Heterozygosity, inbreeding and neonatal traits in Soay sheep on St Kilda

We investigated whether birth weight and neonatal survival, a period within which 24% of all mortalities occur, were correlated with levels of inbreeding in St Kilda Soay sheep, using pedigree inbreeding coefficients and four marker-based estimators of inbreeding. None of the inbreeding estimators, either of the offspring, or of their mothers, explained significant variation in a lamb's birth weight or probability of surviving the neonatal period, suggesting low inbreeding depression for these traits. We evaluated the correlation between the marker-based measures of inbreeding and inbreeding coefficients obtained from the Soay pedigree, where paternal links were inferred using the same panel of microsatellite markers. Even when using a relatively complete portion of the pedigree, in which all individuals had known maternal and paternal grandparents, the correlation was found to be weak (r = -0.207, where mean f = 0.0168). These results add support to the recent prediction that when the mean and variance in inbreeding are low in a population, heterozygosity-fitness correlations can be very weak or even undetectable. The pursuit of more detailed pedigrees offers the best prospect for identifying inbreeding depression within this study population.     

Ageing in a variable habitat: environmental stress affects senescence in parasite resistance in St Kilda Soay sheep

Despite widespread empirical evidence for a general deterioration in the majority of traits with advancing age, it is unclear whether the progress of senescence is chronologically determined, or whether factors such as environmental conditions experienced over the lifespan are more important. We explored the relative importance of ‘chronological’ and ‘environmental’ measures of age to changes in parasite resistance across the lifespan of free-living Soay sheep. Our results show that individuals experience an increase in parasite burden, as indicated by gastrointestinal helminth faecal egg count (FEC) with chronological age. However, chronological age fails to fully explain changes in FEC because a measure of environmental age, cumulative environmental stress, predicts an additional increase in FEC once chronological age has been accounted for. Additionally, we show that in females age-specific changes are dependent upon the environmental conditions experienced across individuals'' life histories: increases in FEC with age were greatest among individuals that had experienced the highest degree of stress. Our results illustrate that chronological age alone may not always correspond to biological age, particularly in variable environments. In these circumstances, measures of age that capture the cumulative stresses experienced by an individual may be useful for understanding the process of senescence.     

Correlated responses of chickens to selection for production of antibodies to sheep erythrocytes

A bi directional selection experiment was conducted to measure 5-day antibody titers to sheep erythrocytes in White Leghorn chickens. There was an immediate response to selection with significant differences between lines for the selected trait found in the S₁ and all subsequent generations. Comparisons of S₆, S₇ and S₈ generation females revealed differences between lines in disease resistance and in certain reproductive traits such as age at first egg, percentage hen-day egg production, percentage fertility and duration of fertility. The implications of these correlated responses are important to selection programs for general disease resistance.     

Grazing decisions of Soay sheep,Ovis aries,on St Kilda: a consequence of parasite distribution?

Relationships between the height of grass swards and herbage intake have been established for a number of large herbivores and sward height plays a major role in determining the selection of herbivore diets. However, tall grass swards also represent a more stable damp microclimate for gastrointestinal parasite larvae and tall swards contain generally greater numbers of parasites than short swards. Herbivores may then trade-off the need to maximise nutrient intake through the selection of tall swards with the need to avoid parasite ingestion. Stratified sward sampling techniques were used to determine the distribution of nutrients and parasites in a heterogeneous sward structure on the island of Hirta, St Kilda, which is grazed by a population of feral Soay sheep. The disparity between the short gap vegetation and the tall tussock vegetation was greatest in spring, when gap vegetation was some three-fold shorter than tussock vegetation; this led to tussocks offering greater nutrient and energy intake rate when compared to gap vegetation in spring. Parasites were concentrated in tussocks in spring, thus creating trade-offs. Such trade-offs were not present in summer when parasites were more evenly distributed across the gap/tussock sward structure and the nutritional advantages associated with grazing tussocks were diminished. The diet selection of the resident population of sheep was determined by recording the number of bites taken from gap and tussock vegetation by randomly selected focal animals over repeated 5-min grazing periods. Overall, all animals avoided grazing tussocks, which were most strongly avoided in the spring, and older animals avoided grazing tussocks to a greater extent. Overall, females with lambs and males avoided tussocks to a similar degree and both avoided them less than barren females. Faecal egg counts (FEC) of female sheep were negatively correlated with tussock selectivity and vice versa for male sheep. The interaction between the grazing behaviour of each sex and FEC is discussed in relation to the immunocompetence and nutrient requirements of the different sexes. The maximum disparity between the costs and benefits of the trade-off studied occurred in late winter/early spring which also represents the time of greatest nutrient demand in the Soay sheep, since many are close to starvation and/or are in the advanced stages of pregnancy. Grazing decisions of individuals at this time, determining relative nutrient and parasite intake, may then be related to subsequent fitness and survival.     

