Differences In Skeletal Morphology Research Articles

Skeletal anatomy of the pectoral fin in mudskipper species from terrestrial and aquatic habitats.

Mudskippers are a group of amphibious fishes in the family Oxudercidae, whose species inhabit a range of habitats from mostly aquatic to mostly terrestrial. Most of our understanding about habitat preference comes from natural history observations, particularly where they are collected (i.e., low intertidal vs. high intertidal regions). Mudskippers have undergone several morphological changes to accommodate a terrestrial life, including major changes to the pectoral and pelvic girdles. These changes result in a novel crutching gait, which mudskippers use to move over land. Though the appendicular morphology and crutching gait of mudskippers have been described in some species, few studies have compared skeletal structures across the family. In our study, we use microcomputed tomography (µCT) scans to compare the skeletal anatomy of 16 species of aquatic and terrestrial mudskippers. Linear discriminant analysis is used to analyze measurements obtained through geometric morphometrics (landmarks). We found bone structures of the pectoral region in the terrestrial group were significantly longer and wider than those in the aquatic group. Furthermore, a significant difference in anatomy is shown between terrestrial and aquatic genera with both axial and appendicular elements contributing to the separation between groups. This work describes the differences in skeletal morphology associated with terrestriality in mudskippers and provides valuable insights into specific anatomical characteristics contributing to their adaptation to novel environments.

Comparative Approaches in Vertebrate Cartilage Histogenesis and Regulation: Insights from Lampreys and Hagfishes

Jawed vertebrates (gnathostomes) have been the dominant lineage of deuterostomes for nearly three hundred fifty million years. Only a few lineages of jawless vertebrates remain in comparison. Composed of lampreys and hagfishes (cyclostomes), these jawless survivors are important systems for understanding the evolution of vertebrates. One focus of cyclostome research has been head skeleton development, as its evolution has been a driver of vertebrate morphological diversification. Recent work has identified hyaline-like cartilage in the oral cirri of the invertebrate chordate amphioxus, making cyclostomes critical for understanding the stepwise acquisition of vertebrate chondroid tissues. Our knowledge of cyclostome skeletogenesis, however, has lagged behind gnathostomes due to the difficulty of manipulating lamprey and hagfish embryos. In this review, we discuss and compare the regulation and histogenesis of cyclostome and gnathostome skeletal tissues. We also survey differences in skeletal morphology that we see amongst cyclostomes, as few elements can be confidently homologized between them. A recurring theme is the heterogeneity of skeletal morphology amongst living vertebrates, despite conserved genetic regulation. Based on these comparisons, we suggest a model through which these mesenchymal connective tissues acquired distinct histologies and that histological flexibility in cartilage existed in the last common ancestor of modern vertebrates.

Exploring the functional morphology of the Gorilla shoulder through musculoskeletal modelling.

Musculoskeletal computer models allow us to quantitatively relate morphological features to biomechanical performance. In non‐human apes, certain morphological features have long been linked to greater arm abduction potential and increased arm‐raising performance, compared to humans. Here, we present the first musculoskeletal model of a western lowland gorilla shoulder to test some of these long‐standing proposals. Estimates of moment arms and moments of the glenohumeral abductors (deltoid, supraspinatus and infraspinatus muscles) over arm abduction were conducted for the gorilla model and a previously published human shoulder model. Contrary to previous assumptions, we found that overall glenohumeral abduction potential is similar between Gorilla and Homo. However, gorillas differ by maintaining high abduction moment capacity with the arm raised above horizontal. This difference is linked to a disparity in soft tissue properties, indicating that scapular morphological features like a cranially oriented scapular spine and glenoid do not enhance the abductor function of the gorilla glenohumeral muscles. A functional enhancement due to differences in skeletal morphology was only demonstrated in the gorilla supraspinatus muscle. Contrary to earlier ideas linking a more obliquely oriented scapular spine to greater supraspinatus leverage, our results suggest that increased lateral projection of the greater tubercle of the humerus accounts for the greater biomechanical performance in Gorilla. This study enhances our understanding of the evolution of gorilla locomotion, as well as providing greater insight into the general interaction between anatomy, function and locomotor biomechanics.

