Shell and hinge morphology of juvenile Limopsis (Bivalvia: Arcoida) - implications for limopsid evolution

Early ontogenetic shells of Limopsis angusta, L. aurita, L. cristata, L. friedbergi and L. minuta are described in detail and their morphogenetic traits discussed in a phylogenetic context. Prodissoconch length is found to range from 170-370 µm discounting the so-called prodissoconch 2 stage of authors. Reports of such a stage refer to the early postlarval 'interdissoconch'. Prodissoconch sizes are indicative of lecithotrophy but not of brooding. Limopsid-like prodissoconchs are rather common among pteriomorphs and at present of little use for phylogenetic interpretations, therefore. The early postlarval hinge is characterized by two tooth generations: an early one, (G1), representing the postlarval continuation of provincular teeth, and a late one, (G2), representing the independent adult dentition. This is a plesiomorphic trait for Pteriomorphia. The postlarval ligament is continuous with the larval resilium; it is neither co-functional with nor is it substituted by the typical duplivincular ligament as in other arcoids. Occasionally developed ridges, gutters or multiplication of ligament sublayers remain restricted to the primordial resilifer. It is concluded that the limopsid adult ligament represents a fusion of adult sublayer repetition within a retained larval resilium, a phenomenon best described as a heterochronic process. Comparisons with homologous characters of other arcoids support the view that Limopsidae evolved from parallelodontid arcoids and that they gave rise to Philobryidae but not to Glycymerididae.     

Evolution of parental care and ovulation behavior in oysters

Approximately half of all living oysters brood offspring in the inhalant chamber of their mantle cavities; the remainder are broadcast spawners which do not engage in parental care of young. Ostreid ovulation involves a complex behavioral sequence that results in the countercurrent passage of newly spawned eggs through the gills (ctenidia) and into the inhalant chamber. We constructed molecular and combined-evidence phylogenetic trees to test hypotheses concerning the directionality of parental care evolution, and the evolutionary significance of the trans-ctenidial ovulation pathway, in the Ostreidae. Representatives of all three ostreid subfamilies, together with gryphaeid and nonostreoidean pterioid outgroups, were sequenced for a 941-nucleotide fragment of the 28S ribosomal gene. Our phylogenetic analyses indicate that (1) the Ostreidae are robustly monophyletic, (2) broadcast spawning and larval planktotrophy are ancestral ostreid traits, (3) trans-ctenidial ovulation predates the evolution of parental care in ostreid lineages, and (4) brooding originated once in the common ancestor of the Ostreinae/Lophinae, involved a modification of the final behavioral step in the ancestral ovulation pathway, and has been retained in all descendent lineages. Our data permit an independent test of fossil-based ostreid phylogenetic hypotheses and provide novel insights into oyster evolution and systematics.     

Systemic administration of IGF-I enhances healing in collagenous extracellular matrices: evaluation of loaded and unloaded ligaments

Background  

Insulin-like growth factor-I (IGF-I) plays a crucial role in wound healing and tissue repair. We tested the hypotheses that systemic administration of IGF-I, or growth hormone (GH), or both (GH+IGF-I) would improve healing in collagenous connective tissue, such as ligament. These hypotheses were examined in rats that were allowed unrestricted activity after injury and in animals that were subjected to hindlimb disuse. Male rats were assigned to three groups: ambulatory sham-control, ambulatory-healing, and hindlimb unloaded-healing. Ambulatory and hindlimb unloaded animals underwent surgical disruption of their knee medial collateral ligaments (MCLs), while sham surgeries were performed on control animals. Healing animals subcutaneously received systemic doses of either saline, GH, IGF-I, or GH+IGF-I. After 3 weeks, mechanical properties, cell and matrix morphology, and biochemical composition were examined in control and healing ligaments.     

