Quantitative analysis of dental microwear in threespine stickleback: a new approach to analysis of trophic ecology in aquatic vertebrates

1. The threespine stickleback Gasterosteus aculeatus is an important model organism in studies of genomic and phenotypic evolution, adaptation and speciation. Fossil Gasterosteus offer the potential to test models derived from studies of extant fishes over true evolutionary time-scales. Competition for food resources, for example, plays an important part in stickleback speciation, causing divergence in food gathering traits and ecological character displacement, but it is not possible to test this model in fossils because evidence of diet is almost never preserved. 2. We demonstrate here that quantitative analysis of dental microwear, a technique previously applied only to mammals, provides a reliable guide to the dietary preferences of stickleback. Teeth from stickleback raised under laboratory conditions exhibit microwear patterns that vary systematically according to substrate coarseness and whether fishes feed on Daphnia within the water column, or on chironomid larvae from the bottom. Furthermore, microwear data exhibit a progressive shift in their distribution that tracks differences in experimental feeding treatments. 3. Microwear in wild populations also exhibits a relationship with feeding. In blind assessments of trophic niche based on microwear patterns we were able to correctly assign all but one equivocal population to trophic group. Microwear data from wild stickleback exhibit a shift in distribution comparable with that observed across the range of treatments in the laboratory and these allow populations to be ranked according to the degree to which they approach fully benthic or fully limnetic feeding. 4. Our results demonstrate that microwear has the potential to be a powerful tool in the analysis of fish trophic ecology, particularly in the analysis of species pairs and niche differentiation. It has advantages over the trophic snapshot provided by analysis of stomach contents in that microwear reflects feeding and food preferences over a longer period of time, and can be applied where these data are unavailable. Furthermore, it is applicable to extinct organisms and fossils, allowing the role of trophic ecology, niche partitioning and competition over evolutionary time-scales to be investigated for the first time.     

Networks and dominance hierarchies: does interspecific aggression explain flower partitioning among stingless bees?

1. The distribution of consumers among resources (trophic interaction network) may be shaped by asymmetric competition. Dominance hierarchy models predict that asymmetric interference competition leads to a domination of high quality resources by hierarchically superior species. 2. In order to determine the competitive dominance hierarchy and its effect on flower partitioning in a local stingless bee community in Borneo, interspecific aggressions were tested among eight species in arena experiments. 3. All species tested were strongly mutually aggressive in the arena, and the observed interactions were often lethal for one or both opponents. Aggression significantly increased with body size differences between fighting pairs and was asymmetric: larger aggressors were superior over smaller species. Additional aggression tests involved dummies with surface extracts, and results suggest that species‐ and colony‐specific surface profiles are important in triggering the aggressive behaviour. 4. Sixteen stingless bee species were observed foraging on 41 species of flowering plants. The resulting bee–flower interaction network showed a high degree of generalisation (network‐level specialisation H₂’ = 0.11), corresponding to a random, opportunistic distribution of bee species among available flower species. 5. Aggressions on flowers were rare and only occurred at a low level. The dominance hierarchy obtained in the arena experiments did not correlate significantly with plant quality, estimated as the number of flowers per plant or as total bee visitation rate. 6. Our findings suggest that asymmetries in interference competition do not necessarily translate into actual resource partitioning in the context of complex interacting communities.     

The Evolutionary Emergence of Vertebrates From Among Their Spineless Relatives

The evolutionary origin of vertebrates has been debated ad nauseam by anatomists, paleontologists, embryologists, and physiologists, but it is only now that molecular phylogenetics is providing a more rigorous framework for the placement of vertebrates among their invertebrate relatives that we can begin to arrive at concrete conclusions concerning the nature of ancient ancestors and the sequence in which characteristic anatomical features were acquired. Vertebrates tunicates and cephalochordates together comprise the chordate phylum, which along with echinoderms and hemichordates constitute the deuterostomes. The origin of vertebrates and of jawed vertebrates is characterized by a doubling of the vertebrate genome, leading to hypotheses that this genomic event drove organismal macroevolution. However, this perspective of evolutionary history, based on living organisms alone, is an artifact. Phylogenetic trees that integrate fossil vertebrates among their living relatives demonstrate the gradual and piecemeal assembly of the gnathostome body plan. Unfortunately, it has not been possible to demonstrate gradual assembly of the vertebrate body plan. This is not because vertebrates are irreducibly complex but because many of the characters that distinguish vertebrates from invertebrates are embryological and cellular and, therefore, inherently unfossilizable.