Intraspecific and interspecific investigations of skeletal DNA methylation and femur morphology in primates.

Epigenetic mechanisms influence the development and maintenance of complex phenotypes and may also contribute to the evolution of species-specific phenotypes. With respect to skeletal traits, little is known about the gene regulation underlying these hard tissues or how tissue-specific patterns are associated with bone morphology or vary among species. To begin exploring these topics, this study evaluates one epigenetic mechanism, DNA methylation, in skeletal tissues from five nonhuman primate species which display anatomical and locomotor differences representative of their phylogenetic groups. First, we test whether intraspecific variation in skeletal DNA methylation is associated with intraspecific variation in femur morphology. Second, we identify interspecific differences in DNA methylation and assess whether these lineage-specific patterns may have contributed to species-specific morphologies. Specifically, we use the Illumina Infinium MethylationEPIC BeadChip to identify DNA methylation patterns in femur trabecular bone from baboons (n = 28), macaques (n = 10), vervets (n = 10), chimpanzees (n = 4), and marmosets (n = 6). Significant differentially methylated positions (DMPs) were associated with a subset of morphological variants, but these likely have small biological effects and may be confounded by other variables associated with morphological variation. Conversely, several species-specific DMPs were identified, and these are found in genes enriched for functions associated with complex skeletal traits. Overall, these findings reveal that while intraspecific epigenetic variation is not readily associated with skeletal morphology differences, some interspecific epigenetic differences in skeletal tissues exist and may contribute to evolutionarily distinct phenotypes. This work forms a foundation for future explorations of gene regulation and skeletal trait evolution in primates.

Postcranial morphological variation between hunter-gatherers and horticulturalists from the lower Paraná River Delta, Argentina.

Although pre-Hispanic hunter-gatherer and horticulturalist (known as Guaraní) societies from the lower Paraná River Delta (Argentina) presented differences in diet, pottery decoration, mortuary practices, and places of origin, differences in skeletal morphology between such groups have never been systematically explored. This work focuses on variations in humeral and femoral external linear measurements and derived structural properties of adult individuals from both societies as well as on variations in body mass and stature. Bone length, epiphyseal size, and midshaft breadth were measured in 82 adult humeri and 100 femora from hunter-gatherer and Guaraní archeological sites. Epiphyseal and midshaft robusticity, residual strength, midshaft shape and area, stature, and body mass were then estimated. Mann-Whitney tests were run to compare the hunter-gatherer and Guaraní samples. Male Guaraní individuals presented stronger humeri and more robust femoral and humeral proximal epiphyses than hunter-gatherers. In addition, female Guaraní individuals showed rounder femoral diaphyses in comparison with female hunter-gatherers. Concerning stature, the Guaraní individuals were found to be shorter than hunter-gatherers, regardless of sex. No statistical differences were found in body mass. Despite the fact that skeletal variations between Guaraní and hunter-gatherers could be a consequence of differences in mechanical loadings and genetic composition, bone robusticity is also positively correlated with increased age, but as the age composition of the Guaraní sample could not be estimated, skeletal variation between the samples could be a consequence of differences in age distribution.

Upper airway features of unilateral cleft lip and palate patients in different growth stages.

