Genetic Variation In Growth Rate Research Articles

Elevational clines predict genetically determined variation in tropical forest seedling performance in Borneo: implications for seed sourcing to support reforestation

While much research has focused on genetic variation in plants in relation to abiotic clines in temperate and boreal forests, few studies have examined similar relationships in tropical forests. Genetic variation in desirable performance traits of trees, such as drought tolerance, fast‐growth, and carbon sequestration rates, is widely used to improve reforestation efforts in nontropical systems. However, evolutionary processes such as local adaptation are poorly understood in tropical forests making it difficult to locate desired phenotypes. To test for genetic variation in growth rate in relationship to climatic clines, we conducted a common garden study over 18 months in a nursery using four dipterocarp tree species, represented by 9–12 half‐sib families, sourced across an elevational gradient ranging from lowland to hill forests (circa 130–470 m above sea‐level) in Malaysian Borneo. We found genetic variation in growth for all four species with fast‐growing half‐sib families growing 42–88% faster than poorly performing half‐sib families. Furthermore, in three species we found that elevation of seedling origin predicted seedling performance; in Shorea fallax and S. johorensis, half‐sib families originating from low elevations performed the best. In S. argentifolia half‐sib families' seedlings from low elevations grew slowly. Because elevation is a good proxy for climate, the finding of elevational clines predicting genetic variation in growth provides evidence of evolution affecting the function of tropical tree species. Our research highlights opportunities to better understand evolutionary processes in tropical forests and to use such information to improve seed source selection in reforestation.

A plant growth model to test for changes in plant–plant interaction over a growing season: the case of kin competition

Kin recognition and kin selection have long been known to occur in animals where they shape altruistic behavior towards relatives. More recently, studies have found that kin recognition and altered behavior towards kin can also occur in plants. However, inferring the underlying mechanism responsible for variation in plant performance in experimental studies is challenging as often, results can be explained by alternative and non‐exclusive mechanisms such as niche differences, kin competition avoidance and genetic variation in growth rate and competitive ability. Plant–plant interactions may change with the life stage of plants, and competition is often most intense towards the end of plants growing season. However, changes in plant–plant interaction intensity across plants life cycle are rarely considered in kin interaction studies. Here, we adapt a model of plant growth over time modified to specifically include effects of kin and non‐kin competition. The model decompose competitive interactions at different stages during plant growth from initial growth to the end of the growing season. It estimates genotype specific variation in growth rate, and how sensitive individual genotypes are to competition from neighbors. Furthermore, it estimates size asymmetry among plants accounting for both variation in growth rate, neighbor relatedness and resource variation (here water availability). We use this model to analyze the results from a competition experiment where plants grew in mini‐populations with neighbor plants that were either kin or non‐kin. We find that when applied to our experiments, this approach can disentangle kin effects from other effects caused by genotypic variation in growth rate and competitive response to neighbors, and thus significantly help to detect whether plants exhibit kin‐cooperative behavior.

Determining the Genetic Control of Common Bean Early-Growth Rate Using Unmanned Aerial Vehicles

Vigorous early-season growth rate allows crops to compete more effectively against weeds and to conserve soil moisture in arid areas. These traits are of increasing economic importance due to changing consumer demand, reduced labor availability, and climate-change-related increasing global aridity. Many crop species, including common bean, show genetic variation in growth rate, between varieties. Despite this, the genetic basis of early-season growth has not been well-resolved in the species, in part due to historic phenotyping challenges. Using a range of UAV- and ground-based methods, we evaluated the early-season growth vigor of two populations. These growth data were used to find genetic regions associated with several growth parameters. Our results suggest that early-season growth rate is the result of complex interactions between several genetic and environmental factors. They also highlight the need for high-precision phenotyping provided by UAVs. The quantitative trait loci (QTLs) identified in this study are the first in common bean to be identified remotely using UAV technology. These will be useful for developing crop varieties that compete with weeds and use water more effectively. Ultimately, this will improve crop productivity in the face of changing climatic conditions and will mitigate the need for water and resource-intensive forms of weed control.

Heritability of growth and leaf loss compensation in a long-lived tropical understorey palm.

IntroductionDefoliation and light competition are ubiquitous stressors that can strongly limit plant performance. Tolerance to defoliation is often associated with compensatory growth, which could be positively or negatively related to plant growth. Genetic variation in growth, tolerance and compensation, in turn, plays an important role in the evolutionary adaptation of plants to changing disturbance regimes but this issue has been poorly investigated for long-lived woody species. We quantified genetic variation in plant growth and growth parameters, tolerance to defoliation and compensation for a population of the understorey palm Chamaedorea elegans. In addition, we evaluated genetic correlations between growth and tolerance/compensation.MethodsWe performed a greenhouse experiment with 711 seedlings from 43 families with twelve or more individuals of C. elegans. Seeds were collected in southeast Mexico within a 0.7 ha natural forest area. A two-third defoliation treatment (repeated every two months) was applied to half of the individuals to simulate leaf loss. Compensatory responses in specific leaf area, biomass allocation to leaves and growth per unit leaf area were quantified using iterative growth models.ResultsWe found that growth rate was highly heritable and that plants compensated strongly for leaf loss. However, genetic variation in tolerance, compensation, and the individual compensatory responses was low. We found strong correlations between family mean growth rates in control and defoliation treatments. We did not find indications for growth-tolerance/compensation trade-offs: genetic correlation between tolerance/compensation and growth rate were not significant.ImplicationsThe high genetic variation in growth rate, but low genetic variation in tolerance and compensation observed here suggest high ability to adapt to changes in environment that require different growth rates, but a low potential for evolutionary adaptation to changes in damage or herbivory. The strong correlations between family mean growth rates in control and defoliation treatments suggest that performance differences among families are also maintained under stress of disturbance.