Phylogenetic effects, the loss of complex characters, and the evolution of development in calyptraeid gastropods

Abstract Despite considerable theoretical and empirical work on the population genetic effects of mode of development in benthic marine invertebrates, it is unclear what factors generate and maintain interspecific variation in mode of development and few studies have examined such variation in a phylogenetic context. Here I combine data on mode of development with a molecular phylogeny of 72 calyptraeid species to test the following hypotheses about the evolution of mode of development: (1) Is the loss of feeding larvae irreversible? (2) Is there a phylogenetic effect on the evolution of mode of development? (3) Do embryos of direct‐developing species lose the structures necessary for larval feeding and swimming and, if so, is the degree of embryonic modification correlated with the genetic distance between species? The results of these analyses suggest that mode of development evolves rapidly and with little phylogenetic inertia. There are three cases of the possible regain of feeding larvae, in all cases from direct development with nurse eggs. It appears that species with planktotrophic, lecithotrophic, or direct development with nurse eggs all have equal evolutionary potential and retain the possibility of subsequent evolution of a different mode of development. However, species with direct development from large yolky eggs appear to be subject to phylogenetic constraints and may not be able to subsequently evolve a different mode of development. Finally, species that have more recently evolved direct development have less highly modified embryos than older direct‐developing species. Since species with nurse eggs generally have fewer embryonic modifications than those from large yolky eggs, this embryological difference may be the underlying cause of the difference in evolutionary potential.     

Notochordal and elastic components of the axial skeleton of fishes and their functions in locomotion

Structural details of the notochord and elastic longitudinal ligaments (dorsal and ventral) of fish are presented with discussions on their possible contribution to the speed, power and modes of swimming by conferring an automatic spring-like resilience to the vertebral axis as a whole. The notochord is also believed to function as a series of ring-like hinges placed intervertebrally which dictates that the centra must be biconcave (amphicoelous) to support and house them. Examination of about 100 species shows that, whilst the dorsal ligament is always present, the ventral is found in primitive teleosts only. The phylogenetic significance of this in relation to the different efferent branchial systems will be submitted (with diagrammatic recording of dissections) for publication in the near future. The dorsal and ventral ligaments are suitably situated to assist the circulation of lymph and blood respectively in the small lateral vessels associated with the main longitudinal one of the appropriate system. Experimental work is required to test the hypotheses presented.     

EVOLUTIONARY LOSS OF LARVAL FEEDING: DEVELOPMENT,FORM AND FUNCTION IN A FACULTATIVELY FEEDING LARVA,BRISASTER LATIFRONS

Species with large eggs and nonfeeding larvae have evolved many times from ancestors with smaller eggs and feeding larvae in numerous groups of aquatic invertebrates and amphibians. This change in reproductive allocation and larval form is often accompanied by dramatic changes in development. Little is known of this transformation because the intermediate form (a facultatively feeding larva) is rare. Knowledge of facultatively feeding larvae may help explain the conditions under which nonfeeding larvae evolve. Two hypotheses concerning the evolutionary loss of larval feeding are as follows: (1) large eggs evolve before modifications in larval development, and (2) the intermediate form (facultatively feeding larva) is evolutionarily short-lived. I show that larvae of a heart urchin, Brisaster latifrons, are capable of feeding but do not require food to complete larval development. Food for larvae appears to have little effect on larval growth and development. The development, form, and suspension feeding mechanism of these larvae are similar to those of obligate-feeding larvae of other echinoids. Feeding rates of Brisaster larvae are similar to cooccurring, obligate-feeding echinoid larvae but are low relative to the large size of Brisaster larvae. The comparison shows that in Brisaster large egg size, independence from larval food, and relatively low feeding rate have evolved before the heterochronies and modified developmental mechanisms common in nonfeeding echinoid larvae. If it is general, the result suggests that hypotheses concerning the origin of nonfeeding larval development should be based on ecological factors that affect natural selection for large eggs, rather than on the evolution of heterochronies and developmental novelties in particular clades. I also discuss alternative hypotheses concerning the evolutionary persistence of facultative larval feeding as a reproductive strategy. These hypotheses could be tested against a phylogenetic hypothesis.     

Trade-offs in larval performance on normal and novel hosts

The evolution of host specialization in phytophagous insects is generally thought to involve genetic trade-offs that prevent individuals from maximizing fitness simultaneously on two or more hosts. Several hypotheses, however, have suggested that trade-offs may not be evident in experiments comparing larval performance on normal and novel hosts. Tests on survivorship, growth rate, and pupal mass among families of the swallowtail butterfly Papilio oregonius on its normal host and on a novel host provide support for these hypotheses, although they do not discriminate among them. Families differed in their relative performance on the hosts, but there was no evidence of a negative genetic correlation between hosts for any of the measures of performance. In addition, there were no correlations among the different measures, corroborating an earlier result suggesting that these different components of performance in the P. machaon species group are under at least partially separate genetic control. These results and similar results published for other insects have now produced a body of studies indicating that genetic trade-offs in individual components of larval performance may not be a major factor preventing shifts onto novel host plants. Trade-offs leading to the evolution of host specialization are more likely to involve coordination among the various components of performance together with ecological factors that allow higher fitness on one host than on others.     

