Positional specification in the segmental growth pattern of an early arthropod

In many arthropods, there is a change in relative segment size during post-embryonic development, but how segment differential growth is produced is little known. A new dataset of the highest quality specimens of the 429 Myr old trilobite Aulacopleura koninckii provides an unparalleled opportunity to investigate segment growth dynamics and its control in an early arthropod. Morphometric analysis across nine post-embryonic stages revealed a growth gradient in the trunk of A. koninckii. We contrastively tested different growth models referable to two distinct hypotheses of growth control for the developing trunk: (i) a segment-specific control, with individual segments having differential autonomous growth progression, and (ii) a regional control, with segment growth depending on their relative position along the main axis. We show that the trunk growth pattern of A. koninckii was consistent with a regional growth control producing a continuous growth gradient that was stable across all developmental stages investigated. The specific posterior-to-anterior decaying shape of the growth gradient suggests it deriving from the linear transduction of a graded signal, similar to those commonly provided by morphogens. A growth control depending on a form of positional specification, possibly realized through the linear interpretation of a graded signal, may represent the primitive condition for arthropod differential growth along the main body axis, from which the diverse and generally more complex forms of growth control in subsequent arthropods have evolved.     

Morphological variations of the Solidago virgaurea L. complex along an elevational gradient on Mt Norikura,central Japan

Morphological variations in vegetative and reproductive organs in the Solidago virgaurea complex were examined for eight elevations between 1600 and 2400 m a.s.l. in Japan. The rosette diameter and stem height were lower at higher elevations. The stem diameter at any stem height was greater at higher elevations, suggesting that the S. virgaurea complex increases mechanical stiffness against strong wind at high elevations. The number of flower heads at any stem height was less at high elevations (2000–2400 m a.s.l.) than at low elevations (1600–1900 m a.s.l.). Leaf nitrogen concentration did not change along the elevational gradient. Leaf mass per area (LMA) tended to decrease with increasing elevation, except for 2400 m a.s.l. The decrease of LMA would contribute to maintaining a positive carbon balance at high elevations. The number of involucral rays of flower heads was mostly four at low elevations (1600–1900 m a.s.l.) and three at high elevations (2000–2400 m a.s.l.). The number of involucral scales and diameter of flower heads were greater at high elevations (2000–2400 m a.s.l.) than at low elevations (1600–1900 m a.s.l.). Therefore, the S . virgaurea complex is suggested to adapt to high elevations that have cool temperature, a short growing season and strong wind conditions by changing its vegetative and reproductive traits.     

Conservative whole‐organ scaling contrasts with highly labile suborgan scaling differences among compound eyes of closely related Formica ants

Static allometries determine how organ size scales in relation to body mass. The extent to which these allometric relationships are free to evolve, and how they differ among closely related species, has been debated extensively and remains unclear; changes in intercept appear common, but changes in slope are far rarer. Here, we compare the scaling relationships that govern the structure of compound eyes of four closely related ant species from the genus Formica. Comparison among these species revealed changes in intercept but not slope in the allometric scaling relationships governing eye area, facet number, and mean facet diameter. Moreover, the scaling between facet diameter and number was conserved across all four species. In contrast, facet diameters from distinct regions of the compound eye differed in both intercept and slope within a single species and when comparing homologous regions among species. Thus, even when species are conservative in the scaling of whole organs, they can differ substantially in regional scaling within organs. This, at least partly, explains how species can produce organs that adhere to genus wide scaling relationships while still being able to invest differentially in particular regions of organs to produce specific features that match their ecology.     

On the use of log‐transformation versus nonlinear regression for analyzing biological power laws

