Supercontraction forces in spider dragline silk depend on hydration rate

Spider dragline silk is a model biological polymer for biomimetic research due to its many desirable and unusual properties. ‘Supercontraction’ describes the dramatic shrinking of dragline silk fibers when wetted. In restrained silk fibers, supercontraction generates substantial stresses of 40–50 MPa above a critical humidity of 70% relative humidity (RH). This stress may maintain tension in webs under the weight of rain or dew and could be used in industry for robotics, sensor technology, and other applications. Our own findings indicate that supercontraction can generate stress over a much broader range than previously reported, from 10 to 140 MPa. Here we show that this variation in supercontraction stress depends upon the rate at which the environment reaches the critical level of humidity causing supercontraction. Slow humidity increase, over several minutes, leads to relatively low supercontraction stress, while fast humidity increase, over a few seconds, typically results in higher supercontraction stress. Slowly supercontracted fibers take up less water and differ in thermostability from rapidly supercontracted fibers, as shown by thermogravimetric analysis. This suggests that spider silk achieves different molecular configurations depending upon the speed at which supercontraction occurs. Ultimately, rate-dependent supercontraction may provide a mechanism to tailor the properties of silk or biomimetic fibers for various applications.     

Molecular insights into the phylogenetic structure of the spider genus Theridion (Araneae, Theridiidae) and the origin of the Hawaiian Theridion-like fauna

The Hawaiian happy face spider ( Theridion grallator Simon, 1900), named for a remarkable abdominal colour pattern resembling a smiling face, has served as a model organism for understanding the generation of genetic diversity. Theridion grallator is one of 11 endemic Hawaiian species of the genus reported to date. Asserting the origin of island endemics informs on the evolutionary context of diversification, and how diversity has arisen on the islands. Studies on the genus Theridion in Hawaii, as elsewhere, have long been hampered by its large size (> 600 species) and poor definition. Here we report results of phylogenetic analyses based on DNA sequences of five genes conducted on five diverse species of Hawaiian Theridion , along with the most intensive sampling of Theridiinae analysed to date. Results indicate that the Hawaiian Islands were colonised by two independent Theridiinae lineages, one of which originated in the Americas. Both lineages have undergone local diversification in the archipelago and have convergently evolved similar bizarre morphs. Our findings confirm para- or polyphyletic status of the largest Theridiinae genera: Theridion , Achaearanea and Chrysso . Convergent simplification of the palpal organ has occurred in the Hawaiian Islands and in two continental lineages. The results confirm the convergent evolution of social behaviour and web structure, both already documented within the Theridiidae. Greater understanding of phylogenetic relationships within the Theridiinae is key to understanding of behavioural and morphological evolution in this highly diverse group.     

Biogeography and diversification of hermit spiders on Indian Ocean islands (Nephilidae: Nephilengys)

The origin of the terrestrial biota of Madagascar and, especially, the smaller island chains of the western Indian Ocean is relatively poorly understood. Madagascar represents a mixture of Gondwanan vicariant lineages and more recent colonizers arriving via Cenozoic dispersal, mostly from Africa. Dispersal must explain the biota of the smaller islands such as the Comoros and the chain of Mascarene islands, but relatively few studies have pinpointed the source of colonizers, which may include mainland Africa, Asia, Australasia, and Madagascar. The pantropical hermit spiders (genus Nephilengys) seem to have colonized the Indian Ocean island arc stretching from Comoros through Madagascar and onto Mascarenes, and thus offer one opportunity to reveal biogeographical patterns in the Indian Ocean. We test alternative hypotheses on the colonization route of Nephilengys spiders in the Indian Ocean and simultaneously test the current taxonomical hypothesis using genetic and morphological data. We used mitochondrial (COI) and nuclear (ITS2) markers to examine Nephilengys phylogenetic structure with samples from Africa, southeast Asia, and the Indian Ocean islands of Madagascar, Mayotte, Réunion and Mauritius. We used Bayesian and parsimony methods to reconstruct phylogenies and haplotype networks, and calculated genetic distances and fixation indices. Our results suggest an African origin of Madagascar Nephilengys via Cenozoic dispersal, and subsequent colonization of the Mascarene islands from Madagascar. We find strong evidence of gene flow across Madagascar and through the neighboring islands north of it, while phylogenetic trees, haplotype networks, and fixation indices all reveal genetically isolated and divergent lineages on Mauritius and Réunion, consistent with female color morphs. These results, and the discovery of the first males from Réunion and Mauritius, in turn falsify the existing taxonomic hypothesis of a single widespread species, Nephilengys borbonica, throughout the archipelago. Instead, we diagnose three Nephilengys species: Nephilengys livida (Vinson, 1863) from Madagascar and Comoros, N. borbonica (Vinson, 1863) from Réunion, and Nephilengys dodo new species from Mauritius. Nephilengys followed a colonization route to Madagascar from Africa, and on through to the Mascarenes, where it speciated on isolated islands. The related golden orb-weaving spiders, genus Nephila, have followed the same colonization route, but Nephila shows shallower divergencies, implying recent colonization, or a moderate level of gene flow across the archipelago preventing speciation. Unlike their synanthropic congeners, N. borbonica and N. dodo are confined to pristine island forests and their discovery calls for evaluation of their conservation status.     

