Quantitative genetic variation in static allometry in the threespine stickleback

The common pattern of replicated evolution of a consistent shape-environment relationship might reflect selection acting in similar ways within each environment, but divergently among environments. However, phenotypic evolution depends on the availability of additive genetic variation as well as on the direction of selection, implicating a bias in the distribution of genetic variance as a potential contributor to replicated evolution. Allometry, the relationship between shape and size, is a potential source of genetic bias that is poorly understood. The threespine stickleback, Gasterosteus aculeatus, provides an ideal system for exploring the contribution of genetic variance in body shape allometry to evolutionary patterns. The stickleback system comprises marine populations that exhibit limited phenotypic variation, and young freshwater populations which, following independent colonization events, have often evolved similar phenotypes in similar environments. In particular, stickleback diversification has involved changes in both total body size and relative size of body regions (i.e., shape). In a laboratory-reared cohort derived from an oceanic Alaskan population that is phenotypically and genetically representative of the ancestor of the diverse freshwater populations in this region, we determined the phenotypic static allometry, and estimated the additive genetic variation about these population-level allometric functions. We detected significant allometry, with larger fish having relatively smaller heads, a longer base to their second dorsal fin, and longer, shallower caudal peduncles. There was additive genetic variance in body size and in size-independent body shape (i.e., allometric elevation), but typically not in allometric slopes. These results suggest that the parallel evolution of body shape in threespine stickleback is not likely to have been a correlated response to selection on body size, or vice versa. Although allometry is common in fishes, this study highlights the need for additional data on genetic variation in allometric functions to determine how allometry evolves and how it influences phenotypic evolution.     

Geographical variation in allometry in the guppy (Poecilia reticulata)

Variation in static allometry, the power relationship between character size and body size among individuals at similar developmental stages, remains poorly understood. We tested whether predation or other ecological factors could affect static allometry by comparing the allometry between the caudal fin length and the body length in adult male guppies (Poecilia reticulata) among populations from different geographical areas, exposed to different predation pressures. Neither the allometric slopes nor the allometric elevations (intercept at constant slope) changed with predation pressure. However, populations from the Northern Range in Trinidad showed allometry with similar slopes but lower intercepts than populations from the Caroni and the Oropouche drainages. Because most of these populations are exposed to predation by the prawn Macrobrachium crenulatum, we speculated that the specific selection pressures exerted by this predator generated this change in relative caudal fin size, although effects of other environmental factors could not be ruled out. This study further suggests that the allometric elevation is more variable than the allometric slope.     

Bat genitalia: allometry, variation and good genes

Male genitalia are typically highly variable across species, for which sexual selection is thought to be responsible. Sexually selected traits characteristically show positive allometry and high phenotypic variation, although genitalia seem to be typified by negative allometry due to stabilizing selection. Additionally, while sexual selection appears to be the primary force responsible for genital evolution, the precise mechanism is unclear, but good-genes selection could be involved. If so, male genital variation should correlate with some male quality measure(s). We investigated the allometry of male Nyctalus noctula genitalia and investigated associations between genital size and three phenotypic measures of male quality (body size, relative body mass, and fluctuating asymmetry (FA)). We found that the penis exhibited positive allometry and high phenotypic variation, and was positively associated with male body size and relative body mass, but not with FA. This pattern is more typical of sexually selected display traits, contrasting with general patterns of genital allometry. The baculum was negatively allometric and was not associated with any quality measure. Our results suggest that the N. noctula penis is under directional sexual selection and is a reliable indicator of male phenotypic quality.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 497–507.     

