normal micelles to reverse ones. The initiation of the foot layer and the endexine lamellae, with their typical central “white lines”, corresponds to the next—“neat”—mesophase, with its typical central gaps between layers. Aperture sites during development show all the main micellar mesophases and their transitional forms. The data received have supported our previous hypothesis.     

Developmental origins of structural diversity in pollen walls of Compositae

Compositae exhibit some of the most complex and diverse pollen grains in flowering plants. This paper reviews the evolutionary and developmental origins of this diversity in pollen structure using recent models based on the behaviour of colloids and formation of micelles in the differentiating microspore glycocalyx and primexine. The developmental model is consistent with observations of structures recovered by pollen wall dissolution. Pollen wall diversity in Compositae is inferred to result from small changes in the glycocalyx, for example ionic concentration, which trigger the self-assembly of highly diverse structures. Whilst the fine details of exine substructure are, therefore, not under direct genetic control, it is likely that genes establish differences in the glycocalyx which define the conditions for self-assembly. Because the processes described here for Compositae can account for some of the most complex exine structures known, it is likely that they also operate in pollen walls with much simpler organisation.     

The not‐so‐Irish spurge: Euphorbia hyberna (Euphorbiaceae) and the Littletonian plant ‘steeplechase’

The disjunct distributions of the Lusitanian flora, which are found only in south‐west Ireland and northern Iberia, and are generally absent from intervening regions, have been of great interest to biogeographers. There has been much debate as to whether Irish populations represent relicts that survived the Last Glacial Maximum (LGM; approximately 21 kya), or whether they recolonized from southern refugia subsequent to the retreat of the ice and, if so, whether this occurred directly (i.e. the result of long distance dispersal) or successively (i.e. in the manner of a ‘steeplechase’, with the English Channel and Irish Sea representing successive ‘water‐jumps’ that have to be successfully crossed). In the present study, we used a combined palaeodistribution modelling and phylogeographical approach to determine the glacial history of the Irish spurge, Euphorbia hyberna, the sole member of the Lusitanian flora that is also considered to occur naturally in south‐western England. Our findings suggest that the species persisted through the LGM in several southern refugia, and that northern populations are the result of successive recolonization of Britain and Ireland during the postglacial Littletonian warm stage, akin to the ‘steeplechase’ hypothesis. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114 , 249–259.     

On the use of advanced modelling techniques to investigate the conformational discrepancy between two X-ray structures of the AppA BLUF domain

The conformation of the blue light utilising flavin domain of the signalling protein AppA in the dark state is a matter of intensive research, both experimental and theoretical, and has not yet unambiguously been determined. Two contradicting X-ray structures of the dark state have been published previously. We aim at resolving this seeming contradiction by exploring conformational pathways between the two X-ray structures using advanced modelling techniques, such as local-elevation searching and sampling, soft-core non-bonded interactions and protocols of successively biasing the sampling of sets of torsional angles which adopt different values in the two alternative X-ray structures. The results suggest a high energetic barrier for a change in the Trp104 side chain from a ‘Trp-in’ to a ‘Trp-out’ conformation or vice versa, and illustrate the complexity to model conformational transitions involving a large number of degrees of freedom.     

The Disruption Hypothesis Does Not Explain Mate‐Choice Copying in the Guppy (Poecilia reticulata)

In some species, female mate choice is non‐independent as, under certain circumstances, females may copy the mate choice of other nearby females. One standard experimental protocol used to test for mate‐choice copying is the mate‐choice ‘reversal’ protocol. In this protocol, a focal female is allowed to choose between two males as potential mates and then is presented with an opportunity to see another female (i.e. the model female) choose the male that she did not initially choose. The focal is subsequently allowed to again choose between the same set of males. An observed reversal of her initial choice in this second preference test has been previously interpreted as evidence for mate‐choice copying. Alternatively, it has recently been proposed that environmental events, such as seeing the mate choice of nearby females that occur within the visual field of a female actively engaged in mate assessment, may ‘disrupt’ her decision‐making behavior and consequently alter the consistency of her mating preference, and may thus cause mate‐choice reversals. The disruption hypothesis predicts that if a model female is placed near the male that the focal female initially chose, the latter's mate preference would be disrupted and she would subsequently and consistently prefer the male that she initially rejected. Here we examined whether the disruption hypothesis explains mate‐choice copying in the guppy (Poecilia reticulata). Our results do not support this hypothesis, but rather provide further support for mate‐choice copying in the guppy.     