REEXAMINING THE RELATIONSHIP BETWEEN BODY SIZE AND TONAL SIGNALS FREQUENCY IN WHALES: A COMPARATIVE APPROACH USING A NOVEL PHYLOGENY

A negative relationship between cetacean body size and tonal sound minimum and maximum frequencies has been demonstrated in several studies using standard statistical approaches where species are considered independent data points. Such studies, however, fail to account for known dependencies among related species—shared similarity due to common ancestry. Here we test these hypotheses by generating the most complete species level cetacean phylogeny to date, which we then use to reconstruct the evolutionary history of body size and standard tonal sounds parameters (minimum, maximum, and center frequency). Our results show that when phylogenetic relationships are considered the correlation between body size (length or mass) and minimum frequency is corroborated with approximately 27% of the variation in tonal sound frequency being explained by body size compared to 86% to 93% explained when phylogenetic relationships are not considered. Central frequency also correlates with body size in toothed whales, but for other tonal sound frequency parameters, including maximum frequency, this hypothesized correlation disappears. Therefore, constraints imposed by body size seem to have played a role in the evolution of minimum frequency but alternative hypotheses are required to explain variation in maximum frequency.     

Can a spider web be too sticky? Tensile mechanics constrains the evolution of capture spiral stickiness in orb-weaving spiders

Owing to habitat loss and fragmentation, large mammal populations all over the world are becoming increasingly small and isolated. It is therefore a conservation priority to understand mechanisms influencing the demography of such populations, which can easily be driven to extinction. The Przewalski's horse Equus ferus przewalskii remains one of the world's most endangered species and reintroduced animals are still vulnerable. Over 9 years, we analysed factors affecting mortality and female fecundity at the individual level in a predator-free, closed population of Przewalski's horses, which grew from 11 to 55 individuals. Similar to other wild equids, the annual growth rate of the population was r =0.169. Typically, adult mortality was much lower than juvenile mortality, the latter being correlated with neither inbreeding coefficient of foals nor population density. We found no link between female fecundity and operational sex ratio of the herd, or inbreeding coefficient, lactation status and body condition of the mares. Although food therefore seemed not to be limiting in this population, density (number of horses ha⁻¹) clearly reduced fecundity, especially in subadult mares. Thus, our results show that space can slow the growth rate of a population before resources become limited, a potential source of concern for increasingly shrinking habitats of endangered large mammals. Possible mechanisms causing this may be found in incest avoidance or other social parameters. Finally, in large herbivores, population density is said to exert influence in a sequential order: juvenile survival first, followed by fecundity of young females, then adult females, and adult survival last. Although we observed no link between density and juvenile survival in the studied population, our results otherwise support this hypothesis.     

Is ACCTRAN better than DELTRAN?

When parsimony ancestral character reconstruction is ambiguous, it is often resolved in favour of the more complex character state. Hence, secondary loss (secondary “absence”) of a complex feature is favoured over parallel gains of that feature as this preserves the stronger hypothesis of homology. We believe that such asymmetry in character state complexity is important information for understanding character evolution in general. However, we here point out an inappropriate link that is commonly made between this approach and the accelerated transformation (ACCTRAN) algorithm. In ACCTRAN, changes are assigned along branches of a phylogenetic tree as close to the root as possible. This has been taken to imply that ACCTRAN will minimize hypotheses of parallel origins of complex traits and thus that ACCTRAN is philosophically better justified than the alternatives, such as delayed transformation (DELTRAN), where changes are assigned along branches as close to the tips as possible. We provide simple examples to show that such views are mistaken and that neither ACCTRAN nor DELTRAN consistently minimize parallel gain of complex traits. We therefore do not see theoretical grounds for favouring the popular ACCTRAN algorithm. © The Willi Hennig Society 2008.     