Cranial allometry and geographic variation in slow lorises (Nycticebus)

A series of 20 craniodental measurements was obtained for two sister taxa: Nycticebus coucang (common slow loris) and N. pygmaeus (pygmy slow loris). Multivariate analysis of variance was performed with adult data to describe patterns of subspecific and specific variation in this genus. The geometric mean of adult cranial dimensions was compared to field data on latitudinal coordinates for available specimens to investigate if size variation in Nycticebus is clinal in nature. Ontogenetic series for larger-bodied N. coucang and smaller-bodied N. pygmaeus were compared to test the hypothesis that species and subspecific variation in skull form results from the differential extension of common patterns of relative growth. A MANOVA provides independent support of Groves's [pp. 44–53 in Proceedings of the Third International Congress on Primatology, Vol. 1 (Basel: S. Karger), 1971] classification of Nycticebus into two species, with four subspecies in the common slow loris and one form of the pygmy slow loris. Within N. coucang, cranial proportions for all four subspecies are ontogenetically scaled, and size differentiation is mainly clinal (Bergmann's Rule). N. c. bengalensis represents the most northerly disposed and the largest form. N. c. javanicus represents the next-largest form and is located in a southerly direction the next-farthest away from the equator. N. c. coucang and N. c. menagensis are both equatorial; however, the latter subspecies is the smallest. A genetic basis for some of the taxonomic variation between N. c. coucang and N. c. menagensis is supported by such nonclinal variation in body size. Variation in the presence/absence of I² is not size-related but rather tracks geographic proximity and isolating factors which predate the most recent inundation of the Sunda Shelf. Although they inhabit a nonequatorial environment, pygmy slow lorises are the smallest of all Nycticebus. As N. pygmaeus is sympatric with N. c. bengalensis, the largest slow loris, it appears that the evolution of its smaller body size represents a case of character displacement. Unlike N. coucang, skull size becomes significantly smaller in more northern N. pygmaeus. This may also reflect character displacement between sympatric sister taxa underlain by a cline-dependent ecological factor which is marked in more northerly latitudes. On the other hand, the negative correlation between body size and latitude in N. pygmaeus could be due to the influence of nonprimate fauna, such as predators, which themselves evince a similar clinal pattern. Analyses of relative growth indicate that skull proportions in the two species of Nycticebus are ontogenetically scaled in two-thirds of the cases. All but one of the seven comparisons (interorbital breadth) which do not indicate ontogenetic scaling represent part of the masticatory complex. This likely reflects a reorganization of N. pygmaeus maxillomandibular proportions linked to smaller size and changes in diet. Am. J. Primatol. 45:225–243, 1998. © 1998 Wiley-Liss, Inc.     

Temporal variation in body size and weapon allometry in the New Zealand giraffe weevil

1. Body size and exaggerated traits can show high phenotypic plasticity in response to environmental variation. Trait size can vary among generations but also fluctuate within a breeding season in response to resource availability. 2. This study documents patterns of temporal variation in body and weapon size, and in weapon allometry over 3 years for a wild population of New Zealand giraffe weevils [Lasiorhynchus barbicornis (Fabricius)], the males of which display an extremely elongated rostrum used as a weapon during contests for females. 3. It was predicted that body size and rostrum allometry would decrease during a breeding season, but in spite of significant annual and seasonal variation there was little evidence to support these predictions. Weapon allometry in males was more variable between years and over the breeding season than females, suggesting that male rostrum size may be more susceptible to environmental change than female rostrum size.     

Life-history variation and allometry for sexual size dimorphism in Pacific salmon and trout

Allometry for sexual size dimorphism (SSD) is common in animals, but how different evolutionary processes interact to determine allometry remains unclear. Among related species SSD (male : female) typically increases with average body size, resulting in slopes of less than 1 when female size is regressed on male size: an allometric relationship formalized as 'Rensch's rule' . Empirical studies show that taxa with male-biased SSD are more likely to satisfy Rensch's rule and that a taxon's mean SSD is negatively correlated with allometric slope, implicating sexual selection on male size as an important mechanism promoting allometry for SSD. I use body length (and life-history) data from 628 (259) populations of seven species of anadromous Pacific salmon and trout (Oncorhynchus spp.) to show that in this genus life-history variation appears to regulate patterns of allometry both within and between species. Although all seven species have intraspecific allometric slopes of less than 1, contrary to expectation slope is unrelated to species' mean SSD, but is instead negatively correlated with two life-history variables: the species' mean marine age and variation in marine age. Second, because differences in marine age among species render SSD and body size uncorrelated, the interspecific slope is isometric. Together, these results provide an example of how evolutionary divergence in life history among related species can affect patterns of allometry for SSD across taxonomic scales.     