The odontode explosion: The origin of tooth‐like structures in vertebrates

Essentially we show recent data to shed new light on the thorny controversy of how teeth arose in evolution. Essentially we show (a) how teeth can form equally from any epithelium, be it endoderm, ectoderm or a combination of the two and (b) that the gene expression programs of oral versus pharyngeal teeth are remarkably similar. Classic theories suggest that (i) skin denticles evolved first and odontode‐inductive surface ectoderm merged inside the oral cavity to form teeth (the ‘outside‐in’ hypothesis) or that (ii) patterned odontodes evolved first from endoderm deep inside the pharyngeal cavity (the ‘inside‐out’ hypothesis). We propose a new perspective that views odontodes as structures sharing a deep molecular homology, united by sets of co‐expressed genes defining a competent thickened epithelium and a collaborative neural crest‐derived ectomesenchyme. Simply put, odontodes develop ‘inside and out’, wherever and whenever these co‐expressed gene sets signal to one another. Our perspective complements the classic theories and highlights an agenda for specific experimental manipulations in model and non‐model organisms.     

The ecology of sex explains patterns of helping in arthropod societies

Across arthropod societies, sib‐rearing (e.g. nursing or nest defence) may be provided by females, by males or by both sexes. According to Hamilton's ‘haplodiploidy hypothesis’, this diversity reflects the relatedness consequences of diploid vs. haplodiploid inheritance. However, an alternative ‘preadaptation hypothesis’ instead emphasises an interplay of ecology and the co‐option of ancestral, sexually dimorphic traits for sib‐rearing. The preadaptation hypothesis has recently received empirical support, but remains to be formalised. Here, we mathematically model the coevolution of sex‐specific helping and sex allocation, contrasting these hypotheses. We find that ploidy per se has little effect. Rather, the ecology of sex shapes patterns of helping: sex‐specific preadaptation strongly influences who helps; a freely adjustable sex ratio magnifies sex biases and promotes helping; and sib‐mating, promiscuity, and reproductive autonomy also modulate the sex and abundance of helpers. An empirical survey reveals that patterns of sex‐specific helping in arthropod taxa are consistent with the preadaptation hypothesis.     

The Adaptive Significance of Avian Mobbing V. An Experimental Test of the ‘Move On’ Hypothesis

This paper presents the first experimental study of the effects of mobbing on predators. Two captive little owls Athene noctua and a tawny owl Strix aluco, having stabilized activity patterns and roosting sites, were singly confronted with one or two mobbing blackbirds Turdus merula. The little owls showed marked stress (gular fluttering, panting and panic flights), abandoned chosen roosts and remained unsettled for several days. A permanent alternative roost was chosen after one mobbing encounter, other encounters resulted in several days of ‘floating’ before returning to the original sites. One little owl used ‘camouflage’ by ‘freezing’ during repeated harassments, the other tended to retaliate if mobbing was close and weak. The tawny owl seemed to ignore the rather weak blackbird mobbing, but abandoned its roost for up to 36 h when mobbed by larger numbers of woodland birds at close range. It is concluded that mobbing causes profound distress, especially in small, vulnerable predators like the little owl and forces it to change its daytime roost. If abandoning the area is not possible, time and energy are expended in increased vigilance as a protection against further mobbing attacks. Thus there is strong support for the ‘Move on’ hypothesis. Results also support the ‘Perception advertisement’ hypothesis, implying that the detected hunter deliberately forfeits the surprise element. Which effect prevails depends on predator evaluation of the signals involved.     

综述——新型纳米生物材料的研发：聚乙二醇化磷脂胶束的纳米结构与功能

梁伟等采用一步自组装法制备粒径在20 nm左右、具有核-壳结构的聚乙二醇化磷脂(PEG-PE)胶束,药物装载后对胶束的粒径无明显影响,但显著提高了胶束的体内外稳定性,被装载的药物主要分布在胶束的核-壳界面处．研究表明药物的理化性质决定了其与载体之间的组装机制及体外药物释放的特性．在不影响细胞膜的完整性及通透性的情况下,PEG-PE胶束通过插膜提高了细胞膜的流动性,进而促进小分子药物的翻转过膜,增加药物的入胞量．与游离药物相比,装载化疗药的胶束可增强药物对肿瘤组织的渗透能力,显著抑制动物皮下移植瘤的生长,延长动物的生存时间．PEG-PE胶束还通过增加药物在淋巴组织中的分布,降低了动物转移模型中的淋巴转移,相应地减少了肿瘤的肺部转移．PEG-PE为美国食品药品管理局(FDA)批准的可用于人体的药物载体材料,具有良好的生物相容性与安全性．因此,PEG-PE胶束作为药物载体具有广阔的发展前景．     