Genome size correlates with reproductive fitness in seed beetles

The ultimate cause of genome size (GS) evolution in eukaryotes remains a major and unresolved puzzle in evolutionary biology. Large-scale comparative studies have failed to find consistent correlations between GS and organismal properties, resulting in the ‘C-value paradox’. Current hypotheses for the evolution of GS are based either on the balance between mutational events and drift or on natural selection acting upon standing genetic variation in GS. It is, however, currently very difficult to evaluate the role of selection because within-species studies that relate variation in life-history traits to variation in GS are very rare. Here, we report phylogenetic comparative analyses of GS evolution in seed beetles at two distinct taxonomic scales, which combines replicated estimation of GS with experimental assays of life-history traits and reproductive fitness. GS showed rapid and bidirectional evolution across species, but did not show correlated evolution with any of several indices of the relative importance of genetic drift. Within a single species, GS varied by 4–5% across populations and showed positive correlated evolution with independent estimates of male and female reproductive fitness. Collectively, the phylogenetic pattern of GS diversification across and within species in conjunction with the pattern of correlated evolution between GS and fitness provide novel support for the tenet that natural selection plays a key role in shaping GS evolution.     

Genetic variance and covariance patterns of larval development in the small white butterflyPieris rapae crucivora Boisduval

Quantitative genetic theory indicates that genetic covariance patterns among life history characters should have played an important role as genetic constraint in life history evolution. Highly positve (and negative) genetic correlations between larval development time (or larval growth rate) and adult size characters were detected by means of sib analysis for the small white butterfly Pieris rapae crucivora. The genetic associations suggested that evolution of developmental characteristics and adult phenotypic traits were constrained by pleiotropy. The positive genetic correlations between development time and adult body size may be compatible with the trade-off between them, but the negative genetic correlations between larval growth rate and adult body size are not predicted from theories of optimal energy allocation. That phenotypic correlations drastically differed from the genetic correlations indicates limitations of evolutionary inferences based only on phenotypic variation.     

Testing the genetic constraint hypothesis in a phylogenetic context: a simulation study

Quantitative genetic theory predicts that when populations diverge by drift the interspecific divergence (D matrix), calculated from species means, will be proportional to the average value of the additive genetic variance-covariance matrix, or G matrix. Most empirical studies in which this hypothesis has been investigated have ignored phylogenetic nonindependence among included taxa. Baker and Wilkinson (2003; also Revell et al. 2007) used a test for constraint in which the D matrix is calculated from phylogenetically independent contrasts (Felsenstein 1985) instead of directly from the species means. I use computer simulations to show that, on average, when the process of evolution is genetic drift, the divergence matrix calculated from independent contrasts (D(IC)) is more highly correlated with G than is the divergence matrix calculated ignoring phylogenetic nonindependence (D). This effect is more pronounced when speciation is initially slow but increases over time than when speciation decreases over time. Finally, when evolution is primarily by drift but phenotype space is bounded (as if by functional constraint) the average correlation is decreased between both G and D or D(IC), however the correlation between G and D(IC) is much larger than between G and D. Although limited in scope, to my knowledge this is the first study to use individual-based quantitative genetic simulations in a phylogenetic context.     

Evolution of Trypanosoma cruzi: clarifying hybridisations,mitochondrial introgressions and phylogenetic relationships between major lineages

Several different models of Trypanosoma cruzi evolution have been proposed. These models suggest that scarce events of genetic exchange occurred during the evolutionary history of this parasite. In addition, the debate has focused on the existence of one or two hybridisation events during the evolution of T. cruzi lineages. Here, we reviewed the literature and analysed available sequence data to clarify the phylogenetic relationships among these different lineages. We observed that TcI, TcIII and TcIV form a monophyletic group and that TcIII and TcIV are not, as previously suggested, TcI-TcII hybrids. Particularly, TcI and TcIII are sister groups that diverged around the same time that a widely distributed TcIV split into two clades (TcIV_S and TcIV_N). In addition, we collected evidence that TcIII received TcIV_S kDNA by introgression on several occasions. Different demographic hypotheses (surfing and asymmetrical introgression) may explain the origin and expansion of the TcIII group. Considering these hypotheses, genetic exchange should have been relatively frequent between TcIII and TcIV_S in the geographic area in which their distributions overlapped. In addition, our results support the hypothesis that two independent hybridisation events gave rise to TcV and TcVI. Consequently, TcIV_S kDNA was first transferred to TcIII and later to TcV and TcVI in TcII/TcIII hybridisation events.     

