Genetic and phenotypic correlations among size-related traits, and heritability variation between body parts in Drosophila buzzatii

Recent studies have shown that body size is a heritable trait phenotypically correlated with several fitness components in wild populations of the cactophilic fly Drosophila buzzatii. To obtain further information on size-related variation, heritabilities as well as genetic and phenotypic correlations among size-related traits of several body parts (head, thorax and wings) were estimated. The study was carried out on an Argentinean natural population in which size-related selection was previously detected. The genetic parameters were estimated using offspring-parent regressions (105 families) in the laboratory G₂ generation of a sample of wild flies. The traits were also scored in Wild-Caught Flies (WCF). Laboratory-Reared Flies (LRF) were larger and less variable than WCF. Although heritability estimates were significant for all traits, heritabilities were higher for thorax-wing traits than for head traits. Phenotypic and genetic correlations were all positive. The highest genetic correlations were found between traits which are both functionally and developmentally related. Genetic and phenotypic correlations estimated in the lab show similar correlation patterns (r = 0.49; TP = 0.02, Mantel's test). However, phenotypic correlations were found to be typically larger in WCF than in LRF. The genetic correlation matrix estimated in the relatively homogeneous lab environment is not simply a constant multiplicative factor of the phenotypic correlation matrix estimated in WCF. This revised version was published online in July 2006 with corrections to the Cover Date.     

Song repertoire size varies with HVC volume and is indicative of male quality in song sparrows (Melospiza melodia)

Complex birdsong is a classic example of a sexually selected ornamental trait. In many species, females prefer males with large song repertoires, possibly because repertoire size is limited by the size of song control nuclei which reflect developmental success. We investigated whether song repertoire size was indicative of brain area and male quality in song sparrows (Melospiza melodia) by determining if repertoire size was related to the volume of song control nucleus HVC, as well as several morphological, immunological and genetic indices of quality. We found that males with large repertoires had larger HVCs and were in better body condition. They also had lower heterophil to lymphocyte ratios, indicating less physiological stress and a robust immune system as measured by the number of lymphocytes per red blood cell. Song repertoire size also tended to increase with neutral-locus genetic diversity, as assessed by mean d2, but was not related to internal relatedness. Our results suggest several mechanisms that might explain the finding of a recent study that song sparrows with large song repertoires have higher lifetime fitness.     

Inbreeding coefficient and heterozygosity–fitness correlations in unhatched and hatched song sparrow nestmates

Heterozygosity–fitness correlations use molecular measures of heterozygosity as proxy estimates of individual inbreeding coefficients (f) to examine relationships between inbreeding and fitness traits. Heterozygosity–fitness correlations partly depend on the assumption that individual heterozygosity and f are strongly and negatively correlated. Although theory predicts that this relationship will be strongest when mean f and variance in f are high, few studies of heterozygosity–fitness correlations include estimates of f based on pedigrees, which allow for more thorough examinations of the relationship between f, heterozygosity and fitness in nature. We examined relationships between pedigree‐based estimates of f, multilocus heterozygosity (MLH) and the probability of survival to hatch in song sparrow nestmates. f and MLH were weakly, but significantly negatively correlated. Inbreeding coefficient predicted the probability of survival to hatch. In contrast, MLH did not predict the probability of survival to hatch nor did it account for residual variation in survival to hatch after statistically controlling for the effects of f. These results are consistent with the expectation that heterozygosity–f correlations will be weak when mean and variance in f are low. Our results also provide empirical support for recent simulation studies, which show that variation in MLH among siblings with equal f can be large and may obscure MLH–fitness relationships.     

硬叶兜兰居群表型变异研究

为了揭示硬叶兜兰表型变异的规律,以滇东南核心分布地区7个硬叶兜兰居群113个体的18个表型数量性状进行研究,通过变异系数、巢式方差分析、相关性和聚类分析,结果显示:除了叶长、叶宽、花梗粗、苞片长和苞片宽5个数量性状不存在显著差异外,其余数量性状在居群水平上都存在显著差异。各性状变异系数在0.088~0.189之间,花梗长、叶长、叶长宽比、附毛长的变异系数较大,而中萼高、花囊高和宽、花梗粗、合萼高、花瓣宽和长相对变异不大;18个性状表型分化系数(Vst)在2.05%~50.62%之间,平均值达到22.02%,暗示有77.98%表型变异存在于居群内。除雄蕊长和宽表现居群内和居群间的变异相当外,果蒴长、花瓣长和宽、中萼高和宽、合萼高和宽的分化系数分别为36.03%、23.74%、24.55%、28.13%、18.00%、20.56%和18.12%,叶长最小,为2.05%,居群间的变异明显小于居群内。性状关联性研究表明:花瓣、合萼、中萼高、花梗长和叶长宽比高度相关,而叶宽、苞片长、雄蕊性状和中萼高与其余性状关联性不密切,这些性状相对独立,稳定性较高;基于数量指标的欧氏距离聚类表明:7个居群可分为2大类:杨柳井居群和马固居群为近缘支;其余5个居群(斗咀、小坝子、夹寒箐、古林箐和田坝)为一类。     