Adaptation and plasticity in aboveground allometry variation of four pine species along environmental gradients

Plant species aboveground allometry can be viewed as a functional trait that reflects the evolutionary trade‐off between above‐ and belowground resources. In forest trees, allometry is related to productivity and resilience in different environments, and it is tightly connected with a compromise between efficiency‐safety and competitive ability. A better understanding on how this trait varies within and across species is critical to determine the potential of a species/population to perform along environmental gradients. We followed a hierarchical framework to assess tree height‐diameter allometry variation within and across four common European Pinus species. Tree height‐diameter allometry variation was a function of solely genetic components –approximated by either population effects or clinal geographic responses of the population's site of origin– and differential genetic plastic responses –approximated by the interaction between populations and two climatic variables of the growing sites (temperature and precipitation)–. Our results suggest that, at the species level, climate of the growing sites set the tree height‐diameter allometry of xeric and mesic species (Pinus halepensis, P. pinaster and P. nigra) apart from the boreal species (P. sylvestris), suggesting a weak signal of their phylogenies in the tree height‐diameter allometry variation. Moreover, accounting for interpopulation variability within species for the four pine species aided to: (1) detect genetic differences among populations in allometry variation, which in P. nigra and P. pinaster were linked to gene pools –genetic diversity measurements–; (2) reveal the presence of differential genetic variation in plastic responses along two climatic gradients in tree allometry variation. In P. sylvestris and P. nigra, genetic variation was the result of adaptive patterns to climate, while in P. pinaster and P. halepensis, this signal was either weaker or absent, respectively; and (3) detect local adaptation in the exponent of the tree height‐diameter allometry relationship in two of the four species (P. sylvestris and P. nigra), as it was a function of populations' latitude and altitude variables. Our findings suggest that the four species have been subjected to different historical and climatic constraints that might have driven their aboveground allometry and promoted different life strategies.     

Baculum variation and allometry in the muskrat (Ondatra zibethicus): a case for sexual selection

Sexual selection is a powerful force that influences the evolution of a variety of traits associated with female mate choice and male–male competition. Although other factors have been implicated, sexual selection may be particularly important in the evolution of the genitalia. Traits under sexual selection typically have high phenotypic variance and positive allometry relative to non-sexual traits. Here, we test the hypothesis that the baculum (os penis) of the muskrat (Ondatra zibethicus) is under sexual selection by examining phenotypic variance and allometry relative to non-sexual traits. Muskrats were sampled from Ontario, Canada, and a variety of traits measured. Measurements included baculum length and width, and three non-sexual traits (skull length, skull width, hind foot length). We used coefficient of variation (CV) and allometric slopes calculated using reduced major axis regression to test our hypotheses. Baculum traits had significantly higher CV’s relative to non-sexual traits. Baculum traits also showed positive allometry, whereas all non-sexual traits had negative allometric relationships. In addition, baculum width had higher CV’s and steeper allometric slopes than baculum length, indicating that, in muskrat, baculum width may be more influenced by sexual selection than baculum length. Positive allometry of the baculum is consistent with other examples of mammalian genitalia, but contrasts with negative allometry found in many insects. Other examples of positive allometry and high phenotypic variance of the baculum have suggested that females may use the baculum as an indicator of male quality. “Good genes” indicator traits may be particularly important in species that mate in an environmental context that prohibits female assessment of male quality. Muskrats mate aquatically, and thus females may be unable to properly assess males prior to copulation.     

