Study on the Rectification of Ionic Diode Based on Cross-Linked Nanocellulose Bipolar Membranes.

Nanocellulose-based membranes have attracted intense attention in bioelectronic devices due to their low cost, flexibility, biocompatibility, degradability, and sustainability. Herein, we demonstrate a flexible ionic diode using a cross-linked bipolar membrane fabricated from positively and negatively charged cellulose nanofibrils (CNFs). The rectified current originates from the asymmetric charge distribution, which can selectively determine the direction of ion transport inside the bipolar membrane. The mechanism of rectification was demonstrated by electrochemical impedance spectroscopy with voltage biases. The rectifying behavior of this kind of ionic diode was studied by using linear sweep voltammetry to obtain current-voltage characteristics and the time dependence of the current. In addition, the performance of cross-linked CNF diodes was investigated while changing parameters such as the thickness of the bipolar membranes, the scanning voltage range, and the scanning rate. A good long-term stability due to the high density cross-linking of the diode was shown in both current-voltage characteristics and the time dependence of current.

The effect of crosslinking on ion transport in nanocellulose-based membranes.

Ion selective membranes are at the heart of energy conversion and harvesting, water treatment, and biotechnologies. The currently available membranes are mostly based on expensive and non-biodegradable polymers. Here, we report a cation-selective and low-cost membrane prepared from renewable nanocellulose and 1,2,3,4-butanetetracarboxylic acid which simultaneously serves as crosslinker and source of anionic surface groups. Charge density and structure of the membranes are studied. By using different degrees of crosslinking, simultaneous control over both the nanochannel structure and surface charge concentration is achieved, which in turn determines the resulting ion transport properties. Increasing negative charge concentration via higher crosslinker content, the obtained ion conductivity reaches up to 8 mS/cm (0.1 M KCl). Optimal ion selectivity, also influenced by the solution pH, is achieved at 20 wt% crosslinker addition (with ion conductivity of 1.6 mS/cm). As regular ~1.4 nm nanochannels were formed at this composition, nanofluidic contribution to ion transport is likely.

Radiotherapy regimens for rectal cancer: long-term outcomes and health-related quality of life in the Stockholm III trial.

BACKGROUND: The Stockholm III trial randomly assigned 840 patients to short-course radiotherapy of 5 × 5 Gy with surgery within 1 week (SRT), short-course radiotherapy of 5 × 5 Gy with surgery after 4-8 weeks (SRT-delay), or long-course radiotherapy of 25 × 2 Gy with surgery after 4-8 weeks (LRT-delay). This study details the long-term oncological outcomes and health-related quality of life (HRQoL). METHODS: Patients with biopsy-proven resectable adenocarcinoma of the rectum were included. Primary outcome was time to local recurrence (LR), and secondary endpoints were distant metastases (DMs), overall survival (OS), recurrence-free survival (RFS), and HRQoL. Patients were analysed in a three-arm randomization and a short-course radiotherapy comparison. RESULTS: From 1998 to 2013, 357, 355, and 128 patients were randomized to the SRT, SRT-delay, and LRT-delay groups respectively. Median follow-up time was 5.7 (range 5.3-7.6) years. Comparing patients in the three-arm randomization, the incidence of LR was three of 129 patients, four of 128, and seven of 128, and DM 31 of 129 patients, 38 of 128, and 38 of 128 in the SRT, SRT-delay, and LRT-delay groups respectively. In the short-course radiotherapy comparison, the incidence of LR was 11 of 357 patients and 13 of 355, and DM 88 of 357 patients and 82 of 355 in the SRT and SRT-delay groups respectively. No comparisons showed statistically significant differences. Median OS was 8.1 (range 6.9-11.2), 10.3 (range 8.2-12.8), and 10.5 (range 7.0-11.3) years after SRT, SRT-delay, and LRT-delay respectively. Median OS was 8.1 (range 7.2-10.0) years after SRT and 10.2 (range 8.5-11.7) years after SRT-delay. There were no statistically significant differences in HRQoL. CONCLUSION: After a follow-up of 5 years, delaying surgery for 4-8 weeks after radiotherapy treatment with 5 × 5 Gy was oncologically safe. Long-term HRQoL was similar among the treatment arms. TRIAL REGISTRATION NUMBER: NTC00904813.