Biomechanics of changes in ACL and PCL material properties or prestrains in flexion under muscle force-implications in ligament reconstruction

The effects of changes in cruciate ligament material and prestrain on knee joint biomechanics following ligament reconstruction surgery by a tendon are not adequately known. A 3D nonlinear finite element model of the entire knee joint was used to investigate the joint response at different flexion angles under a quadriceps force while varying ACL and PCL initial strains or material properties. The ACL and PCL forces as well as tibiofemoral contact forces/areas substantially increased with greater ACL or PCL initial strains or stiffness. The patellofemoral contact force slightly increased whereas the tibial extensor moment slightly decreased with tenser or stiffer ACL. Reverse trends were predicted with slacker ACL. Results confirm the hypotheses that changes in the prestrain of one cruciate ligament substantially influence the force in the other cruciate ligament and the entire joint and that the use of the patellar tendon (PT) as a replacement for cruciate ligaments markedly alters the joint biomechanics with trends similar to those predicted when increasing prestrains. Forces in both ACL and PCL ligaments increased as one of them became tenser or stiffer and diminished as it became slacker. These results have important consequences in joint biomechanics following ligament injuries or replacement and tend to recommend the use of grafts with smaller prestrains (i.e. slacker than intact) when using the PT as the replacement material with stiffness greater than that of replaced ligament itself.     

Biomechanics of changes in ACL and PCL material properties or prestrains in flexion under muscle force-implications in ligament reconstruction

The effects of changes in cruciate ligament material and prestrain on knee joint biomechanics following ligament reconstruction surgery by a tendon are not adequately known. A 3D nonlinear finite element model of the entire knee joint was used to investigate the joint response at different flexion angles under a quadriceps force while varying ACL and PCL initial strains or material properties. The ACL and PCL forces as well as tibiofemoral contact forces/areas substantially increased with greater ACL or PCL initial strains or stiffness. The patellofemoral contact force slightly increased whereas the tibial extensor moment slightly decreased with tenser or stiffer ACL. Reverse trends were predicted with slacker ACL. Results confirm the hypotheses that changes in the prestrain of one cruciate ligament substantially influence the force in the other cruciate ligament and the entire joint and that the use of the patellar tendon (PT) as a replacement for cruciate ligaments markedly alters the joint biomechanics with trends similar to those predicted when increasing prestrains. Forces in both ACL and PCL ligaments increased as one of them became tenser or stiffer and diminished as it became slacker. These results have important consequences in joint biomechanics following ligament injuries or replacement and tend to recommend the use of grafts with smaller prestrains (i.e. slacker than intact) when using the PT as the replacement material with stiffness greater than that of replaced ligament itself.     

Cladistic analysis of the Sepsidae (Cyclorrhapha: Diptera) based on a comparative scanning electron microscopic study of larvae

The result of a phylogenetic analysis of the Sepsidae based on larval characters is presented. It is shown that cyclorrhaphan larvae can be as rich a source of characters as Nematocera immatures when investigated using an SEM. The cladistic analysis comprised fifty-two species in sixteen genera of the Sepsidae and five outgroup species and used fifty-seven morphological characters. It found seven parsimonious trees which only differed with respect to the arrangement of some species within the genus Themira. The basal dichotomies of the phylogenetic trees are particularly well supported, indicating the conservative nature of larval characters. Orygma is confirmed as the sister group of all the remaining sepsids, the Sepsinae. There is good larval evidence that Ortalischema is the sister group of all remaining Sepsinae and that the Toxopodinae constitute an early radiation within the Sepsidae. According to larval data, some genera are paraphyletic ( Themira, Palaeosepsis ), but adult characters appear to contradict these findings. Among the traditionally recognized higher taxa within the Sepsidae, Hennig's Themira species-group and Steysbal's Sepsini have to be rejected as polyphyletic. However, Hennig's Sepsis species-group is confirmed as monophyletic and will probably constitute one major element of a future phylogenetic system of the Sepsidae. States of the strongly modified fore-legs of some adult sepsid males are mapped onto the phylogenetic tree, largely confirming Šulc's ideas about the evolution of these features. The origin and evolution of male sternites with brushes and a gland on the tibiae of the males ('osmeterium') are discussed. Whereas adult characters point to a sister-group relationship between the Sepsidae and the Ropalomeridae, larval characters appear to indicate a sister-group relationship between the Coelopidae and the Sepsidae. The evidence for both hypotheses is critically evaluated.     