A molecular analysis of relationships and biogeography within a species complex of Holarctic fish (genus Osmerus)

Episodes of trans-Arctic faunal exchange and isolation between the north Pacific and Atlantic ocean basins have been implicated as important historic geological events contributing to extant patterns of genetic diversity and structure in Holarctic faunas. We made a further test of the significance of such biogeographic events by examining mitochondrial DNA (mtDNA) restriction fragment length and cytochrome b sequence polymorphism among north Pacific and Arctic, north-western Atlantic (north-eastern North American), and north-eastern Atlantic (European) regional forms of the boreal smelt, genus Osmerus. Our analyses also assessed whether the regional forms within this ‘species complex’: (i) represent a single widely distributed and polytypic species, or is composed of three geographically distinct species, and (ii) resulted from a single split from north Pacific ancestral Osmerus or two independent Pacific-Atlantic divergences. MtDNA sequence divergence estimates among forms ranged from 5.6–8.9% and from 6.1–8.5% based on restriction fragment and 300 base pairs of cytochrome b sequencing, respectively. Divergence within forms averaged less than 0.5% for fragment analysis and no differences were detected from sequence analysis. Provisional dating of lineage separations in Osmerus based on our sequence divergence estimates suggested a mid-Pliocene to early Pleistocene time frame for diversification among the forms. These estimated lineage separation dates support the idea that geological events in ‘Beringia’ and the surrounding trans-Arctic area (e.g. opening of the Bering Seaway, Pleistocene glacial advances), occurring over a similar time frame, have influenced radiation in Osmerus. Phenetic and parsimony analyses of the sequence divergence estimates and of sequence polymorphisms suggested that the north Pacific/Arctic form and the northwestern Atlantic form shared a common ancestor more recently than either has with the north-eastern Atlantic form, thus supporting the hypothesis that the species complex has arisen from two independent Pacific-Atlantic divergences probably beginning during the mid-Pliocene.     

Compactness Determines the Success of Cube and Octahedron Self-Assembly

Nature utilizes self-assembly to fabricate structures on length scales ranging from the atomic to the macro scale. Self-assembly has emerged as a paradigm in engineering that enables the highly parallel fabrication of complex, and often three-dimensional, structures from basic building blocks. Although there have been several demonstrations of this self-assembly fabrication process, rules that govern a priori design, yield and defect tolerance remain unknown. In this paper, we have designed the first model experimental system for systematically analyzing the influence of geometry on the self-assembly of 200 and 500 µm cubes and octahedra from tethered, multi-component, two-dimensional (2D) nets. We examined the self-assembly of all eleven 2D nets that can fold into cubes and octahedra, and we observed striking correlations between the compactness of the nets and the success of the assembly. Two measures of compactness were used for the nets: the number of vertex or topological connections and the radius of gyration. The success of the self-assembly process was determined by measuring the yield and classifying the defects. Our observation of increased self-assembly success with decreased radius of gyration and increased topological connectivity resembles theoretical models that describe the role of compactness in protein folding. Because of the differences in size and scale between our system and the protein folding system, we postulate that this hypothesis may be more universal to self-assembling systems in general. Apart from being intellectually intriguing, the findings could enable the assembly of more complicated polyhedral structures (e.g. dodecahedra) by allowing a priori selection of a net that might self-assemble with high yields.     

When an epithelium ceases to exist – an ultrastructural study on the fate of the embryonic coelom in Epiperipatus biolleyi (Onychophora, Peripatidae)

It is an accepted fact that fusion between the coelomic cavities and the primary body cavity occurs during development in the Arthropoda. However, such a fusion is much disputed in the Onychophora. In order to clarify this subject, the fate of embryonic coelomic cavities has been studied in an onychophoran. Ultrastructural investigations in this paper provide evidence that embryonic coelomic cavities fuse with spaces of the primary body cavity in Epiperipatus biolleyi. During embryogenesis, the somatic and splanchnic portions of the mesoderm separate and the former coelomic linings are transformed into mesenchymatic tissue. The resulting body cavity therefore represents a mixture of primary and secondary (coelomic) body cavities, i.e. the ‘mixocoel’. The nephridial anlage is already present, when the ‘mixocoel’ is formed, although there is no trace of a sacculus yet. The lumen of the nephridial anlage, thus, communicates with the newly formed ‘mixocoel’. Accordingly, the lumen of the nephridial sacculus cannot be regarded as a kind of ‘persisting coelomic cavity’ in E. biolleyi. Our findings support the hypothesis that the ‘mixocoel’ was already present in the common stem species of the Onychophora and Euarthropoda.