The Use of Layer-by-Layer Self-Assembly and Nanocellulose to Prepare Advanced Functional Materials.

The current knowledge about the formation of layer-by-layer (LbL) self-assemblies using combinations of nanocelluloses (NCs) and polyelectrolytes is reviewed. Herein, the fundamentals behind the LbL formation, with a major focus on NCs, are considered. Following this, a special description of the limiting factors for the formation of LbLs of only NCs, both anionic and cationic, and the combination of NCs and polyelectrolytes/nanoparticles is provided. The ability of the NCs and polyelectrolytes to form dense films with excellent mechanical properties and with tailored optical properties is then reviewed. How low-density, wet stable networks of cellulose nanofibrils can be used as substrates for the preparation of antibacterial, electrically interactive, and fire-retardant materials by forming well-defined LbLs inside these networks is then considered. A short outlook of the possible uses of LbLs containing NCs is given to conclude.

Layer-by-Layer Self-Assembled Nanostructured Electrodes for Lithium-Ion Batteries.

Gaining control over the nanoscale assembly of different electrode components in energy storage systems can open the door for design and fabrication of new electrode and device architectures that are not currently feasible. This work presents aqueous layer-by-layer (LbL) self-assembly as a route towards design and fabrication of advanced lithium-ion batteries (LIBs) with unprecedented control over the structure of the electrode at the nanoscale, and with possibilities for various new designs of batteries beyond the conventional planar systems. LbL self-assembly is a greener fabrication route utilizing aqueous dispersions that allow various Li+ intercalating materials assembled in complex 3D porous substrates. The spatial precision of positioning of the electrode components, including ion intercalating phase and electron-conducting phase, is down to nanometer resolution. This capable approach makes a lithium titanate anode delivering a specific capacity of 167 mAh g-1 at 0.1C and having comparable performances to conventional slurry-cast electrodes at current densities up to 100C. It also enables high flexibility in the design and fabrication of the electrodes where various advanced multilayered nanostructures can be tailored for optimal electrode performance by choosing cationic polyelectrolytes with different molecular sizes. A full-cell LIB with excellent mechanical resilience is built on porous insulating foams.

Self-Assembled Polyester Dendrimer/Cellulose Nanofibril Hydrogels with Extraordinary Antibacterial Activity.

Cationic dendrimers are intriguing materials that can be used as antibacterial materials; however, they display significant cytotoxicity towards diverse cell lines at high generations or high doses, which limits their applications in biomedical fields. In order to decrease the cytotoxicity, a series of biocompatible hybrid hydrogels based on cationic dendrimers and carboxylated cellulose nanofibrils were easily synthesized by non-covalent self-assembly under physiological conditions without external stimuli. The cationic dendrimers from generation 2 (G2) to generation 4 (G4) based on trimethylolpronane (TMP) and 2,2-bis (methylol)propionic acid (bis-MPA) were synthesized through fluoride promoted esterification chemistry (FPE chemistry). FTIR was used to show the presence of the cationic dendrimers within the hybrid hydrogels, and the distribution of the cationic dendrimers was even verified using elemental analysis of nitrogen content. The hybrid hydrogels formed from G3 and G4 showed 100% killing efficiency towards Escherichia coli (E. coli), Staphylococcus aureus (S. aureus) and Pseudomonas aeruginosa (P. aeruginosa) with bacterial concentrations ranging from 105 CFU/mL to 107 CFU/mL. Remarkably, the hybrid hydrogels also showed good biocompatibility most probably due to the incorporation of the biocompatible CNFs that slowed down the release of the cationic dendrimers from the hybrid hydrogels, hence showing great promise as an antibacterial material for biomedical applications.

Dendritic Polyampholyte-Assisted Formation of Functional Cellulose Nanofibril Materials.