To compare growth-related changes of skeletal and upper airway features of unilateral cleft lip and palate subjects (UCLP) with non-cleft control (NCC) subjects by using lateral cephalograms. The sample comprised 238 subjects, collected cross-sectionally, divided into 2 groups: 94 with UCLP, and 144 NCC, subdivided into 4 groups according to their growth stages by using cervical vertebral maturation stage (CVMS). The subgroups were defined as early childhood (stage 1), prepubertal (stage 2: CVMS I and II), pubertal (stage 3: CVMS III and IV), and postpubertal (stage 4: CVMS V and VI). The maxilla was more retrognathic at stages 2, 3, and 4 in females with UCLP. The mandible was more retrognathic in UCLP at stage 1 in males, and stages 2 and 3 in females. ANB (angle between NA plane and NB plane) was significantly smaller in UCLP subjects at stage 4 for both sexes. A vertical growth pattern was seen in UCLP subjects except males at stages 2 and 3, and females at stage 2. Posterior airway space was significantly narrower at all stages in males and after stage 1 in females. Middle airway space was significantly wider at all stages in females and after stage 1 in males. Epiglottic airway space was significantly narrower in males at stage 3. Age- and sex-dependent differences in skeletal morphology and upper-airway widths of the UCLP subjects were identified when compared with controls.

What Fossils Can and Can't Tell Us about Hominin Locomotor Evolution: Insights from Experimental Skeletal Biomechanics

Lacking a time machine, the best way to glean information about hominin locomotor evolution is by making inferences about the behavior of our ancient ancestors from the anatomy of their fossilized remains. Such inferences are only credible, however, if underlying assumptions about links between skeletal form and function are valid. In order to faithfully translate the static fossil record into a behaviorally dynamic picture, experiments are required to test and validate these assumptions. This talk provides an overview of the critical role that experimental skeletal biomechanics plays in the epistemology of paleoanthropology and highlights examples of research that have been especially influential for our understanding of what fossils can and cannot tell us about the locomotor behavior of ancient hominins.Experimental biomechanics in paleoanthropology has generally proceeded on two fronts. First, comparative biomechanical studies of living primates have been carried out within phylogenetic frameworks to explicitly test hypotheses of evolutionary change in various skeletal structures and their functions. Some of the most significant advances in interpreting hominin fossil anatomy have come from studies comparing the skeletal biomechanics of humans and those of our closest living relatives, chimpanzees. Two key lines of evidence from these studies are kinematics, or the measurement of motion of the body, and electromyography, the measurement of the neuromuscular control of motion. Application of these techniques has allowed researchers to relate skeletal anatomy to muscle activity patterns and the body movements they produce, rather than just infer function based on differences in skeletal morphology between species. Key studies have illuminated the relationship between pelvis and lower limb anatomy and the evolution of bipedalism.Second, experiments with animal models such as rodents, sheep, pigs and fowl have been conducted to establish basic principles about how bone structure changes throughout life in response to loads sustained during locomotion. The responsiveness of the skeleton to loading is of interest because if bones are shaped by locomotor activity during life, then it might be possible to infer the lifetime locomotor patterns of hominins based on fossil morphology. Although many experiments have shown that loads produced by locomotion can indeed affect bone structure, the process has often been observed to be complex in ways that would seem to greatly undermine the accuracy of inferences about lifetime physical activity from fossil structure. Important issues raised by these studies include the weak correspondence between bone structure and its in vivo loading environment, the large influence of genetics on bone structure independent of loading and the age‐dependency of bone's responsiveness to loading.While paleoanthropological investigations of hominin locomotor evolution once relied primarily on speculation about the functional significance of fossil anatomy, today researchers are equipped with a variety of experimental models and tools to empirically evaluate their ideas. Experimental biomechanics provides a means of testing hypotheses about skeletal form and function that cannot be determined from fossil anatomy alone.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

The effects of captive versus wild rearing environments on long bone articular surfaces in common chimpanzees (Pan troglodytes).

