Joseph C. Spagna

Phylogeny of entelegyne spiders: Affinities of the family Penestomidae (NEW RANK), generic phylogeny of Eresidae, and asymmetric rates of change in spinning organ evolution (Araneae, Araneoidea, Entelegynae)

Penestomine spiders were first described from females only and placed in the family Eresidae. Discovery of the male decades later brought surprises, especially in the morphology of the male pedipalp, which features (among other things) a retrolateral tibial apophysis (RTA). The presence of an RTA is synapomorphic for a large clade of spiders exclusive of Eresidae. A molecular data matrix based on four loci was constructed to test two alternative hypotheses: (1) penestomines are eresids and the RTA is convergent, or (2) penestomines belong within the RTA clade. Taxon sampling concentrated on the Eresidae and the RTA clade, especially outside of the Dionycha and Lycosoidea. Evolution of the cribellum, conventionally characterized as a primitive araneomorph spinning organ lost multiple times, is explored. Parsimony optimization indicates repeated appearances of the cribellum. Exploration of asymmetric rates of loss and gain in both a likelihood framework and using a Sankoff matrix under parsimony reveals that cribellum homology is supported when losses are two times more likely than gains. We suggest that when complicated characters appear (under parsimony optimization) to evolve multiple times, investigators should consider alternative reconstructions featuring a relatively high rate of loss. Evolution of other morphological characters is also investigated. The results imply revised circumscription of some RTA-clade families, including Agelenidae, Amaurobiidae, Cybaeidae, Dictynidae and Hahniidae. Some nomenclatural changes are formally proposed here; others await further investigation. The family Penestomidae (NEW RANK) is established. Tamgrinia, not Neoramia, is the cribellate sister clade of the ecribellate Agelenidae. Tamgrinia and the subfamily Coelotinae are transferred from the family Amaurobiidae to the family Agelenidae. Zanomys and its relatives are not coelotines but belong to a clade tentatively identified as Macrobuninae.

The evolution of genome size in ants

BackgroundDespite the economic and ecological importance of ants, genomic tools for this family (Formicidae) remain woefully scarce. Knowledge of genome size, for example, is a useful and necessary prerequisite for the development of many genomic resources, yet it has been reported for only one ant species (Solenopsis invicta), and the two published estimates for this species differ by 146.7 Mb (0.15 pg).ResultsHere, we report the genome size for 40 species of ants distributed across 10 of the 20 currently recognized subfamilies, thus making Formicidae the 4th most surveyed insect family and elevating the Hymenoptera to the 5th most surveyed insect order. Our analysis spans much of the ant phylogeny, from the less derived Amblyoponinae and Ponerinae to the more derived Myrmicinae, Formicinae and Dolichoderinae. We include a number of interesting and important taxa, including the invasive Argentine ant (Linepithema humile), Neotropical army ants (genera Eciton and Labidus), trapjaw ants (Odontomachus), fungus-growing ants (Apterostigma, Atta and Sericomyrmex), harvester ants (Messor, Pheidole and Pogonomyrmex), carpenter ants (Camponotus), a fire ant (Solenopsis), and a bulldog ant (Myrmecia). Our results show that ants possess small genomes relative to most other insects, yet genome size varies three-fold across this insect family. Moreover, our data suggest that two whole-genome duplications may have occurred in the ancestors of the modern Ectatomma and Apterostigma. Although some previous studies of other taxa have revealed a relationship between genome size and body size, our phylogenetically-controlled analysis of this correlation did not reveal a significant relationship.ConclusionThis is the first analysis of genome size in ants (Formicidae) and the first across multiple species of social insects. We show that genome size is a variable trait that can evolve gradually over long time spans, as well as rapidly, through processes that may include occasional whole-genome duplication. The small genome sizes of ants, combined with their ecological, evolutionary and agricultural importance, suggest that some of these species may be good candidates for future whole-genome sequencing projects.

Patterns of habitat affinity and Austral/Holarctic parallelism in dictynoid spiders (Araneae:Entelegynae)

The ability to survive in a terrestrial environment was a major evolutionary hurdle for animals that, once passed, allowed the diversification of most arthropod and vertebrate lineages. Return to a truly aquatic lifestyle has occurred only rarely among terrestrial lineages, and is generally associated with modifications of the respiratory system to conserve oxygen and allow extended periods of apnea. Among chelicerates, in particular spiders, where the circulatory system also serves as a hydrostatic skeleton, very few taxa have exploited aquatic environments, though these environments are abundant and range from freshwater ponds to the marine intertidal and relictual (salt) lakes. The traditional systematic positions of the taxa inhabiting these environments are controversial. Partitioned Bayesian analysis using a doublet model for stems in the nearly complete 18S rRNA gene (~1800 nt) and in the D2 and D3 regions of the 28S rRNA gene (~690 nt), and standard models for loops and full protein-coding histone H3 (349 nt) partitions (totalling 3133 bp when aligned) of dictynoid spiders and related lineages revealed that the only truly aquatic spider species, Argyroneta aquatica (Clerck, 1767) (Cybaeidae Banks, 1892), belongs in a clade containing other taxa with unusual habitat affinities related to an aquatic existence, including occupation of semi-aquatic (intertidal) areas (Desidae Pocock, 1985: Paratheuma spp.) and highly alkaline salt-crusts (Dictynidae O. Pickard-Cambridge, 1871: Saltonia incerta (Banks, 1898)). In a contrasting pattern, other spiders that also occupy intertidal zones, including some other members of the family Desidae (Desis spp., Badumna longinqua (L. Koch, 1867)), are an independently derived clade found primarily in the southern hemisphere. Use of the doublet model reduced some branch-support values in the single-gene trees for rRNA data, but resulted in a robust combined-data phylogeny from 18S rRNA, 28S rRNA, and histone H3. This combination of results – reduction in support in single-gene trees and gain in support in combined-data trees –is consistent with use of the doublet model reducing problematic signal from non-independent base pairs in individual data partitions, resulting in improved resolution in the combined-data analyses.