A new platform of functional hybrid materials from anionically charged high-aspect-ratio cellulose nanofibrils (CNFs) and a dendritic polyampholyte, Helux, is herein proposed. The polyampholytic character of Helux enabled facile and efficient nanoscale mixing with the CNFs, and the resulting composite mixtures of CNFs and Helux displayed thixotropic behavior and formed physical and reversibly cross-linked gels when left unperturbed for short spans of time. The gel could be chemically cross-linked into self-supporting solid hydrogels containing impressive water contents of 99.6% and a storage modulus of 1.8 kPa by thermal activation. Non-cross-linked mixtures of CNF/Helux were assembled into composites, such as films by solvent casting and aerogels with densities as low as 4 kg/m3 by lyophilizing ice-templated CNF/Helux mixtures. The resulting materials exhibited excellent wet stability due to the heat-activated cross-linking and were readily available for postfunctionalization via amidation chemistry using Helux-accessible amines in aqueous conditions. The mechanical performance of the films was not jeopardized by the addition of Helux. Additionally, by varying the amount of Helux, the compressive elastic modulus of aerogels was tunable in both the non-cross-linked and cross-linked states. The fast and efficient nanoscale mixing of anionic CNFs and a polymer containing cationic groups is unique, novel, and promising as a functional material platform. Sustainable CNFs guided by heterofunctional dendritic polyampholytes are envisaged to act as a pillar toward high-performance applications, including biomedicine and biomaterials.

Multifunctional Nanocomposites with High Strength and Capacitance Using 2D MXene and 1D Nanocellulose.

The family of two-dimensional (2D) metal carbides and nitrides, known as MXenes, are among the most promising electrode materials for supercapacitors thanks to their high metal-like electrical conductivity and surface-functional-group-enabled pseudocapacitance. A major drawback of these materials is, however, the low mechanical strength, which prevents their applications in lightweight, flexible electronics. A strategy of assembling freestanding and mechanically robust MXene (Ti3 C2 Tx ) nanocomposites with one-dimensional (1D) cellulose nanofibrils (CNFs) from their stable colloidal dispersions is reported. The high aspect ratio of CNF (width of ≈3.5 nm and length reaching tens of micrometers) and their special interactions with MXene enable nanocomposites with high mechanical strength without sacrificing electrochemical performance. CNF loading up to 20%, for example, shows a remarkably high mechanical strength of 341 MPa (an order of magnitude higher than pristine MXene films of 29 MPa) while still maintaining a high capacitance of 298 F g-1 and a high conductivity of 295 S cm-1 . It is also demonstrated that MXene/CNF hybrid dispersions can be used as inks to print flexible micro-supercapacitors with precise dimensions. This work paves the way for fabrication of robust multifunctional MXene nanocomposites for printed and lightweight structural devices.

Layer-by-Layer Assembly of High-Performance Electroactive Composites Using a Multiple Charged Small Molecule.

Layer-by-layer (LbL) assembly is a versatile tool for fabricating multilayers with tailorable nanostructures. LbL, however, generally relies on polyelectrolytes, which are mostly insulating and induce large interlayer distances. We demonstrate a method in which we replace polyelectrolytes with the smallest unit capable of LbL self-assembly: a molecule with multiple positive charges, tris(3-aminopropyl)amine (TAPA), to fabricate LbL films with negatively charged single-walled carbon nanotubes (CNTs). TAPA introduces less defects during the LbL build-up and results in more efficient assembly of films with denser micromorphology. Twenty bilayers of TAPA/CNT showed a low sheet resistance of 11 kΩ, a high transparency of 91% at 500 nm, and a high electronic conductivity of 1100 S/m on planar substrates. We also fabricated LbL films on porous foams with a conductivity of 69 mS/m and used them as electrodes for supercapacitors with a high specific capacitance of 43 F/g at a discharging current density of 1 A/g.

Tumour regression after radiotherapy for rectal cancer - Results from the randomised Stockholm III trial.