The physical environments of captive and wild animals frequently differ in substrate types and compliance. As a result, there is an assumption that differences in rearing environments between captive and wild individuals produce differences in skeletal morphology. Here, this hypothesis is tested using a sample of 42 captive and wild common chimpanzees (Pan troglodytes). Articular surface areas of the humerus, radius, ulna, femur, and tibia were calculated from linear breadth measurements, adjusted for size differences using Mosimann shape variables, and compared across sex and environmental groups using two-way ANOVA. Results indicate that the articular surfaces of the wrist and knee differ between captive and wild chimpanzees; captive individuals have significantly larger distal ulna and tibial plateau articular surfaces. In both captive and wild chimpanzees, males have significantly larger femoral condyles and distal radius surfaces than females. Finally, there is an interaction effect between sex and rearing in the articular surfaces of the femoral condyles and distal radius in which captive males have significantly larger surface areas than all other sex-rearing groups. These data suggest that long bone articular surfaces may be sensitive to differences experienced by captive and wild individuals, such as differences in diet, body mass, positional behaviors, and presumed loading environments. Importantly, these results only find differences due to rearing environment in some long bone articular surfaces. Thus, future work on skeletal morphology could cautiously incorporate data from captive individuals, but should first investigate potential intraspecific differences between captive and wild individuals.

Growth-related changes of skeletal and upper-airway features in bilateral cleft lip and palate patients

The craniofacial morphology of subjects with cleft lip and palate differs from that of subjects without clefts. Subjects with bilateral cleft lip and palate tend to have maxillary retrognathism, a smaller mandible with an obtuse gonial angle, greater anterior upper and lower facial heights, and retroclined maxillary incisors. The purposes of this research were to compare the skeletal and upper-airway features of subjects with bilateral cleft lip and palate with the same features in control subjects without clefts and to determine the growth-related changes at different growth stages. The sample comprised 212 subjects divided into 2 groups: 68 with bilateral cleft lip and palate, and 144 controls without clefts; each group was further divided into 4 subgroups according to growth stage using the cervical vertebral maturation stage method. The subgroups were defined as early childhood (stage 1), prepubertal (stage 2), pubertal (stage 3), and postpubertal (stage 4). The cephalometric variables were evaluated with 2-way analysis of variance and the Bonferroni test. Maxillary position showed no significant differences between the male groups. The maxilla was more prognathic at stage 2 and became more retrognathic at stages 3 and 4 in the females. The mandible was more retrusive in the bilateral cleft lip and palate subjects at stage 1 in males and at stages 3 and 4 in females. ANB was larger at stages 1 and 2, and it became similar to the controls at stages 3 and 4 in male and female bilateral cleft lip and palate subjects. Vertical growth was seen in the bilateral cleft lip and palate subjects regardless of sex, and no change was observed with age. Posterior airway space was narrower in all stages (except for stage 1 in females). Middle airway space was wider after stage 1 in the male and female bilateral cleft lip and palate subjects. Inferior airway space was narrower in the male bilateral cleft lip and palate patients at the early childhood and pubertal stages. Age- and sex-dependent differences in skeletal morphology and upper-airway widths of the bilateral cleft lip and palate subjects were identified when compared with the control subjects without clefts.

Multivariate analysis of neognath skeletal measurements: implications for body mass estimation in Mesozoic birds

The abundant fossils of avian stem taxa unearthed during the last years make it necessary to review and improve the models for estimating body mass used in palaeoecological studies. In this article, single and multiple regression functions based on osteological measurements were obtained from a large data set of extant flying birds for estimating the body mass of 42 Mesozoic specimens from stem taxa Archaeopterygidae, Jeholornithidae, Sapeornithidae, Confuciusornithidae, and Enantiornithes, and basal members of Ornithuromorpha. Traditionally, body mass has been estimated in fossil vertebrates using univariate scaling functions. In contrast, multiple regression functions have been used less frequently. Both predictive methods can be affected by different sources of error from statistics, phylogenetic relationships, ecological adaptations, and bone preservation; however, although some studies have addressed these biases, few have tested them within the context of a single data set. In our data set, we find that the models with greater predictive strength and applicability for new specimens, especially for stem taxa, are those derived from multiple regression analyses. For this reason, we suggest that multiple regression analyses may provide improved predictive strength for stem group specimens. Moreover, the methodology used for selecting variables allowed us to obtain specific sets of predictors for each fossil stem group that presumably minimized the variation resulting from historical contingency (i.e. differences in skeletal morphology arising from phylogeny), locomotor adaptations, and diagenetic compaction. The loss of generalizability in the multiple regression models resulting from collinearity effects was negligible on the body mass estimates derived from our data set. Therefore, the body mass values obtained for Mesozoic specimens are accurate and can be used in future studies in a number of palaeobiological and evolutionary aspects of extinct birds, particularly the first stages of avian flight.