Terrestrial locomotion in arachnids

In this review, we assess the current state of knowledge on terrestrial locomotion in Arachnida. Arachnids represent a single diverse (>100,000 species) clade containing well-defined subgroups (at both the order and subordinal levels) that vary morphologically around a basic body plan, yet exhibit highly disparate limb usage, running performance, and tarsal attachment mechanisms. Spiders (Araneae), scorpions (Scorpiones), and harvestmen (Opiliones) have received the most attention in the literature, while some orders have never been subject to rigorous mechanical characterization. Most well-characterized taxa move with gaits analogous to the alternating tripod gaits that characterize fast-moving Insecta - alternating tetrapods or alternating tripods (when one pair of legs is lifted from the ground for some other function). However, between taxa, there is considerable variation in the regularity of phasing between legs. Both large and small spiders appear to show a large amount of variation in the distribution of foot-ground contact, even between consecutive step-cycles of a single run. Mechanisms for attachment to vertical surfaces also vary, and may depend on tufts of adhesive hairs, fluid adhesives, silks, or a combination of these. We conclude that Arachnida, particularly with improvements in microelectronic force sensing technology, can serve as a powerful study system for understanding the kinematics, dynamics, and ecological correlates of sprawled-posture locomotion.

Gait characteristics of two fast-running spider species (Hololena adnexa and Hololena curta), including an aerial phase (Araneae: Agelenidae)

Abstract Funnel-web spinning spiders of the genus Hololena are capable of fast movements in a horizontal plane across a variety of challenging surfaces. We used two species, H. curta (McCook 1894) and H. adnexa (Chamberlin & Gertsch 1929), in experiments designed to reveal how they achieve remarkable speeds, occasionally exceeding 70 body lengths (∼50 cm) per second. In high-speed recordings we found that spiders used their legs in alternating sets of four, distributed in staggered pairs along the body axis, resulting in an alternating-tetrapod gait. Increases in speed showed positive linear relationships with both frequency and stride length. There were also inverse, linear relationships in both species between speed and duty factor, meaning that increases in speed are associated with a decrease in the relative amount of time spent by the legs on the ground during each full leg cycle. By examining their duty factor vs. speed regressions, we found that spiders of both species were capable of aerial phases during high-speed running, with the transitional speed occurring at an average of 54 body lengths per second. We conclude that further experimentation with high-speed spiders and insects will likely show that a variety of species exhibits dynamically stable locomotion, including aerial phases.

Evidence of behavioral co-option from context-dependent variation in mandible use in trap-jaw ants (Odontomachus spp.)

Evolutionary co-option of existing structures for new functions is a powerful yet understudied mechanism for generating novelty. Trap-jaw ants of the predatory genus Odontomachus are capable of some of the fastest self-propelled appendage movements ever recorded; their devastating strikes are not only used to disable and capture prey, but produce enough force to launch the ants into the air. We tested four Odontomachus species in a variety of behavioral contexts to examine if their mandibles have been co-opted for an escape mechanism through ballistic propulsion. We found that nest proximity makes no difference in interactions with prey, but that prey size has a strong influence on the suite of behaviors employed by the ants. In trials involving a potential threat (another trap-jaw ant species), vertical jumps were significantly more common in ants acting as intruders than in residents (i.e. a dangerous context), while horizontal jumps occurred at the same rate in both contexts. Additionally, horizontal jump trajectories were heavily influenced by the angle at which the substrate was struck and appear to be under little control by the ant. We conclude that while horizontal jumps may be accidental side-effects of strikes against hard surfaces, vertical escape jumps are likely intentional defensive behaviors that have been co-opted from the original prey-gathering and food-processing functions of Odontomachus jaws.

Finding an upper limit for gap costs in direct optimization parsimony

Exploring a large number of parameter sets in sensitivity analyses of direct optimization parsimony can be costly in terms of time and computing resources, and there is little a priori guidance available for reasonable limits to these search parameters. For this reason, we sought a general‐purpose upper limit for gap costs in the direct optimization program POY to streamline this process. To test the performance of POY as gap costs increase, we simulated data onto a pre‐set topology using a GTR + I + G model modified to include gaps by adding them according to a negative‐binomial model. Gaps were then removed and the data were analysed in POY at increasing gap costs. Increasing gap costs consistently resulted in reduced phylogenetic accuracy across trees of different relative branch lengths. Decoupling gap insertion and gap extension costs recovered a fraction of the accuracy lost by having both high gap insertion and gap extension costs, but only in trees with long internal nodes. To determine whether loss of phylogenetic accuracy was node‐specific, we designed a small dataset with a constrained node, where all possible combinations of cost substitution and different percentages of gap versus nucleotide changes were explored. These analyses showed that the effects of gap insertion and extension are node‐specific, and the minimum threshold for convergence on gap‐supported nodes is similar to the threshold for accuracy loss found in the larger simulated datasets. Subsequent analyses of empirical data revealed that a similar pattern of loss with gap cost increase can occur with ribosomal genes (18S, 28S, 16S and 12S) but this pattern was not seen in the intron data (myoglobin II) examined. In conjunction with previously published congruence‐based studies, the results suggest that POY sensitivity analyses can be streamlined and made more accurate if gap insertion and extension costs follow, as a guideline, a limit of four times the highest base‐transformation cost.