Synergistic selection and graded traits

Fitness interactions where benefits are shared only between individuals with similar traits are often referred to as synergistic. Examples include defence characters, like insect warning colouration and plant unpalatability, and joint activities needing the active participation of all group members, such as cooperative hunting. Previous analyses, assuming discrete variation in the trait, have shown that synergistic selection can be a sufficient explanation for the evolutionary stability of such traits. Here, we investigate the consequences of graded variation in the trait responsible for synergistic effects. Classifying the synergism as unbiased when an individual receives maximum associational benefit by having the same trait value as its neighbours, and letting a positive (negative) bias represent the maximum above (below) this value, we show that only positively biased synergistic selection can enhance a graded trait. Thus for graded traits, a synergistic benefit is not in itself sufficient for evolutionary stability. We study possible reasons for synergistic bias in a simple model of plant defences against herbivores, and suggest that the processes of herbivore avoidance learning and diet selection are probable causes of positive bias. We propose that mammalian herbivores exposed to a given level of toxicity will show stronger feeding aversion to higher toxicity, resulting in positively biased synergistic selection of plant defence traits. Positive bias produced by avoidance learning may, in a similar way, also select for defence signals.     

Modeling linkage disequilibrium in natural populations: the example of the Soay sheep population of St. Kilda, Scotland

The use of linkage disequilibrium to localize the genes underlying quantitative traits has received considerable attention in the livestock genetics community over the past few years. This has resulted in the investigation of linkage disequilibrium structures of several domestic livestock populations to assess their potential use in fine-mapping efforts. However, the linkage disequilibrium structure of free-living populations has been less well investigated. As the direct evaluation of linkage disequilibrium can be both time consuming and expensive the use of simulations that include as many aspects of population history as possible is advocated as an alternative. A simulation of the linkage disequilibrium structure of the Soay sheep population of St. Kilda, Scotland, is provided as an example. The simulated population showed significant decline of linkage disequilibrium with genetic distance and low levels of background linkage disequilibrium, indicating that the Soay sheep population is a viable resource for linkage disequilibrium fine mapping of quantitative trait loci.     

The evolution of threshold traits: effects of selection on fecundity and correlated response in wing dimorphism in the sand cricket

The quantitative genetic basis of traits can be determined using a pedigree analysis or a selection experiment. Each approach is valuable and the combined data can contribute more than either method alone. Analysis using both sib analysis and selection is particularly essential when there are likely to be nonlinearities in the functional relationships among traits. A class of traits for which this occurs is that of threshold traits, which are characterized by a dichotomous phenotype that is determined by a threshold of sensitivity and a continuously distributed underlying trait called the liability. In this case, traits that are correlated with the liability may show a nonlinear relationship due to the dichotomy of expression at the phenotypic level. For example, in wing dimorphic insects fecundity of the macropterous (long-winged) females appears in part to be determined by the allocation of resources to the flight muscles, which are almost invariably small or absent in the micropterous (short-winged, flightless) females. Pedigree analysis of the cricket Gryllus firmus has shown that wing morph, fecundity and the trade-off between the two have additive genetic (co)variance. It has also been shown that selection on proportion macroptery produced an asymmetric correlated response of fecundity. The present paper details the results of direct selection on fecundity and the correlated response in proportion macroptery. Selection for increased fecundity resulted in increased fecundity within both wing morphs and a correlated decrease in proportion macroptery. Similarly, selection for decreased fecundity resulted in a decrease within morphs and a correlated increase in the proportion of macropterous females. This provides additional evidence that the trade-off between fecundity and wing morphology has a genetic basis and will thus modulate the evolution of the two traits.     