Morphological and taxonomic revision of species of Squatina from the Southwestern Atlantic Ocean (Chondrichthyes: Squatiniformes: Squatinidae).

The morphology and taxonomy of species of Squatina from the southwestern Atlantic Ocean are revised. Species previously considered valid, Squatina argentina (Marini, 1930), Squatina guggenheim Marini, 1936 and Squatina occulta Vooren and da Silva, 1991, are investigated and described in detail, including a morphometric and meristic study of specimens from their recorded range. The taxonomic status of the doubtful nominal species Squatina punctata Marini, 1936 was also evaluated. This species was previously considered a junior synonym of S. argentina, a junior synonym of S. guggenheim, or a senior synonym of S. occulta. Although there is much morphological similarity between Squatina species, significant differences in dermal denticle patterns, dorsal coloration, tooth formula, and size at maturity are reported, enabling the recognition of S. argentina, S. guggenheim and S. occulta as valid species, and relegating S. punctata to the synonymy of S. guggenheim. Differences in skeletal morphology between valid species are described and considered supportive of the taxonomic hypothesis, corroborating a previous study of neurocrania. Additionally, an unidentified specimen is reported, as Squatina sp., from the continental shelf of Bahia state, Brazil, recognized by having more vertebral centra and a conspicuous dermal denticle morphology on interspiracular region, features not present in other South America angelshark species. A report on the only known syntype of Squatina dumeril Le Sueur, 1818 is presented, describing features that are still preserved and designating it as lectotype. Lateral-line sensory canals, skeleton, and cranial and hypobranchial muscles for the three valid species of Squatina from the southwestern Atlantic, as well as the brain and cranial nerves of S. guggenheim, are described and illustrated.

Craniofacial morphology in Austrian Early Bronze Age populations reflects sex-specific migration patterns.

The Early Bronze Age (2.300-1.500 BC) in lower Austria consists of three synchronous regional manifestations (Únetice, Unterwölbling, and Wieselburg cultures). The bearers of these cultures inhabited a relatively small geographic area and shared similar ecological conditions, but previous studies revealed population differences in skeletal morphology. We analyzed the cranial morphology of 171 individuals of these populations with a geometric morphometric approach in order to compare different migration scenarios. We find significant mean form differences between populations and between sexes. In a principal component analysis, the Wieselburg population, located southwest of the Danube, largely separates from the Únetice population north of the Danube, whereas the southwestern Unterwölbling group, which played a central role in trading bronze objects, overlaps with both. The Böheimkirchen group, inhabiting the southwestern Danubian area in the later phase of the Early Bronze Age, differs from the chronologically older Unterwölbling group. Geographic distance between six sites and position relative to the river Danube accounted for 64% of form distance variation; the effect of the river Danube was considerably larger than hat of geographic distance per se. As predicted for a patrilocal system in which females have a larger marriage domain than males, we found that female mean forms are more similar to each other than male mean forms. Geographic conditions explained more than twice as much variation in females as in males, suggesting that female migration was more affected by geographical constraints than male migration was.