East Coast fever: quantitative studies of Theileria parva in cattle

A series of experiments is described in which infective material obtained by grinding adult Rhipicephalus appendiculatus ticks containing mature Theileria parva parasites was titrated in East Coast fever-susceptible cattle. The reactions of the cattle to the various inocula, and the rate of multiplication of macroschizonts in their lymph nodes, were studied. In the final experiment, conclusive evidence was produced to support the observations of previous workers that the prepatent period, time to onset of febrile response, and time to death of the animals was dose-dependent, whereas the production of intraerythrocytic piroplasms was totally time-dependent. Furthermore, the effective rate of multiplication of macroschizonts was shown for the first time to be dose-dependent. It was not possible to detect macroschizonts before the fifth day after inoculation, and an occult phase of the parasite's life cycle, between the infective particle and the uninuclear macroschizont, is postulated. The discrepancies between the results of the present work and those of previous workers are discussed.     

Genome duplication, extinction and vertebrate evolution

Vertebrate evolution has been punctuated by three episodes of widespread gene or genome duplication, which have been linked with the origin of vertebrates, gnathostomes and teleosts, respectively. These three events coincide with bursts of character acquisition and increases in phenotypic complexity, and many researchers have suggested a causal relationship between the two. However, this pattern is derived from data for living taxa only; we argue here that, when fossils are taken into account, bursts of character acquisition disappear and gen(om)e duplication in vertebrate phylogeny can no longer be correlated with the origin of body plans. If patterns of character acquisition or morphological gaps between higher taxa are a reflection of phenotypic complexity, then more inclusive data sets incorporating fossil taxa provide no support for hypotheses linking gen(om)e duplications and the evolution of complexity in vertebrates.     

Architecture and functional morphology of the skeletal apparatus of ozarkodinid conodonts

Ozarkodinid conodonts were one of the most successful groups of agnathan vertebrates. Only the oropharyngeal feeding apparatus of conodonts was mineralized, and the skeletal elements were generally disarticulated on the death and decay of the body. Occasionally, however, they were preserved in association as ''natural assemblages'', fossilized in situ after post-mortem collapse of the apparatus. From analysis of element arrangement in natural assemblages of Idiognathodus from the Pennsylvanian of Illinois we have produced a precise scale model of the feeding apparatus of ozarkodinid conodonts. At the front lay an axial Sa element, flanked by two groups of four close-set elongate Sb and Sc elements which were inclined obliquely inwards and forwards; above these elements lay a pair of arched and inward pointing M elements. Behind the S-M array lay transversely oriented and bilaterally opposed Pb and Pa elements. Our model sheds new light on food acquisition in conodonts. We propose that the anterior S and M elements of ozarkodinid conodonts were attached to cartilaginous plates. In order for the animal to feed, these plates were first everted, and then drawn back and upward over the anterior edge of an underlying cartilage. These movements produced a highly effective grasping action, the cusps and denticles of the elements converging to grab and impale any food item that lay anterior to the open array. According to this hypothesis, the anterior part of the conodont apparatus is comparable to, and possibly homologous with, the lingual apparatus of extant agnathans; the elements themselves, however, have no direct homologues. <br>     

Testing hypotheses of element loss and instability in the apparatus composition of complex conodonts: articulated skeletons of Hindeodus

Knowledge of the conodont skeleton, in terms of the morphology of the elements and the positions they occupy, provides the foundation for understanding of homology, taxonomy and evolutionary relationships in conodonts. This knowledge also underpins analyses of conodont functional morphology and feeding. Direct evidence of skeletal anatomy and apparatus architecture comes from natural assemblages: fossils that preserve together the articulated remains of the conodont apparatus, either collapsed onto a bedding plane or as clusters of elements in which juxtaposed and overlapping elements have been fused together by diagenetic minerals. Here we describe six clusters of the biostratigraphically important conodont Hindeodus parvus from the Lower Triassic Shangsi section, Sichuan Province, South China. Five of these clusters represent the partial remains of articulated skeletons, providing direct evidence of the number and arrangement of elements in the apparatus. Combined with data from previously published natural assemblages this provides a test of the hypothesis that Triassic conodonts had a reduced dentition. Hindeodus parvus possessed a complete raptorial array of two M and nine S elements (unpaired S₀; symmetrically paired S₁, S₂, S₃, S₄); the paired P₁ locations were occupied by carminiscaphate elements, but the apparatus lacked P₂ elements. This is consistent with broader evidence for a particularly high degree of integration and constraint operating on the S–M array of morphologically complex conodonts, leading to conserved architecture of the array over a period of more than 250 million years. The loss of elements from the P domain implies a change in food processing ability and, given the predominance of data from P elements in conodont taxonomy and biostratigraphy, the hypothesis of element loss from the P domain has significant implications for the broader understanding of conodont diversity and evolutionary patterns.