BACKGROUND AND PURPOSE: Neoadjuvant radiotherapy (RT) in rectal cancer induces tumour regression with a possible complete response (pCR). The optimal fractionation and timing to surgery is not established. The Stockholm III trial randomly assigned 840 patients to 5 × 5 Gy surgery within one week (SRT), 5 × 5 Gy with surgery after 4-8 weeks, and 2 Gy × 25 with surgery after 4-8 weeks (LRT-delay). The aim of this substudy was to assess tumour regression and correlation to survival. MATERIAL AND METHODS: All available microscopy slides were assessed by one pathologist, blinded to treatment, regarding tumour regression, graded according to the Dworak system (TRG), TNM-stage and other standard histopathology characteristics. Patients' data were collected from the Swedish ColoRectal Cancer Registry. Outcomes were TRG, pCR-rates, overall survival (OS) and time to recurrence (TTR). RESULTS: 318, 285 and 94 patients were included in the SRT, SRT-delay and LRT-delay groups. Median follow up was 5.7 years. There were significantly lower tumour stages after SRT-delay. pCR was seen in 1 (0.3%), 29 (10.4%) and 2 (2.2%) patients in SRT, SRT-delay and LRT-delay, respectively. The pCR and Dworak grade 4 were associated with superior survival. pCR vs no-pCR Hazard Ratio (95% Confidence Interval) OS: 0.51 (0.26-0.99) p = 0.046, TTR: 0.27 (0.09-0.86) p = 0.027. CONCLUSION: SRT-delay induces pCR in about 10% of the patients and is in this aspect superior to 25 × 2 Gy. A complete tumour response, TRG 4 using the Dworak system, or a pCR, is associated with superior OS and TTR.

Cross-Linked and Shapeable Porous 3D Substrates from Freeze-Linked Cellulose Nanofibrils.

Chemically cross-linked highly porous nanocellulose aerogels with complex shapes have been prepared using a freeze-linking procedure that avoids common post activation of cross-linking reactions and freeze-drying. The aerogel shapes ranged from simple geometrical three-dimensional bodies to swirls and solenoids. This was achieved by molding or extruding a periodate oxidized cellulose nanofibril (CNF) dispersion prior to chemical cross-linking in a regular freezer or by reshaping an already prepared aerogel by plasticizing the structure in water followed by reshaping and locking the aerogel into its new shape. The new shapes were most likely retained by new cross-links formed between CNFs brought into contact by the deformation during reshaping. This self-healing ability to form new bonds after plasticization and redrying also contributed to the mechanical resilience of the aerogels, allowing them to be cyclically deformed in the dry state, reswollen with water, and redried with good retention of mechanical integrity. Furthermore, by exploiting the shapeability and available inner structure of the aerogels, a solenoid-shaped aerogel with all surfaces coated with a thin film of conducting polypyrrole was able to produce a magnetic field inside the solenoid, demonstrating electromagnetic properties. Furthermore, by biomimicking the porous interior and stiff exterior of the beak of a toucan bird, a functionalized aerogel was created by applying a 300 µm thick stiff wax coating on its molded external surfaces. This composite material displayed a 10-times higher elastic modulus compared to that of the plain aerogel without drastically increasing the density. These examples show that it is possible to combine advanced shaping with functionalization of both the inner structure and the surface of the aerogels, radically extending the possible use of CNF aerogels.

Interfacial Polymerization of Cellulose Nanocrystal Polyamide Janus Nanocomposites with Controlled Architectures.

The widespread use of renewable nanomaterials has been limited due to poor integration with conventional polymer matrices. Often, chemical and physical surface modifications are implemented to improve compatibility, however, this comes with environmental and economic cost. This work demonstrates that renewable nanomaterials, specifically cellulose nanocrystals (CNCs), can be utilized in their unmodified state and presents a simple and versatile, one-step method to produce polyamide/CNC nanocomposites with unique Janus-like properties. Nanocomposites in the form of films, fibers, and capsules are prepared by dispersing as-prepared CNCs in the aqueous phase prior to the interfacial polymerization of aromatic diamines and acyl chlorides. The diamines in the aqueous phase not only serve as a monomer for polymerization, but additionally, adsorb to and promote the incorporation of CNCs into the nanocomposite. Regardless of the architecture, CNCs are only present along the surface facing the aqueous phase, resulting in materials with unique, Janus-like wetting behavior and potential applications in filtration, separations, drug delivery, and advanced fibers.