Molecular tools, classic questions - an interview with Clifford Tabin

Clifford J. Tabin has made pioneering contributions to several fields in biology, including retroviruses, oncogenes, developmental biology and evolution. His father, a physicist who worked in the Manhattan project, kindled his interest in science. Cliff later chose to study biology and started his research career when the world of recombinant DNA was opening up. In Robert Weinbergs lab, he constructed the Moloney leukaemia virus (MLV-tk), the first recombinant retrovirus that could be used as a eukaryotic vector. He also discovered the amino acid changes leading to the activation of Ras, the first human oncogene discovered. As an independent researcher, he began in the field of urodele limb regeneration, and described the expression of retinoic acid receptor and Hox genes in the blastema. Moving to the chick model, his was one of the labs that simultaneously cloned the first vertebrate hedgehog cognates and showed that sonic hedgehog functions as a morphogen in certain developmental contexts, in particular as an organizing activity during limb development. Comparative studies by Ann Burke in his lab showed that differences in boundaries of Hox gene expression across vertebrate phylogeny correlated with differences in skeletal morphology. The Tabin lab also discovered a genetic pathway responsible for mediating left-right asymmetry in vertebrates; helped uncover the pathways leading to dorsoventral limb patterning; made contributions to our understanding of skeletal morphogenesis and identified developmental mechanisms that might underpin the diversification of the beak in Darwins finches. Despite being a professor of genetics at Harvard, Tabin says: "I have never done a genetics experiment in my life!". This is changing with his latest project: the genetics of Mexican cavefish. I interviewed Cliff on the 3rd October, 2007, in his office at Harvard.

Determining Skeletal Parameters in Angle Classes II, Division 1 and II, Division 2*

Aim of the study was to find out whether significant differences in skeletal morphology exist between Angle Class II, Division 1 and II, Division 2. Fifty-nine patients without previous orthodontic treatment showing Class II occlusion and distobasal jaw relation were evaluated by cephalometric analysis. Significant differences existed in skeletal morphology between patients with Angle Class II, Division 1 and II, Division 2, with only minor gender-specific differences: angleML/NSL in Class II, Division 2 males (p < 0.01) and females (p < 0.05) was smaller compared to Class II, Division 1 patients. angleML/NL was also noticeably smaller in Class II, Division 2 males (p < 0.001) and females (p < 0.01) compared to Class II, Division 1 patients. Patients with Class II, Division 2 had a smaller gonial angle (angleArGoMe), with the difference being significant only in males (p < 0.05).

Men, bone and estrogen: unresolved issues.

Gonadal function has long been known to be important for skeletal health in men. Prepubertal hypogonadism is clearly associated with impairment in peak bone mass development and adult-onset hypogonadism with accelerated bone loss. Gonadal failure results in deficits in both androgen and estrogen action, but traditionally androgens were assumed to have the most important skeletal effect in men. Recently that model has been reconsidered as a variety of kinds of evidence have appeared to document a critical role for estrogen in bone physiology. As a result of this fresh perspective a host of interesting new dilemmas and hypotheses are being examined, including those related to the mechanisms of sex steroid action in bone, the origins of gender differences in skeletal morphology and physiology, and the role of estrogen in diagnostic and therapeutic strategies in men with metabolic bone disorders.

Subspecific genetic differences in the saddle-back tamarin (Saguinus fuscicollis) postcranial skeleton.

Saddle-back tamarins (Saguinus fuscicollis) have been differentiated into subspecies based on coat color and facial morphology. Morphology of first-generation genetic hybrids can provide an understanding of population differences and evolution that may otherwise be difficult to obtain. This study compares postcranial morphology in two hybrids of saddle-back tamarins (S. f. illigeri x S. f. lagonotus and S. f. illigeri x S. f. leucogenys) to the purebred subspecies. We examine whether postcranial morphology differs between the purebred subspecies and their hybrids, and whether hybrids exhibit heterosis indicative of genetic differences between the parental subspecies. Fifty postcranial dimensions, or traits, were measured on 287 S. fuscicollis skeletons. Measurements were corrected for sex differences and environmental differences (wild-derived or laboratory-born). Females were larger than males for most postcranial dimensions, especially in the pelvis. Significant environmental differences were found throughout the postcranial skeleton, with wild-derived animals generally larger than animals born in the laboratory. Significant heterosis was found for six postcranial traits in the S. f. illigeri x S. f. leucogenys cross, and heterosis was found for 15 postcranial traits in the S. f. illigeri x S. f. lagonotus cross. These results suggest that subspecies differences in skeletal morphology are generally due to genetic rather than environmental differences. The subspecies appear to represent geographic isolates that differ in gene frequencies and directional dominance for skeletal traits.