Immediate and long-term effects of testosterone on song plasticity and learning in juvenile song sparrows

Steroid sex hormones play critical roles in the development of brain regions used for vocal learning. It has been suggested that puberty-induced increases in circulating testosterone (T) levels crystallize a bird's repertoire and inhibit future song learning. Previous studies show that early administration of T crystallizes song repertoires but have not addressed whether new songs can be learned after this premature crystallization. We brought 8 juvenile song sparrows (Melospiza melodia) into the laboratory in the late summer and implanted half of them with subcutaneous T pellets for a two week period in October. Birds treated with T tripled their singing rates and crystallized normal songs in 2 weeks. After T removal, subjects were tutored by 4 new adults. Birds previously treated with T tended toward learning fewer new songs post T, consistent with the hypothesis that T helps to close the song learning phase. However, one T-treated bird proceeded to learn several new songs in the spring, despite singing perfectly crystallized songs in the fall. His small crystallized fall repertoire and initial lag behind other subjects in song development suggest that this individual may have had limited early song learning experience. We conclude that an exposure to testosterone sufficient for crystallization of a normal song repertoire does not necessarily prevent future song learning and suggest that early social experiences might override the effects of hormones in closing song learning.     

Territory establishment,song learning strategies and survival in song sparrows

In most songbirds, the processes of song learning and territory establishment overlap in the early life and a young bird usually winds up with songs matching those of his territorial neighbors in his first breeding season. In the present study, we examined the relationships among the timing of territory establishment, the pattern of song learning and territorial success in a sedentary population of song sparrows (Melospiza melodia). Males in this population tend to learn their songs from their neighbors and consequently they show high song sharing with neighbors and use these shared songs preferentially in interactions with them. Males also show significant variation in the timing of territory establishment, ranging from their natal summer to the next spring. Using a three-year dataset, we found that the timing of territory establishment did not systematically affect the composition of the song repertoire of the tutee: early establishers and late establishers learned equally as much from their primary tutors and had a similar number of tutors and similar repertoire sizes, nor did timing of territory establishment affect subsequent survival on territory. Therefore, the song-learning program of song sparrows seems versatile enough to lead to high song sharing even for birds that establish territories relatively late.     

Cultural and genetic evolution in mountain white-crowned sparrows: song dialects are associated with population structure

Bird song often varies geographically within a species; when this geographic variation has distinct boundaries, the shared song types are referred to as song dialects. How dialects are produced and their adaptive significance are longstanding problems in biology, with implications for the role of culture in the evolution and ecology of diverse organisms, including humans. Here we test the hypothesis that song dialect, a culturally transmitted trait, is related to the population genetic structure of mountain white-crowned sparrows (Zonotrichia leucophrys oriantha). To address this, we compared microsatellite allele frequencies from 18 sample sites representing eight dialect regions in the Sierra Nevada. Pairwise genetic distances were not significantly correlated with geographic distances either within or between dialects, nor did dialect groups form distinct genetic groups according to neighbor-joining or UPGMA analysis, and most variation in allele frequencies occurred among individuals rather than at higher levels. However, most of the remaining variation was attributable to differences among, rather than within, dialect regions, and this among-dialect component of variance was statistically significant. Moreover, when controlling for the effect of geographic distance, song dissimilarity and genetic distance between site pairs were significantly correlated. Thus, song dialects appear to be associated with reductions in, but not strict barriers to, gene flow among dialect regions.     

Sexual variation in heritability and genetic correlations of morphological traits in house sparrow (Passer domesticus)

Estimates of genetic components are important for our understanding of how individual characteristics are transferred between generations. We show that the level of heritability varies between 0.12 and 0.68 in six morphological traits in house sparrows (Passer domesticus L.) in northern Norway. Positive and negative genetic correlations were present among traits, suggesting evolutionary constraints on the evolution of some of these characters. A sexual difference in the amount of heritable genetic variation was found in tarsus length, wing length, bill depth and body condition index, with generally higher heritability in females. In addition, the structure of the genetic variance-covariance matrix for the traits differed between the sexes. Genetic correlations between males and females for the morphological traits were however large and not significantly different from one, indicating that sex-specific responses to selection will be influenced by intersexual differences in selection differentials. Despite this, some traits had heritability above 0.1 in females, even after conditioning on the additive genetic covariance between sexes and the additive genetic variances in males. Moreover, a meta-analysis indicated that higher heritability in females than in males may be common in birds. Thus, this indicates sexual differences in the genetic architecture of birds. Consequently, as in house sparrows, the evolutionary responses to selection will often be larger in females than males. Hence, our results suggest that sex-specific additive genetic variances and covariances, although ignored in most studies, should be included when making predictions of evolutionary changes from standard quantitative genetic models.     