Xiao and colleagues re‐examined 471 datasets from the literature in a major study comparing two common procedures for fitting the allometric equation y = ax^b to bivariate data (Xiao et al., 2011). One of the procedures was the traditional allometric method, whereby the model for a straight line fitted to logarithmic transformations of the original data is back‐transformed to form a two‐parameter power function with multiplicative, lognormal, heteroscedastic error on the arithmetic scale. The other procedure was standard nonlinear regression, whereby a two‐parameter power function with additive, normal, homoscedastic error is fitted directly to untransformed data by nonlinear least squares. Xiao and colleagues articulated a simple (but explicit) protocol for fitting and comparing the alternative models, and then used the protocol to examine each of the datasets in their compilation. The traditional method was said to provide a better fit in 69% of the cases and an equivalent fit in another 15%, so the investigation appeared to validate findings from a large majority of prior studies on allometric variation. However, focus for the investigation by Xiao and colleagues was overly narrow, and statistical models apparently were not validated graphically in the scale of measurement. The present study re‐examined a subset of the cases using a larger pool of candidate models and graphical validation, and discovered complexities that were overlooked in their investigation. Some datasets that appeared to be described better by the traditional method actually were unsuited for use in an allometric analysis, whereas other datasets were not described adequately by a two‐parameter power function, regardless of how the model was fitted. Thus, conclusions reached by Xiao and colleagues are not well supported and their paradigm for fitting allometric equations is unreliable. Future investigations of allometric variation should adopt a more holistic approach and incorporate graphical validation on the original arithmetic scale. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 1167–1178.     

Ontogenetic and interspecific organ weight allometry in Old World monkeys

The importance of allometry as an analytic tool is well recognized in the literature of primate morphology. However, a number of recent studies have illustrated how interpretive difficulties can arise when researchers confound different types of allometric data. Such confusion is due less to carelessness than to uncertainty about how different types of allometry are related. The present study examines the relationship between two types—ontogenetic and interspecific allometry–in the case of organ weight scaling in six species of Old World monkeys. Accepting the interpretation of interspecific allometry as a reflection of functional scaling constraints, the results of this analysis indicate how ontogenetic patterns have been modified in different-sized species to maintain compliance with these constraints. Specifically, for the heart and lungs it appears that vertical transpositions of individual species' ontogenies are dictated by isometric interspecific allometry, while in the case of the kidneys and liver, the relation of negative allometry across species entails alteration of the relative growth coefficients of the individual species. While these conclusions can at present only be applied to organ weight scaling, the approach of examining interspecific patterns in light of developmental differences between species should prove very helpful in our efforts to understand the phenomena of size and scaling.     

Allometric and non‐allometric consequences of inbreeding on Drosophila melanogaster wings

Inbreeding is expected to increase the variability in size and shape within populations. The distinct effects of inbreeding on size and shape suggest that they are governed by different developmental pathways. One unresolved question is whether the non‐allometric shape component is partially unconstrained developmentally and therefore whether shape is evolvable. In the present study, we utilized a mass outbred population of Drosophila melanogaster maintained at standard laboratory conditions. Eight lines with equivalent expected levels of inbreeding (F ≈ 0.67) were obtained by restricting the size of each population to two pairs for nine generations. Nine landmarks were measured on Drosophila wings of the inbreed lines and compared with those of the mass population. Wing landmarks comprise an excellent model system for studying evolution of size and shape. Landmark measurements were analyzed with a Procrustes generalized least squares procedure. To visualize global shape changes among samples, we reconstructed the mean shape and the shape changes related to both the allometric and non‐allometric components. An increased variability in the non‐allometric shape component was found with inbreeding. This indicated that shape was not entirely developmentally constrained, and therefore that shape appears to be evolvable. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 626–634.     

Antler and Body Weight Allometry in Red Deer: A Comparison of Statistical Estimators

In allometric expressions, both variables are measured with error. Such structural relationships require specific statistical techniques for precise and valid estimation of equation parameters. The relative growth pattern for antler weight and body weight was examined for 1700 red deer (Cervus elaphus L.) stags from the Harz mountain district in West Germany. Coefficients for the allometric equation were determined using several statistical estimation procedures and the results were compared and evaluated. There is evidence that the magnitude of the exponent in the allometric expression increases with the onset of sexual maturity.     

The complex ecology of genitalia: Gonopodium length and allometry in the Trinidadian guppy

Male genitalia present an extraordinary pattern of rapid divergence in animals with internal fertilization, which is usually attributed to sexual selection. However, the effect of ecological factors on genitalia divergence could also be important, especially so in animals with nonretractable genitalia because of their stronger interaction with the surrounding environment in comparison with animals with retractable genitalia. Here, we examine the potential of a pervasive ecological factor (predation) to influence the length and allometry of the male genitalia in guppies. We sampled guppies from pairs of low‐predation (LP) and high‐predation (HP) populations in seven rivers in Trinidad, and measured their body and gonopodium length. A key finding was that HP adult males do not have consistently longer gonopodia than do LP adult males, as had been described in previous work. However, we did find such divergence for juvenile males: HP juveniles have longer gonopodia than do LP juveniles. We therefore suggest that an evolutionary trend toward the development of longer gonopodia in HP males (as seen in the juveniles) is erased after maturity owing to the higher mortality of mature males with longer gonopodia. Beyond these generalities, gonopodium length and gonopodium allometry were remarkably variable among populations even within a predation regime, thus indicating strong context dependence to their development/evolution. Our findings highlight the complex dynamics of genitalia evolution in Trinidadian guppies.     