Skeletal site-dependent expression of the androgen receptor in human osteoblastic cell populations.

There are obvious sexual differences in adult skeletal morphology which for the most part are related to differences in size. Higher androgen serum levels in males exert potent osteoanabolic effects and therefore may contribute to this sexual dimorphism of the skeleton. The presence of androgen receptors (AR) in bone cells is a prerequisite for a direct osteoanabolic action of androgens. To investigate the possibility that, in addition to gender-related differences in androgen serum levels, there are also gender-related differences in the osteoblastic expression pattern of the androgen receptor, we examined AR mRNA expression, androgen binding sites, and mitogenic responses to the androgen dihydrotestosterone (DHT) in human osteoblastic cell (HOC) populations. HOCs were isolated from bone biopsy specimens derived from different skeletal sites of healthy adult males and females (2-69 years old). We found that male and female HOCs of all examined ages express similar AR mRNA levels and similar numbers of androgen binding sites. Using whole-cell-binding assays, we observed 3129-8417 androgen binding sites per femoral HOC with apparent KDs of 1.45-2.83 nM depending on the age of the investigated HOC population. Mandibular and cortical HOC of both sexes expressed higher AR mRNA levels, significantly more androgen binding sites per cell, and exhibited significantly greater mitogenic responses to DHT than iliac crest-derived and trabecular HOC of the same skeletal system and the same skeletal-site, respectively. In early adulthood, HOCs of both sexes appear to express somewhat higher AR mRNA levels and to possess more androgen binding sites than prepubertal and senescent HOC. Because sex hormone serum levels rise in puberty, we investigated the regulation of the AR mRNA expression by various steroids. We found that dexamethasone (dexa) and in some experiments also 17beta-estradiol (E2) and 1,25-dihydroxyvitamin D3 (D3) increased AR mRNA levels and androgen binding in HOC cultures. A pretreatment with dexa, E2, and D3 significantly increased the mitogenic response of HOCs to DHT. We conclude that (1) higher androgen serum levels in males together with a higher AR expression at certain skeletal sites may contribute to the development of sex-related differences in skeletal morphology, (2) glucocorticoids induce AR gene expression in HOC cultures, and (3) glucocorticoids, E2, and D3 enhance the mitogenic action of DHT.

QUANTITATIVE GENETICS OF BRYOZOAN PHENOTYPIC EVOLUTION. II. ANALYSIS OF SELECTION AND RANDOM CHANGE IN FOSSIL SPECIES USING RECONSTRUCTED GENETIC PARAMETERS.

The roles of natural selection and random genetic change in the punctuated phenotypic evolution of eight Miocene-Pliocene tropical American species of the cheilostome bryozoan Metrarabdotos are analyzed by quantitative genetic methods. Trait heritabilities and genetic covariances reconstructed by partitioning within- and among-colony phenotypic variance are similar to those previously obtained for living species of the cheilostome Stylopoma using breeding data. The hypothesis that differences in skeletal morphology between species of Metrarabdotos are entirely due to mutation and genetic drift cannot be rejected for reasonable rates of mutation maintained for periods brief enough to account for the geologically abrupt appearances of these species in the fossil record. Except for one pair of species, separated by the largest morphologic distance, directional selection acting alone would require unrealistically high rates of selective mortality to be maintained for these periods. Thus, directional selection is not strongly implicated in the divergence of Metrarabdotos species. Within species, rates of net phenotypic change are slow enough to require stabilizing selection, but mask large, relatively rapid fluctuations, all of which, however, can be attributed to chance departures from the mean phenotype by mutation and genetic drift, rather than to tracking environmental fluctuation by directional selection. The results are consistent with genetic models involving shifts between multiple adaptive peaks on which phenotypes remain more or less static through long-term stabilizing selection. Regardless of the degree to which directional selection may be involved in peak shifts, phenotypic differentiation is thus related to processes different than the pervasive stabilizing selection acting within species.