Genetically Engineered Mucoadhesive Spider Silk.

Mucoadhesion is defined as the adhesion of a material to the mucus gel covering the mucous membranes. The mechanisms controlling mucoadhesion include nonspecific electrostatic interactions and specific interactions between the materials and the mucins, the heavily glycosylated proteins that form the mucus gel. Mucoadhesive materials can be used to develop mucosal wound dressings and noninvasive transmucosal drug delivery systems. Spider silk, which is strong, biocompatible, biodegradable, nontoxic, and lightweight would serve as an excellent base for the development of such materials. Here, we investigated two variants of the partial spider silk protein 4RepCT genetically engineered in order to functionalize them with mucoadhesive properties. The pLys-4RepCT variant was functionalized with six cationically charged lysines, aiming to provide nonspecific adhesion from electrostatic interactions with the anionically charged mucins, while the hGal3-4RepCT variant was genetically fused with the Human Galectin-3 Carbohydrate Recognition Domain which specifically binds the mucin glycans Galß1-3GlcNAc and Galß1-4GlcNAc. First, we demonstrated that coatings, fibers, meshes, and foams can be readily made from both silk variants. Measured by the adsorption of both bovine submaxillary mucin and pig gastric mucin, the newly produced silk materials showed enhanced mucin binding properties compared with materials of wild-type (4RepCT) silk. Moreover, we showed that pLys-4RepCT silk coatings bind mucins through electrostatic interactions, while hGal3-4RepCT silk coatings bind mucins through specific glycan-protein interactions. We envision that the two new mucoadhesive silk variants pLys-4RepCT and hGal3-4RepCT, alone or combined with other biofunctional silk proteins, constitute useful new building blocks for a range of silk protein-based materials for mucosal treatments.

Sulfoethylated nanofibrillated cellulose: Production and properties.

Sulfoethylated nanofibrillated cellulose (NFCSulf) was produced by an industrially relevant process. The properties of the NFCSulf were compared with those of carboxymethylated nanofibrillated cellulose (NFCCarb), which has been identified as an attractive NFC for several industrial applications. The investigations revealed that NFCSulf is characterized by a higher degree of fibrillation and has superior redispersion properties. Furthermore, NFCSulf displays higher stability in varying pH values as compared to NFCCarb. Hence, NFCSulf may be a more attractive alternative than NFCCarb in applications such as rheological modifiers or adsorbing components in personal care products, in which the performance of NFC must remain unaffected in varying ambient conditions. The superior properties of NFCSulf compared to NFCCarb were proposed to be due to the combination of the unique chemical characteristics of the sulfoethylated reagent, and the larger size of the sulfonate group compared to the carboxymethyl group.

Optimal fractionation of preoperative radiotherapy and timing to surgery for rectal cancer (Stockholm III): a multicentre, randomised, non-blinded, phase 3, non-inferiority trial.