Testing adaptive hypotheses on the evolution of larval life history in acorn and stalked barnacles

Despite strong selective pressure to optimize larval life history in marine environments, there is a wide diversity with regard to developmental mode, size, and time larvae spend in the plankton. In the present study, we assessed if adaptive hypotheses explain the distribution of the larval life history of thoracican barnacles within a strict phylogenetic framework. We collected environmental and larval trait data for 170 species from the literature, and utilized a complete thoracican synthesis tree to account for phylogenetic nonindependence. In accordance with Thorson's rule, the fraction of species with planktonic‐feeding larvae declined with water depth and increased with water temperature, while the fraction of brooding species exhibited the reverse pattern. Species with planktonic‐nonfeeding larvae were overall rare, following no apparent trend. In agreement with the “size advantage” hypothesis proposed by Strathmann in 1977, egg and larval size were closely correlated. Settlement‐competent cypris larvae were larger in cold water, indicative of advantages for large juveniles when growth is slowed. Planktonic larval duration, on the other hand, was uncorrelated to environmental variables. We conclude that different selective pressures appear to shape the evolution of larval life history in barnacles.     

Ligament structure, physiology and function

Ligaments are specialized connective tissues with very interesting biomechanical properties. They have the ability to adapt to the complex functions that each are required to perform. While ligaments were once thought to be inert, they are in fact responsive to many local and systemic factors that influence their function within the organism. Injury to a ligament results in a drastic change in its structure and physiology and creates a situation where ligament function is restored by the formation of scar tissue that is biologically and biomechanically inferior to the tissue it replaces. This article will briefly review the basic structure, physiology and function of normal versus healing knee ligaments, referring specifically to what is known about two of the most extensively studied and clinically relevant knee ligaments, the anterior cruciate (ACL) and medial collateral (MCL) ligaments of the knee. Those readers wishing for more comprehensive sources of information on ligament biology and biomechanics are referred to many excellent reviews on these topics.     

Testing geographical pathways of speciation in a recent island radiation

Determining the mode, or geographical context, of speciation is a critical first step to understanding the evolutionary mechanisms that cause new species to arise. In this study, we estimated phylogenetic relationships in the cerasina species group of the Hawaiian cricket genus Laupala (Orthoptera: Gryllidae) to test competing phylogeographical hypotheses and thus infer the mode of speciation. A previous phylogenetic result based on nuclear sequence data suggested that populations of L. cerasina on the Big Island of Hawaii are the result of two independent colonizations from Maui, implying parallel speciation and convergent song evolution, and contradicting systematic hypotheses based on behavioural and morphological data. We used amplified fragment length polymorphisms to investigate further the relationships among species and populations in the cerasina species group. Results of these analyses provide a robust estimate of phylogenetic relationships and support the phylogeographical history indicated by behavioural and morphological data.     

Origins of the other metazoan body plans: the evolution of larval forms

Bilaterian animal body plan origins are not solely about adult forms. Most animals have larvae with body plans, ontogenies and ecologies distinct from adults. There are two primary hypotheses for larval origins. The first hypothesis suggests that the first animals were small pelagic forms similar to modern larvae, with adult bilaterian body plans evolved subsequently. The second hypothesis suggests that adult bilaterian body plans evolved first and that larval body plans arose by interpolation of features into direct-developing ontogenies. The two hypotheses have different consequences for understanding parsimony in evolution of larvae and of developmental genetic mechanisms. If primitive metazoans were like modern larvae and distinct adult forms evolved independently, there should be little commonality of patterning genes among adult body plans. However, sharing of patterning genes is observed. If larvae arose by co-option of adult bilaterian-expressed genes into independently evolved larval forms, larvae may show morphological convergence, but with distinct patterning genes, and this is observed. Thus, comparative studies of gene expression support independent origins of larval features. Precambrian and Cambrian embryonic fossils are also consistent with direct development of the adult as being primitive, with planktonic larvae arising during the Cambrian. Larvae have continued to co-opt genes and evolve new features, allowing study of developmental evolution.