Distinct evolutionary patterns of morphometric sperm traits in passerine birds

The striking diversity of sperm shape across the animal kingdom is still poorly understood. Postcopulatory sexual selection is an important factor driving the evolution of sperm size and shape. Interestingly, morphometric sperm traits, such as the length of the head, midpiece and flagellum, exhibit a strong positive phenotypic correlation across species. Here we used recently developed comparative methods to investigate how such phenotypic correlations between morphometric sperm traits may evolve. We compare allometric relationships and evolutionary trajectories of three morphometric sperm traits (length of head, midpiece and flagellum) in passerine birds. We show that these traits exhibit strong phenotypic correlations but that allometry varies across families. In addition, the evolutionary trajectories of the midpiece and flagellum are similar while the trajectory for head length differs. We discuss our findings in the light of three scenarios accounting for correlated trait evolution: (i) genetic correlation; (ii) concerted response to selection acting simultaneously on different traits; and (iii) phenotypic correlation between traits driven by mechanistic constraints owing to selection on sperm performance. Our results suggest that concerted response to selection is the most likely explanation for the phenotypic correlation between morphometric sperm traits.     

Variation,selection and evolution of function-valued traits

We describe an emerging framework for understanding variation, selection and evolution of phenotypic traits that are mathematical functions. We use one specific empirical example – thermal performance curves (TPCs) for growth rates of caterpillars – to demonstrate how models for function-valued traits are natural extensions of more familiar, multivariate models for correlated, quantitative traits. We emphasize three main points. First, because function-valued traits are continuous functions, there are important constraints on their patterns of variation that are not captured by multivariate models. Phenotypic and genetic variation in function-valued traits can be quantified in terms of variance-covariance functions and their associated eigenfunctions: we illustrate how these are estimated as well as their biological interpretations for TPCs. Second, selection on a function-valued trait is itself a function, defined in terms of selection gradient functions. For TPCs, the selection gradient describes how the relationship between an organism's performance and its fitness varies as a function of its temperature. We show how the form of the selection gradient function for TPCs relates to the frequency distribution of environmental states (caterpillar temperatures) during selection. Third, we can predict evolutionary responses of function-valued traits in terms of the genetic variance-covariance and the selection gradient functions. We illustrate how non-linear evolutionary responses of TPCs may occur even when the mean phenotype and the selection gradient are themselves linear functions of temperature. Finally, we discuss some of the methodological and empirical challenges for future studies of the evolution of function-valued traits.     

Predicting trait values and measuring selection in complex life histories: reproductive allocation decisions in Soay sheep

Accurate prediction of life history phenomena and characterisation of selection in free-living animal populations are fundamental goals in evolutionary ecology. In density regulated, structured populations, where individual state influences fate, simple and widely used approaches based on individual lifetime measures of fitness are difficult to justify. We combine recently developed structured population modelling tools with ideas from modern evolutionary game theory (adaptive dynamics) to understand selection on allocation of female reproductive effort to singletons or twins in a size-structured population of feral sheep. In marked contrast to the classical selection analyses, our model-based approach predicts that the female allocation strategy is under negligible directional selection. These differences arise because classical selection analysis ignores components of offspring fitness and fails to consider selection over the complete life cycle.     

Genetic parameter estimates for pre-weaning weight traits in Dorper sheep

Genetic parameters were estimated for birth-, 42-day, and 100-day (weaning) weight in the Dorper flock of the Glen Agricultural Institute in South Africa. Direct heritability estimates of 0.11, 0.28 and 0.20 and maternal heritability estimates of 0.10, 0.10 and 0.10 were obtained for body weights at birth, 42 and 100 days, respectively. The corresponding genetic correlation estimates between direct and maternal effects were 0.35, −0.63 and −0.58, respectively. Both direct and maternal genetic correlation estimates among the traits were of moderate to high magnitude and positive. It is concluded that the traits can be improved by selection with no serious antagonisms among traits studied.     

Accounting for female space sharing in St. Kilda Soay sheep (Ovis aries) results in little change in heritability estimates

When estimating heritability in free‐living populations, it is common practice to account for common environment effects, because of their potential to generate phenotypic covariance among relatives thereby biasing heritability estimates. In quantitative genetic studies of natural populations, however, philopatry, which results in relatives being clustered in space, is rarely accounted for. The two studies that have been carried out so far suggest absolute declines in heritability estimates of up to 43% when accounting for space sharing by relatives. However, due to methodological limitations these estimates may not be representative. We used data from the St. Kilda Soay sheep population to estimate heritabilities with and without accounting for space sharing for five traits for which there is evidence for additive genetic variance (birthweight, birth date, lamb August weight, and female post‐mortem jaw and metacarpal length). We accounted for space sharing by related females by separately incorporating spatial autocorrelation, and a home range similarity matrix. Although these terms accounted for up to 18% of the variance in these traits, heritability estimates were only reduced by up to 7%. Our results suggest that the bias caused by not accounting for space sharing may be lower than previously thought. This suggests that philopatry does not inevitably lead to a large bias if space sharing by relatives is not accounted for. We hope our work stimulates researchers to model shared space when relatives in their study population share space, as doing so will enable us to better understand when bias may be of particular concern.     