Mate choice for nonadditive genetic benefits and the maintenance of genetic diversity in song sparrows

The lek paradox asserts that strong directional selection via female choice should deplete additive genetic variation in fitness and consequently any benefit to females expressing the preference. Recently, we have provided a novel resolution to the paradox by showing that nonadditive genetic effects such as overdominance can be inherited from parent to offspring, and populations with females that express a mating preference for outbred males maintain higher genetic variation than populations with females that mate randomly. Here, we test our dynamic model using empirical data previously published from a small island population of song sparrows (Melospiza melodia). The model assumes that fitness and male trait expression display overdominance effects. The results demonstrate that female choice for outbred males mediated by directional selection on song repertoire size provides a heritable benefit to offspring through reduced inbreeding depression. Within the population, we estimate the heritability of the inbreeding coefficient to be 0.18 ± 0.08 (SD). Furthermore, we show that mate choice for outbred males increases fitness‐related genetic variation in the population by 12% and thereby reduces inbreeding depression by 1% per generation in typical years and upwards of 15% in severe years. Thus, mate choice may help to stave off population extinction in this and other small populations.     

The genetics of phenotypic plasticity. III. Genetic correlations and fluctuating asymmetries

We examined the relationship of three aspects of development, phenotypic plasticity, genetic correlations among traits, and developmental noise, for thorax length, wing length, and number of sternopleural bristles in Drosophila melanogaster. We used 14 lines which had previously been selected on either thorax length or plasticity of thorax length in response to temperature. A half-sib mating design was used and offspring were raised at 19° C or 25° C. We found that genetic correlations were stable across temperatures despite the large levels of plasticity of these traits. Plasticities were correlated among developmentally related traits, thorax and wing length, but not among unrelated traits, lengths and bristle counts. Amount of developmental noise, measured as fluctuating asymmetry and within-environmental variation, was positively correlated with amount of plasticity only for some traits, thorax length and bristle number, and only at one temperature, 25° C.     

Selection on heritable seasonal phenotypic plasticity of body mass

The ability to cope with environmental change is fundamental to a species' evolution. Organisms can respond to seasonal environmental variation through phenotypic plasticity. The substantial plasticity in body mass of temperate species has often been considered a simple consequence of change in environmental quality, but could also have evolved as an adaptation to seasonality. We investigated the genetic basis of, and selection acting on, seasonal plasticity in body mass for wild bighorn sheep ewes (Ovis canadensis) at Ram Mountain, Alberta, under two contrasting environmental conditions. Heritability of plasticity, estimated as mass-specific summer and winter mass changes, was low but significant. The additive genetic variance component of relative summer mass change was greater under good environmental conditions (characterized by a population increase and high juvenile survival) than under poor conditions (population decrease and low juvenile survival). Additive genetic variance of relative winter mass change appeared independent of environmental conditions. We found evidence of selection on summer (relative) and winter (relative and absolute) mass change. For a given mass, more plastic individuals (with greater seasonal mass changes) achieve greater fitness through reproduction in the following year. However, genetic correlations between mass parameters were positive. Our study supports the hypothesis that seasonal plasticity in body mass in vertebrates is an adaptation that evolved under natural selection to cope with environmental variation but genetic correlations with other traits might limit its evolutionary potential.     

Seasonal changes in song nuclei and song behavior in Gambel's white-crowned sparrows

In males of several songbird species, the morphology of forebrain nuclei that control song changes seasonally. The only seasonally breeding songbird in which seasonal changes in the structure of song control nuclei have been reported not to occur is the nonmigratory Nuttall's subspecies of white-crowned sparrow. In the present study, we manipulated photoperiod and plasma testosterone concentrations in captive male white-crowned sparrows of the migratory Gambel's subspeices. Males exposed to photoperiods and plasma testosterone concentrations typical of those experienced by wild breeding males had larger song control nuclei than males held on a winter photoperiod. We also found seasonal change in stereotypy of spectral and temporal parameters of song in wild Gambel's white-crowned sparrows. We hypothesize that seasonal changes in song control nuclei may correlate with seasonal changes in song stereotypy. © 1995 John Wiley & Sons, Inc.     

Modular genetic architecture of floral morphology in Nicotiana: quantitative genetic and comparative phenotypic approaches to floral integration

Animal‐pollinated flowers are complex structures that may require a precise configuration of floral organs for proper function. As such, they represent an excellent system with which we can examine the role of phenotypic integration and modularity in morphological evolution. We use complementary quantitative genetic and comparative phenotypic approaches to examine correlations among floral characters in Nicotiana alata, N. forgetiana and their artificial fourth‐generation hybrids. Flowers of both species share basic patterns of genetic and phenotypic correlations characterized by at least two integrated character suites that are relatively independent of each other and are not disrupted by four generations of recombination in hybrids. We conclude that these integrated character suites represent phenotypic modules that are the product of a modular genetic architecture. Intrafloral modularity may have been critical for rapid specialization of these species to different pollinators.