BACKGROUND: Radiotherapy reduces the risk of local recurrence in rectal cancer. However, the optimal radiotherapy fractionation and interval between radiotherapy and surgery is still under debate. We aimed to study recurrence in patients randomised between three different radiotherapy regimens with respect to fractionation and time to surgery. METHODS: In this multicentre, randomised, non-blinded, phase 3, non-inferiority trial (Stockholm III), all patients with a biopsy-proven adenocarcinoma of the rectum, without signs of non-resectability or distant metastases, without severe cardiovascular comorbidity, and planned for an abdominal resection from 18 Swedish hospitals were eligible. Participants were randomly assigned with permuted blocks, stratified by participating centre, to receive either 5 × 5 Gy radiation dose with surgery within 1 week (short-course radiotherapy) or after 4-8 weeks (short-course radiotherapy with delay) or 25 × 2 Gy radiation dose with surgery after 4-8 weeks (long-course radiotherapy with delay). After a protocol amendment, randomisation could include all three treatments or just the two short-course radiotherapy treatments, per hospital preference. The primary endpoint was time to local recurrence calculated from the date of randomisation to the date of local recurrence. Comparisons between treatment groups were deemed non-inferior if the upper limit of a double-sided 90% CI for the hazard ratio (HR) did not exceed 1·7. Patients were analysed according to intention to treat for all endpoints. This study is registered with ClinicalTrials.gov, number NCT00904813. FINDINGS: Between Oct 5, 1998, and Jan 31, 2013, 840 patients were recruited and randomised; 385 patients in the three-arm randomisation, of whom 129 patients were randomly assigned to short-course radiotherapy, 128 to short-course radiotherapy with delay, and 128 to long-course radiotherapy with delay, and 455 patients in the two-arm randomisation, of whom 228 were randomly assigned to short-course radiotherapy and 227 to short-course radiotherapy with delay. In patients with any local recurrence, median time from date of randomisation to local recurrence in the pooled short-course radiotherapy comparison was 33·4 months (range 18·2-62·2) in the short-course radiotherapy group and 19·3 months (8·5-39·5) in the short-course radiotherapy with delay group. Median time to local recurrence in the long-course radiotherapy with delay group was 33·3 months (range 17·8-114·3). Cumulative incidence of local recurrence in the whole trial was eight of 357 patients who received short-course radiotherapy, ten of 355 who received short-course radiotherapy with delay, and seven of 128 who received long-course radiotherapy (HR vs short-course radiotherapy: short-course radiotherapy with delay 1·44 [95% CI 0·41-5·11]; long-course radiotherapy with delay 2·24 [0·71-7·10]; p=0·48; both deemed non-inferior). Acute radiation-induced toxicity was recorded in one patient (<1%) of 357 after short-course radiotherapy, 23 (7%) of 355 after short-course radiotherapy with delay, and six (5%) of 128 patients after long-course radiotherapy with delay. Frequency of postoperative complications was similar between all arms when the three-arm randomisation was analysed (65 [50%] of 129 patients in the short-course radiotherapy group; 48 [38%] of 128 patients in the short-course radiotherapy with delay group; 50 [39%] of 128 patients in the long-course radiotherapy with delay group; odds ratio [OR] vs short-course radiotherapy: short-course radiotherapy with delay 0·59 [95% CI 0·36-0·97], long-course radiotherapy with delay 0·63 [0·38-1·04], p=0·075). However, in a pooled analysis of the two short-course radiotherapy regimens, the risk of postoperative complications was significantly lower after short-course radiotherapy with delay than after short-course radiotherapy (144 [53%] of 355 vs 188 [41%] of 357; OR 0·61 [95% CI 0·45-0·83] p=0·001). INTERPRETATION: Delaying surgery after short-course radiotherapy gives similar oncological results compared with short-course radiotherapy with immediate surgery. Long-course radiotherapy with delay is similar to both short-course radiotherapy regimens, but prolongs the treatment time substantially. Although radiation-induced toxicity was seen after short-course radiotherapy with delay, postoperative complications were significantly reduced compared with short-course radiotherapy. Based on these findings, we suggest that short-course radiotherapy with delay to surgery is a useful alternative to conventional short-course radiotherapy with immediate surgery. FUNDING: Swedish Research Council, Swedish Cancer Society, Stockholm Cancer Society, and the Regional Agreement on Medical Training and Clinical Research in Stockholm.

Intertidal zone management in the Western Indian Ocean: assessing current status and future possibilities using expert opinions.