Wood mechanics, allometry, and life-history variation in a tropical rain forest tree community

Wood density plays a central role in the life-history variation of trees, and has important consequences for mechanical properties of wood, stem and branches, and tree architecture. Wood density, modulus of rupture, modulus of elasticity, and safety factors for buckling and bending were determined for saplings of 30 Bolivian rain forest tree species, and related to two important life-history axes: juvenile light demand and maximum adult stature. Wood density was strongly positively related to wood strength and stiffness. Species safety factor for buckling was positively related to wood density and stiffness, but tree architecture (height : diameter ratio) was the strongest determinant of mechanical safety. Shade-tolerant species had dense and tough wood to enhance survival in the understorey, whereas pioneer species had low-density wood and low safety margins to enhance growth in gaps. Pioneer and shade-tolerant species showed opposite relationships between species traits and adult stature. Light demand and adult stature affect wood properties, tree architecture and plant performance in different ways, contributing to the coexistence of rain forest species.     

Linking stand-level self-thinning allometry to the tree-level leaf biomass allometry

Long-term experimental plots of Norway spruce and European beech are investigated for a link between stand-level self-thinning and tree-level leaf biomass allometry. Self-thinning refers to the finding of Reineke (1933), who postulated for unthinned forest stands that with β = −1.605; i.e. an increase of mean (quadratic) diameter d _q by 1% results in a decrease of tree number N by 1.605%. On the individual tree level, leaf biomass (w _L) can be related allometrically to the tree diameter d: w _L = ad ^α. If we assume that (a) the stands have reached the ceiling leaf area, (b) the specific leaf area (leaf area/leaf weight) is constant, and (c) differences resulting from the use of mean quadratic diameter or individual tree diameter are negligible, then the decrease in the stands’ leaf biomass due to the trees lost in self-thinning must be compensated by an equivalent increase in the remaining trees’ leaf biomass. This means, the absolute slope of the individual trees’ leaf biomass allometry α and the self-thinning allometry β would be equal and just have the opposite sign: α = −β. The analysis of the two long-term plots reveals that α is stronger than β, both for spruce (β = −1.744, α = 1.840) and especially for beech (β = −1.791, α = 2.181). The cause is traced back to a changing average specific leaf area during stand development [assumption (b) is wrong]. The results do not only bridge a gap between tree and stand allometry, but also emphasize an important effect for the understanding and modelling of the resource allocations in trees and forests.     

Information and allometry

Many life-history parameters have condition-dependent optima, but individuals are often required to set the values of such parameters relatively early in development, before the relevant conditions can be assessed with full accuracy. If cues are available that predict such future conditions, then the condition-dependent parameter should evolve to assume values that deviate from the mean in the direction implied by the cues, but these deviations should regress towards the mean to the degree that the cues are less than fully reliable. Under mild assumptions, the slopes of the resulting relationships between condition-dependent life-history parameters and the variable conditions on which their optima depend will be the ideal slopes (those that would maximize fitness if the parameter could be chosen on the basis of full information) devalued by the squared correlation between the condition and the parameter.     