ALLOMETRIC CONSTRAINTS AND THE EVOLUTION OF ALLOMETRY

Morphological traits often covary within and among species according to simple power laws referred to as allometry. Such allometric relationships may result from common growth regulation, and this has given rise to the hypothesis that allometric exponents may have low evolvability and constrain trait evolution. We formalize hypotheses for how allometry may constrain morphological trait evolution across taxa, and test these using more than 300 empirical estimates of static (within‐species) allometric relations of animal morphological traits. Although we find evidence for evolutionary changes in allometric parameters on million‐year, cross‐species time scales, there is limited evidence for microevolutionary changes in allometric slopes. Accordingly, we find that static allometries often predict evolutionary allometries on the subspecies level, but less so across species. Although there is a large body of work on allometry in a broad sense that includes all kinds of morphological trait–size relationships, we found relatively little information about the evolution of allometry in the narrow sense of a power relationship. Despite the many claims of microevolutionary changes of static allometries in the literature, hardly any of these apply to narrow‐sense allometry, and we argue that the hypothesis of strongly constrained static allometric slopes remains viable.     

Phyletic size change and brain/body allometry: A consideration based on the African pongids and other primates

A problematic aspect of brain/body allometry is the frequency of interspecific series which exhibit allometry coefficients of approximately 0.33. This coefficient is significantly lower than the 0.66 value which is usually taken to be the interspecific norm. A number of explanations have been forwarded to account for this finding. These include (1) intraspecificallometry explanations, (2) nonallometric explanations, and (3) Jerison’s “extraneurons” hypothesis, among others. The African apes, which exhibit a lowered interspecific allometry coefficient, are used here to consider previous explanations. These are found to be inadequate in a number of ways, and an alternative explanation is proposed. This explanation is based on patterns of brain and body size change during ontogeny and phytogeny. It is argued that the interspecific allometry coefficient in African apes parallels the intraspecific one because similar ontogenetic modifications of body growth separate large and small forms along each curve. In both cases, body size differences are produced primarily by growth in later postnatal periods, during which little brain growth occurs. Data on body growth, neonatal scaling, and various lifehistory traits support this explanation. This work extends previous warnings that sizecorrected estimates of relative brain size may not correspond very closely to our understanding of the behavioral capacities of certain species in lineages characterized by rapid change in body size.     

On the development, habitat selection and taxonomy of Helix (Jacosta) siphnica Kobelt (Gastropoda, Helicellinae)

The environmental factors affecting the distribution, variation, and development and taxonomy of Helix (Jacosta) siphnica Kobelt, 1883, an endemic landsnail of Siphnos Island (Cyclades, Greece), are examined.
Two types of populations can be distinguished due to different strategies of shell development. In the first, and more frequent, type, the keeled and depressed juvenile shells, become rounded in the later developmental stages. In the second type, in all developmental stages, only keeled shells are found. In this study changes in the allometric relationships of shell height, shell diameter and whorls are correlated with the development of the genital system and changes in habitat selection. The keeled forms are found in drier and more stony environments. The degradation of the forests to dry maquis and frygana and desertification in the Aegean insular ecosystems are probably connected with the development of keeled forms. Keeled populations of Trochoidea siphnica are regarded as an example of neoteny leading to paedomorphosis by retardation.     

Relative growth and morphological sexual maturity of the freshwater crab Trichodactylus fluviatilis Latreille 1828 (Decapoda,Trichodactylidae) from west central São Paulo State,Brazil

An analysis of relative growth was performed to determine the size at morphological sexual maturity of Trichodactylus fluviatilis Latreille 1828. Samples were collected by trapping at night from January 2010 through June 2011 from the Barreiro River (22°34′42″S; 49°11′92″W), west central São Paulo State, Brazil. The following measurements were obtained (mm): carapace width (CW) (independent variable), cheliped propodus length (PL), cheliped propodus height (PH), cheliped propodus width (PW), first pleopod length, and abdomen width (AW). The males showed positive allometry in the juvenile and adult stages for the following relationships: PL vs. CW, PH vs. CW, and PW vs. CW. In the females, a positive allometry was observed for AW vs. CW in the juvenile stage, and a negative allometry was observed for this relationship in the adult stage. The differential growth of the chelipeds in males could be related to territorial conflict and courtship, whereas the changes in the growth of the female abdomen most likely favor higher reproductive success.     