This expert opinion study examined the current status of the intertidal zone in the Western Indian Ocean (WIO) and ranked and discussed future management approaches. Information was gathered from scientists, practitioners, and managers active in the WIO region through a questionnaire and a workshop. The experts stated that the productive intertidal environment is highly valuable for reasons such as recreation, erosion protection, and provision of edible invertebrates and fish. Several anthropogenic pressures were identified, including pollution, harbor activities, overexploitation, and climate change. The experts considered the WIO intertidal zone as generally understudied, undermanaged, and with poor or no monitoring. The most important management strategies according to the expert opinions are to develop and involve local people in integrated coastal zone management (ICZM), to increase knowledge on species-environment relationships, and to develop awareness campaigns and education programs. To improve coastal environmental management and conservation, we argue that the intertidal zone should be treated as one organizational management unit within the larger framework of ICZM.

Patchiness and co-existence of indigenous and invasive mussels at small spatial scales: the interaction of facilitation and competition.

Ecological theory predicts that two species with similar requirements will fail to show long-term co-existence in situations where shared resources are limiting, especially at spatial scales that are small relative to the size of the organisms. Two species of intertidal mussels, the indigenous Perna perna and the invasive Mytilus galloprovincialis, form mixed beds on the south coast of South Africa in a situation that has been stable for several generations of these species, even though these populations are often limited by the availability of space. We examined the spatial structure of these species where they co-exist at small spatial scales in the absence of apparent environmental heterogeneity at two sites, testing: whether conspecific aggregation of mussels can occur (using spatial Monte-Carlo tests); the degree of patchiness (using Korcak B patchiness exponent), and whether there was a relationship between percent cover and patchiness. We found that under certain circumstances there is non-random conspecific aggregation, but that in other circumstances there may be random distribution (i.e. the two species are mixed), so that spatial patterns are context-dependent. The relative cover of the species differed between sites, and within each site, the species with higher cover showed low Korcak B values (indicating low patchiness, i.e. the existence of fewer, larger patches), while the less abundant species showed the reverse, i.e. high patchiness. This relationship did not hold for either species within sites. We conclude that co-existence between these mussels is possible, even at small spatial scales because each species is an ecological engineer and, while they have been shown to compete for space, this is preceded by initial facilitation. We suggest that a patchy pattern of co-existence is possible because of a balance between direct (competitive) and indirect (facilitative) interactions.

THE MAINTENANCE OF A CLINE IN THE MARINE SNAIL LITTORINA SAXATILIS: THE ROLE OF HOME SITE ADVANTAGE AND HYBRID FITNESS.

Steep clinal transitions in one or several inherited characters between genetically distinct populations are usually referred to as hybrid zones. Essentially two different mechanisms may maintain steep genetic clines. Either selection acts against hybrids that are unfit over the entire zone due to their mixed genetic origin (endogenous selection), or hybrids and parental types attain different fitness values in different parts of the cline (exogenous selection). Survival rate estimates of hybrids and parental forms in different regions of the cline may be used to distinguish between these models to assess how the cline is maintained. We used reciprocal transplants to test the relative survival rates of two parental ecotypes and their hybrids over microscale hybrid zones in the direct-developing marine snail Littorina saxatilis (Olivi) on the rocky shores of Galicia, Spain. One of the parental forms occupies upper and the other lower shores, and the hybrids are found at various proportions (1-38%) along with both parental forms in a midshore zone a few meters wide. The survival rate over one month was 39-52% of the native ecotype on upper shores, but only 2-8% for the lower-shore ecotype. In contrast, only 4-8% of the upper-shore ecotype but 53% of large (> 6 mm) and 8% of small (3-6 mm) native lower-shore ecotype survived in the lower shores. In the midshores, both the two parental ecotypes and the hybrids survived about equally well. Thus there is a considerable advantage for the native ecotypes in the upper and lower shores, while in the hybrid zone none of the morphs, hybrids included, are favored. This indicates that the dimorphism of L. saxatilis is maintained by steep cross-shore selection gradients, thus supporting the selection-gradient model of hybrid zones. We performed field and laboratory experiments that suggest physical factors and predation as important selective agents. Earlier studies indicate assortative mating between the two ecotypes in the midshore. This is unexpected in a hybrid zone maintained by selection gradients, and it seems as if the reproductive barrier compresses the hybrid zone considerably.