Patterns of allozyme variation in western Eurasian Fagus

Fagus sylvatica L. is one of the most widespread broad‐leaved trees in the temperate forests of the northern hemisphere. In addition to two subspecies, F. sylvatica ssp. sylvatica in Europe and F. sylvatica ssp. orientalis in south‐western Asia, two further taxa were described: F. moesiaca (Maly) Czeczott in the south‐western Balkans and F. taurica Popl. in Crimea. The opinions about the number and ranks of taxa within this complex are highly controversial. To assess the degree of genetic differentiation among them, and to reveal geographical patterns of genetic diversity and their relationships to history and biogeography of beech populations, genetic variation at 12 allozyme loci was studied in 279 populations in western Eurasia. A Bayesian analysis of population structure revealed the existence of two clusters, which fairly well coincided with F. sylvatica ssp. sylvatica and F. sylvatica ssp. orientalis, whereby the populations from the south‐western Balkans and Crimea contained a mixture of these two gene pools. On the other hand, a neighbour‐joining tree based on pairwise F_ST failed to separate the subspecies into well‐defined distinct clades. Populations of F. sylvatica ssp. orientalis proved to be incomparably more differentiated than ssp. sylvatica (F_ST = 0.157 and 0.032, respectively). Asian populations also showed higher levels of allelic richness both on population and taxon levels than the European ones (the number of alleles after rarefaction was 3.40 and 4.27 in F. sylvatica ssp. sylvatica and ssp. orientalis, respectively). This indicates that the gene pool of F. sylvatica ssp. orientalis has not been depleted by reduced population sizes during the Pleistocene glaciations, as is the case of F. sylvatica ssp. sylvatica. Genetic similarities between isolated regional populations are explained by shared ancestral polymorphisms and/or range overlaps with subsequent hybridization in the past. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 154 , 165–174.     

Characteristics and seasonal variation in the semen of Markhoz bucks in western Iran

The aims of the present study were to evaluate if seasonality in semen characteristics and plasma testosterone concentrations exist in Markhoz male goats. Ten Markhoz (Angora) bucks were housed and fed according to standard recognized practices. During the observation period, semen was collected monthly with the aid of an electro-ejaculator and examined microscopically immediately after collection. Physical parameters of semen and the semen index were recorded. Blood samples were also taken monthly throughout the observation period and the plasma testosterone concentration monitored. Bucks demonstrated a higher semen quality (P < 0.05) in autumn and summer (semen index of 965 × 10⁶ and 752 × 10⁶ ml⁻¹, respectively), compared to spring and winter (semen index of 606 × 10⁶ and 512 × 10⁶, respectively). This coincided with a higher (P < 0.05) plasma testosterone concentration in autumn and summer (8.1 and 10.1 ng ml⁻¹, respectively), compared to that obtained in spring (3.0 ng ml⁻¹) and winter (2.5 ng ml⁻¹). During autumn and summer, the ejaculate volume (average of 1.2 and 1.0 ml), sperm output (1159 × 10⁶ and 1005 × 10⁶ sperm ml⁻¹), sperm mass motility (4.2 and 4.3), sperm progressive motility (83.9 and 82.0%) and percentage live sperm (90.7 and 88.2%, respectively) of the bucks were higher (P < 0.05) than in the spring (0.6 ml, 880 × 10⁶ sperm ml⁻¹, 3.3, 71.5% and 80.2%) and winter (0.7 ml, 863 × 10⁶ sperm ml⁻¹, 4.0, 71.5% and 84.9%, respectively). During autumn and summer, the percentage of sperm abnormalities (5.0 and 9.2%) was significantly lower than that in spring (12.9%) and winter (11.2%). The semen pH was slightly alkaline being significantly (P < 0.05) lower in the autumn (7.1) than in spring (7.3). Data showed season of the year to influence all semen parameters evaluated—indicating that optimal buck performance may be obtained in late summer and autumn. It can thus be said that Markhoz bucks have distinct seasonal spermatogenic activity, with poorer semen characteristics being recorded during winter and spring. This may be a critical obstacle when implementing an intensive breeding system of three kidding seasons in 2 years, with natural mating being implemented.     

Size variation,allometry and mating success in Aotearoa|New Zealand kelp flies (Coelopidae)

Evolutionary Ecology - The need to respond quickly to the presence of an ephemeral resource required for breeding is often a feature of scramble competition mating systems. Scramble competition...     