Relative growth and morphological sexual maturity of the mangrove crab Aratus pisonii (H. Milne Edwards, 1837) (Decapoda,Brachyura, Sesarmidae) on the southern coast of the state of São Paulo,Brazil

Growth in crustaceans is characterized by ontogenetic differentiation during the development of their body structures, so studies on relative growth are widely applied in this group. In this study, the growth pattern of the body structures was verified through the analysis of relative growth, and then, morphological sexual maturity of the mangrove crab Aratus pisonii (H. Milne Edwards, 1837) was estimated. The carapace width (CW), cheliped propodus length (PL), cheliped propodus height (PH), propodus width (PW), abdomen width (AW), and first pleopod length (FPL) of the crabs were measured. The relationships that best showed changes in the allometric coefficient among demographic categories were FPL vs. CW for males and AW vs. CW for females. This study verified the increased size of the cheliped in terms of length, width, and height, which occurred mainly in adult males. This increase reflects the importance of this structure in the reproductive processes of A. pisonii. For females, the increase in abdomen growth reflects their reproductive potential, since it is a structure that provides protection for eggs.     

Investment in higher order central processing regions is not constrained by brain size in social insects

The extent to which size constrains the evolution of brain organization and the genesis of complex behaviour is a central, unanswered question in evolutionary neuroscience. Advanced cognition has long been linked to the expansion of specific brain compartments, such as the neocortex in vertebrates and the mushroom bodies in insects. Scaling constraints that limit the size of these brain regions in small animals may therefore be particularly significant to behavioural evolution. Recent findings from studies of paper wasps suggest miniaturization constrains the size of central sensory processing brain centres (mushroom body calyces) in favour of peripheral, sensory input centres (antennal and optic lobes). We tested the generality of this hypothesis in diverse eusocial hymenopteran species (ants, bees and wasps) exhibiting striking variation in body size and thus brain size. Combining multiple neuroanatomical datasets from these three taxa, we found no universal size constraint on brain organization within or among species. In fact, small-bodied ants with miniscule brains had mushroom body calyces proportionally as large as or larger than those of wasps and bees with brains orders of magnitude larger. Our comparative analyses suggest that brain organization in ants is shaped more by natural selection imposed by visual demands than intrinsic design limitations.     

Variation in foot size and shape in some British land snails and its functional significance

Live weight, and the length, breadth and surface area of the extended foot sole were measured in 24 species of British terrestrial snails. Allometric relationships between these variables are described, concentrating on the relationship between foot surface area and weight. Intraspecific foot area/weight relationships deviate from isometric expectation for several species; this may be attributed to density changes, foot-shape variation or the physical constraint imposed by shell aperture size.
The rate of increase of foot sole area with live weight among species is greater than expected under isometry, indicating that larger species partially compensate for the increase in foot loading (weight per unit foot area). The rate of whorl expansion of the shell is related to the deviations from the interspecific foot area/weight relationship, reinforcing the possibility that shell aperture area may limit foot size in some species. Foot ratio (foot length:foot width) is negatively related to live weight and foot loading.
The findings are discussed in relation to the known behaviour and microdistribution patterns of the species.     

Phenotypic and genetic variation in male genitalia in the seedbug, Lygaeus equestris (Heteroptera)

Male genitalia evolve through sexual selection and, in insects, tend to show negative static allometry, low phenotypic variation, and are usually relatively small. Much less is known about the genetic variation and heritability of male genitalia. Additionally, in instances where the intromittent organ is greatly elongated, it is unclear whether typical patterns of genital scaling and variation also apply. In the present study, we investigated the allometry, variation, and heritability of male genital length in the seedbug, Lygaeus equestris , a species with a greatly elongated intromittent organ (i.e. almost as long as male body size). We found that genital length was negatively allometric, in spite of its great length, and was no more variable than nongenital traits. Additionally, genital length was significantly heritable and had considerable evolvability.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 400–405.