Impact of production strategies and animal performance on economic values of dairy sheep traits

The objective of this study was to carry out a sensitivity analysis on the impact of various production strategies and performance levels on the relative economic values (REVs) of traits in dairy sheep. A bio-economic model implemented in the program package ECOWEIGHT was used to simulate the profit function for a semi-extensive production system with the Slovak multi-purpose breed Improved Valachian and to calculate the REV of 14 production and functional traits. The following production strategies were analysed: differing proportions of milk processed to cheese, customary weaning and early weaning of lambs with immediate sale or sale after artificial rearing, seasonal lambing in winter and aseasonal lambing in autumn. Results of the sensitivity analysis are presented in detail for the four economically most important traits: 150 days milk yield, conception rate of ewes, litter size and ewe productive lifetime. Impacts of the differences in the mean value of each of these four traits on REVs of all other traits were also examined. Simulated changes in the production circumstances had a higher impact on the REV for milk yield than on REVs of the other traits investigated. The proportion of milk processed to cheese, weaning management strategy for lambs and level of milk yield were the main factors influencing the REV of milk yield. The REVs for conception rate of ewes were highly sensitive to the current mean level of the trait. The REV of ewe productive lifetime was most sensitive to variation in ewe conception rate, and the REV of litter size was most affected by weaning strategy for lambs. On the basis of the results of sensitivity analyses, it is recommended that economic values of traits for the overall breeding objective for dairy sheep be calculated as the weighted average of the economic values obtained for the most common production strategies of Slovak dairy sheep farms and that economic values be adjusted after substantial changes in performance levels of the traits.     

Survival and behaviour of castrated Soay sheep (Ovis aries) in a feral island population on Hirta, St. Kilda, Scotland

The free-living population of Soay sheep on the island of Hirta, St. Kilda, in the Outer Hebrides, has been intensively studied since 1959. The present study was initiated to throw light on the causes of the high mortality rate of adult rams in comparison to that of ewes. In 1978, 1979 and 1980, a total of 72 male lambs was castrated within a day or two of birth. The survival of these castrates has been much longer than that of the entire rams, marked as controls, and longer than that of ewes of the cohorts of the same years. The daily activity pattern of the castrates was similar to that of ewes rather than that of rams. In particular, during the rut the castrates spent most of the daylight hours grazing, in contrast to the rams who were continuously moving and involved in agonistic and sexual encounters. This study substantiates the earlier assertion that the costs in energy of reproduction are a major cause of mortality in temperate zone ungulates. The social organization of some castrates was similar to that of females in that they remained with the home-range group of ewes into which they were born, but other individuals resembled males in that these castrates clubbed together in their own groups. These patterns of behaviour persisted throughout life.     

Quantitative trait loci underlying milk production traits in sheep

Improvement of milk production traits in dairy sheep is required to increase the competitiveness of the industry and to maintain the production of high quality cheese in regions of Mediterranean countries with less favourable conditions. Additional improvement over classical selection could be reached if genes with significant effects on the relevant traits were specifically targeted by selection. However, so far, few studies have been undertaken to detect quantitative trait loci (QTL) in dairy sheep. In this study, we present a complete genome scan performed in a commercial population of Spanish Churra sheep to identify chromosomal regions associated with phenotypic variation observed in milk production traits. Eleven half-sib families, including a total of 1213 ewes, were analysed following a daughter design. Genome-wise multi-marker regression analysis revealed a genome-wise significant QTL for milk protein percentage on chromosome 3. Eight other regions, localized on chromosomes 1, 2, 20, 23 and 25, showed suggestive significant linkage associations with some of the analysed traits. To our knowledge, this study represents the first complete genome scan for milk production traits reported in dairy sheep. The experiment described here shows that analysis of commercial dairy sheep populations has the potential to increase our understanding of the genetic determinants of complex production-related traits.     

Natural selection and non-metric traits in skeletal populations

The hypothesis of this article is that the frequency of non-metric traits varies at different reproductive periods because of differential mortality associated with non-metric traits. 450 male and 203 female crania were examined. Mortality in the reproduction period is connected with the elimination of individuals with certain variants of non-metric traits. It seems that key genes which perform the stimulating function in the appearance of traits in a given category of formation are under selective pressure. It is assumed that natural selection acts both on the morphology of organisms and on developmental process through environmental conditions. Selection not only optimises the fitness of morphological features but also the processes leading to the formation of features during ontogeny. Both these mechanisms influence the interpopulational variability of non-metric traits.     