Comparing interspecific and intraspecific allometry in the Anatidae

Summary Interspecific scaling relationships (e.g. of limb size with body mass) in vertebrates are usually assumed to be functional (e.g. biomechanical) attributes. In this paper on the Anatidae, we study the scaling of wing length and tarsus length with body mass, relationships that can be expected to have biomechanical significance. At an interspecific level, both wing length and tarsus are positively allometric, a finding consistent with results from previous comparative avian studies. These trends remained significant in regressions controlling for the effects of phylogeny, but interspecific slopes were less steep within tribes than in the whole family (a taxon-level effect). We are not aware of any biomechanical resasons that explain these patterns satisfactorily. Intraspecific (static) allometries in Green-winged Teal (Anas crecca) and Marbled Teal (Marmaronetta angustirostris) are different: wing length is negatively allometric and tarsus is isometric. These anomalies suggest that interspecific and intraspecific scaling relationships do not share common causes. Our results bring into question the significance of interspecific allometries in vertebrate morphology, which may to some extent be non-functional by-products of morphological optimisation processes within species and ecological differences between them.

---

Vergleich inter- und intraspezifisher Allometrien bei Anatiden

Zusammenfassung Bei Vertebraten werden auf dem interspezifischem Niveau Größenbeziehungen (z. B. Gliedmaßenlänge mit Körpergewicht) üblicherweise funktional (z. B. bio-mechanisch) erklärt. In der vorliegende Studie untersuchen wir die Größenbeziehungen von Flügel- und Tarsuslänge mit der Körpermasse bei Anatiden. Es ist zu erwarten, daß diese Beziehungen biomechanische Bedeutung haben. Auf dem interspefischen Niveau (morphometrische Daten von 42 westpaläarktischen Arten wurden untersucht) sind sowohl Flügellänge als auch Tarsuslänge positiv allometrisch, d. h. länger mit zunehmender Masse. Dieses Ergebnis stimmt mit früheren, vergleichenden Studien an Vögeln überein. Diese Trends waren auch in Regressionsanalysen signifikant, in denen auf die Effekte der Phylogenie kontrolliert wurde. Allerdings, waren die interspezifischen Steigungen innerhalb der Triben geringer als in ganzen Familien (Effekt des taxonomischen Niveaus). Für diese Muster haben wir keine befriedigende biomechanische Erklärung. Intraspezifische (statische) Allometrien in einjährigen Krickenten (Anas crecca) und Marmelenten (Marmaronetta angustirostris) unterscheiden sich: die Flügellänge ist hier negativ allometrisch (d. h. relativ kürzer bei steigender Masse), die Tarsuslänge ist isometrisch. Diese Anomalien weisen darauf hin, dass inter-und intraspezifischen Größenbeziehungen keine gemeinsame Erklärung zugrunde liegen. Unsere Ergebnisse stellen die Bedeutung von interspezifischen Allometrien in der Morphologie von Vertebraten in Frage. Sie könnten zum Teil nichtfunktionelle Nebenprodukte von morphologischer Spezialisierung innerhalb von Arten, aber auch von ökologischen Unterschieden zwischen Arten sein.

     

Habitat-dependent geographical variation in ontogenetic allometry of the shiner perch Cymatogaster aggregata Gibbons (Teleostei: Embiotocidae)

Studies of intraspecific morphological variation in fishes have traditionally focused on freshwater rather than marine species. In addition, such studies typically focus on adults, although causes and intensities of selective pressures most likely vary through an individual's lifetime. In this study, body and head shape of a marine species, shiner perch Cymatogaster aggregata Gibbons were compared among localities along the Pacific Northwest coast of North America. Evidence was found for intraspecific variation in ontogenetic allometry, and for a closer correlation of body shape with environment rather than geographical proximity. This correlation with environment was more evident in younger fish, thereby demonstrating the importance of analysing multiple life stages. A common garden experiment suggests both environmental and genetic bases for the observed differences. Recognizing intraspecific ecomorphological complexity and its specificity to habitat and/or life stage can have important consequences for understanding the role of local adaptation and population dynamics in macroecology.