Dogs,cats, and kin: A molecular species-level phylogeny of Carnivora

Phylogenies underpin comparative biology as high-utility tools to test evolutionary and biogeographic hypotheses, inform on conservation strategies, and reveal the age and evolutionary histories of traits and lineages. As tools, most powerful are those phylogenies that contain all, or nearly all, of the taxa of a given group. Despite their obvious utility, such phylogenies, other than summary ‘supertrees’, are currently lacking for most mammalian orders, including the order Carnivora. Carnivora consists of about 270 extant species including most of the world’s large terrestrial predators (e.g., the big cats, wolves, bears), as well as many of man’s favorite wild (panda, cheetah, tiger) and domesticated animals (dog, cat). Distributed globally, carnivores are highly diverse ecologically, having occupied all major habitat types on the planet and being diverse in traits such as sociality, communication, body/brain size, and foraging ecology. Thus, numerous studies continue to address comparative questions within the order, highlighting the need for a detailed species-level phylogeny. Here we present a phylogeny of Carnivora that increases taxon sampling density from 28% in the most detailed primary-data study to date, to 82% containing 243 taxa (222 extant species, 17 subspecies). In addition to extant species, we sampled four extinct species: American cheetah, saber-toothed cat, cave bear and the giant short-faced bear. Bayesian analysis of cytochrome b sequences data-mined from GenBank results in a phylogenetic hypothesis that is largely congruent with prior studies based on fewer taxa but more characters. We find support for the monophyly of Carnivora, its major division into Caniformia and Feliformia, and for all but one family within the order. The only exception is the placement of the kinkajou outside Procyonidae, however, prior studies have already cast doubt on its family placement. In contrast, at the subfamily and genus level, our results indicate numerous problems with current classification. Our results also propose new, controversial hypotheses, such as the possible placement of the red panda (Ailuridae) sister to canids (Canidae). Our results confirm previous findings suggesting that the dog was domesticated from the Eurasian wolf (Canis lupus lupus) and are congruent with the Near East domestication of the cat. In sum, this study presents the most detailed species-level phylogeny of Carnivora to date and a much needed tool for comparative studies of carnivoran species. To demonstrate one such use, we perform a phylogenetic analysis of evolutionary distinctiveness (EDGE), which can be used to help establish conservation priorities. According with those criteria, and under one of the many possible sets of parameters, the highest priority Carnivora species for conservation of evolutionary diversity include: monk seals, giant and red panda, giant otter, otter civet, Owston’s palm civet, sea otter, Liberian mongoose, spectacled bear, walrus, binturong, and the fossa.     

Phylogenetic position and composition of Zygiellinae and Caerostris,with new insight into orb‐web evolution and gigantism

Orb‐weaving spiders are good objects for evolutionary research, but phylogenetic relationships among and within orb‐weaving lineages are poorly understood. Here we present the first species‐level molecular phylogeny that includes the enigmatic orb weavers ‘Zygiellidae’ and Caerostris. Zygiellidae is interesting for the evolution of the sector web, and Caerostris is noteworthy for web gigantism and extraordinary silk biomechanics. We assembled a molecular data set using mitochondrial (COI, 16S) and nuclear (H3, 18S, 28S, ITS2) gene fragments for 112 orbicularian exemplars, focusing on taxa with diverse web architecture and size. We show that ‘Zygiellidae’ contains the Holarctic Zygiella genus group (Leviellus, Parazygiella, Stroemiellus, and Zygiella) and the Australasian Phonognatha and Deliochus. As this clade is placed with Araneidae in all analyses we treat it as a subfamily, Zygiellinae. Using the new phylogeny, we show that the sector web evolved eight times, and coevolved with the silk tube retreat, but that these features are not zygielline synapomorphies. Zygiellinae, Caerostris, and some other araneids form a basal grade of araneids that differ from ‘classical’ araneids in web‐building and preying behaviour. We also confirm that Caerostris represents the most striking case of spider‐web gigantism. © 2015 The Linnean Society of London     

Shark tales: a molecular species-level phylogeny of sharks (Selachimorpha, Chondrichthyes)

Sharks are a diverse and ecologically important group, including some of the ocean's largest predatory animals. Sharks are also commercially important, with many species suffering overexploitation and facing extinction. However, despite a long evolutionary history, commercial, and conservation importance, phylogenetic relationships within the sharks are poorly understood. To date, most studies have either focused on smaller clades within sharks, or sampled taxa sparsely across the group. A more detailed species-level phylogeny will offer further insights into shark taxonomy, provide a tool for comparative analyses, as well as facilitating phylogenetic estimates of conservation priorities. We used four mitochondrial and one nuclear gene to investigate the phylogenetic relationships of 229 species (all eight Orders and 31 families) of sharks, more than quadrupling the number of taxon sampled in any prior study. The resulting Bayesian phylogenetic hypothesis agrees with prior studies on the major relationships of the sharks phylogeny; however, on those relationships that have proven more controversial, it differs in several aspects from the most recent molecular studies. The phylogeny supports the division of sharks into two major groups, the Galeomorphii and Squalimorphii, rejecting the hypnosqualean hypothesis that places batoids within sharks. Within the squalimorphs the orders Hexanchiformes, Squatiniformes, Squaliformes, and Pristiophoriformes are broadly monophyletic, with minor exceptions apparently due to missing data. Similarly, within Galeomorphs, the orders Heterodontiformes, Lamniformes, Carcharhiniformes, and Orectolobiformes are broadly monophyletic, with a couple of species 'misplaced'. In contrast, many of the currently recognized shark families are not monophyletic according to our results. Our phylogeny offers some of the first clarification of the relationships among families of the order Squaliformes, a group that has thus far received relatively little phylogenetic attention. Our results suggest that the genus Echinorhinus is not a squaliform, but rather related to the saw sharks, a hypothesis that might be supported by both groups sharing 'spiny' snouts. In sum, our results offer the most detailed species-level phylogeny of sharks to date and a tool for comparative analyses.