Bad coat, ripped genes: cryptic selection on coat colour varies with ontogeny in Soay sheep

A fundamental goal for evolutionary biologists is to connect the processes of natural selection and genetic drift with the maintenance of genetic variation in the wild. There are now scores of examples in which mapping phenotypes to genotypes has identified the molecular basis of traits in natural populations, but documenting the fitness consequences of these genes has proven more elusive. A long-term study of several thousand unmanaged sheep on the remote Scottish island of Hirta in the St. Kilda archipelago provides a unique opportunity to make these connections between genotype, phenotype and fitness. Since 1985, there has been a steady decline in the frequency of sheep with a rare uniform ('self'-) pigmentation pattern as opposed to the more common wild-type pattern of dark upper body and pale belly. In this issue of Molecular Ecology, Gratten et al. (2012) link variation at the agouti signalling protein (ASIP) gene with 25 years of pedigree information and individual fitness data to investigate the contributions of natural selection and genetic drift to this decline. Consistent with the decrease in frequency of self-type sheep, Gratten et al. demonstrate that recessive genetic variants at ASIP are associated with reduced lifetime fitness. But surprisingly, these variants show the opposite trend to the self-type phenotype by increasing throughout the study period. This paradoxical result occurs because heterozygotes that harbour the majority of self-type alleles have increased in frequency. This pattern would not have been detectable if only phenotype had been monitored because the pigmentation pattern of heterozygotes is indistinguishable from that of wild-type homozygotes. The study provides an excellent example of the importance of measuring selection at both the phenotypic and genetic level, and demonstrates how long-term pedigrees can be used to link alleles to phenotypes and ultimately to fitness.     

Assortment and the analysis of natural selection on social traits

A central problem in evolutionary biology is to determine whether and how social interactions contribute to natural selection. A key method for phenotypic data is social selection analysis, in which fitness effects from social partners contribute to selection only when there is a correlation between the traits of individuals and their social partners (nonrandom phenotypic assortment). However, there are inconsistencies in the use of social selection that center around the measurement of phenotypic assortment. Here, we use data analysis and simulations to resolve these inconsistencies, showing that: (i) not all measures of assortment are suitable for social selection analysis; and (ii) the interpretation of assortment, and how to detect nonrandom assortment, will depend on the scale at which it is measured. We discuss links to kin selection theory and provide a practical guide for the social selection approach.     

湖羊、东湖杂种羊POMC基因外显子3单核苷酸多态性及其与生长性状的关联分析

阿黑皮素原(Pro-opiomelanocortin, POMC)在动物采食和能量平衡调控中发挥重要作用, 文章对绵羊POMC基因外显子3进行扩增和测序, 筛选多态性位点, 并分析多态位点与湖羊和东弗里生×湖羊杂种羊生长性状的相关性。测序后发现湖羊POMC基因外显子3有2个单碱基突变(g.273 T/C和g.456 G/A), 根据273位点处发生的T/C突变, 建立PCR-RFLP分析方法, 并对162只湖羊和130只东湖杂种羊进行检测分析。结果发现, 在湖羊群体中检测到TT(0.469)、TC(0.438)和CC(0.093)3种基因型, 而在东湖杂种羊群体中仅检测到TT(0.754)和TC(0.246)两种基因型。POMC基因外显子3的273位点多态性与生长性状的相关性研究结果显示：湖羊群体中CC基因型个体的2月龄断奶重、4月龄尻高及TC基因型个体4月龄体长和管围均显著高于TT型个体(P<0.05); CC基因型个体的4月龄重、6月龄重极显著高于TT和TC基因型个体(P<0.01); CC基因型个体的4月龄体高和体长极显著高于TT型个体(P<0.01), 且显著高于TC基因型个体(P<0.05)。此外, CC型个体的管围极显著高于TT基因型个体(P<0.01)。东湖杂种羊群体中TC基因型个体的2月龄断奶重、4月龄重及4月龄体高、体长、胸深和管围都显著高于TT型个体(P<0.05), TC型个体的6月龄重极显著高于TT型个体(P<0.01)。研究结果表明, POMC基因外显子3与绵羊生长性状相关, C等位基因对体重及体尺性状的增加更有利。该结果为进一步探讨POMC基因作为绵羊生长性状的辅助选育标记奠定了基础。