Sensitivity of the breeding values for growth rate and worm egg count to environmental worm burden in Australian Merino sheep

The objective of this study was to explore the sensitivity of breeding values for growth rate and worm egg count (WEC, cube root transformed) to environmental worm burden, measured as the average WEC for each contemporary group (CGWEC). Growth rate and WEC were measured on 7,818 naturally infected Merino lambs in eight flocks across Australia, linked through common use of AI sires. Through bivariate analysis, genetic correlations of 0.55±0.23 and 0.30±0.16 were found for growth rate and WEC between low and high CGWEC, respectively. In a second analysis, breeding values for growth rate and WEC were regressed on CGWEC with a random regression model. The heritability for growth rate varied from 0.23 to 0.16 from low to high CGWEC, and the heritability for WEC varied from 0.25 to 0.36. Results suggest that breeding values for both growth rate and WEC are sensitive to environmental worm burden. Animals expressed less genetic variation for growth rate and more genetic variation for WEC in high CGWEC than in low CGWEC. This form of genotype-by-environment interaction should therefore be considered in genetic evaluation of both growth rate and WEC, to increase the efficiency of selection for animals that are more parasite resistant and more resilient to environmental worm challenge.

Genetic and phenotypic aspects of growth rate and efficiency-related traits in sheep

A large set of Baluchi sheep growth data collected at Abbasabad Sheep Breeding Station, Mashhad, Khorasan Razavi, Iran, was used to estimate phenotypic and genetic parameters for growth rate from birth to weaning (GRa), weaning to six months of age (GRb), and weaning to 12 months of age (GRc) and corresponding Kleiber ratios (KRa, KRb, KRc), efficiency of growth (EFa, EFb, EFc) and relative growth rate (RGRa, RGRb, RGRc). Genetic parameters were estimated by REML procedure fitting a series of six univariate animal models including various combinations of maternal effects. The most appropriate model for each trait was determined by the Akaike Information Criterion (AIC). In addition, a multi-variate analysis was done to estimate (co)variance components between traits. Estimates of direct heritabilities (h2) were, respectively, 0.06, 0.03 and 0.10 for GRa, GRb, and GRc; 0.06, 0.06, and 0.05 for EFa, EFb and EFc; 0.07, 0.04 and 0.06 for KRa, KRb and KRc; and 0.07, 0.05 and 0.05 for RGRa, RGRb and RGRc. For traits measured in the pre-weaning growth phase, maternal genetic effect was significant. The estimates of maternal heritability (m2) for GRa, KRa, EFa and RGRa were 0.11, 0.11, 0.04 and 0.04, respectively. Estimates of the additive genetic coefficients of variation (CVA) were used as a measure of genetic variability and ranged between 1.68% (RGRa) and 14.67% (EFb). Genetic correlations between traits ranged from −0.81 (EFa-KRc) to 0.94 (ADGa-KRa) and phenotypic correlations ranged from −0.72 (KRa-EFc) to 0.99 (GRb-RGRb). Overall, results showed little additive genetic variation in growth rate or efficiency-related traits of Baluchi sheep, indicating little opportunity for genetic improvement of these traits in this breed through selection. In addition, significant maternal effects on traits related to pre-weaning growth indicated the importance of including maternal effects in genetic evaluation of traits measured early in life.

Direct and indirect genetic effects in life-history traits of flour beetles (Tribolium castaneum).

Indirect genetic effects (IGEs) are the basis of social interactions among conspecifics, and can affect genetic variation of nonsocial and social traits. We used flour beetles (Tribolium castaneum) of two phenotypically distinguishable populations to estimate genetic (co)variances and the effect of IGEs on three life-history traits: development time (DT), growth rate (GR), and pupal body mass (BM). We found that GR was strongly affected by social environment with IGEs accounting for 18% of the heritable variation. We also discovered a sex-specific social effect: male ratio in a group significantly affected both GR and BM; that is, beetles grew larger and faster in male-biased social environments. Such sex-specific IGEs have not previously been demonstrated in a nonsocial insect. Our results show that beetles that achieve a higher BM do so via a slower GR in response to social environment. Existing models of evolution in age-structured or stage-structured populations do not account for IGEs of social cohorts. It is likely that such IGEs have played a key role in the evolution of developmental plasticity shown by Tenebrionid larvae in response to density. Our results document an important source of genetic variation for GR, often overlooked in life-history theory.

Ontogenetic changes in genetic variances of age-dependent plasticity along a latitudinal gradient.

The expression of phenotypic plasticity may differ among life stages of the same organism. Age-dependent plasticity can be important for adaptation to heterogeneous environments, but this has only recently been recognized. Whether age-dependent plasticity is a common outcome of local adaptation and whether populations harbor genetic variation in this respect remains largely unknown. To answer these questions, we estimated levels of additive genetic variation in age-dependent plasticity in six species of damselflies sampled from 18 populations along a latitudinal gradient spanning 3600 km. We reared full sib larvae at three temperatures and estimated genetic variances in the height and slope of thermal reaction norms of body size at three points in time during ontogeny using random regression. Our data show that most populations harbor genetic variation in growth rate (reaction norm height) in all ontogenetic stages, but only some populations and ontogenetic stages were found to harbor genetic variation in thermal plasticity (reaction norm slope). Genetic variances in reaction norm height differed among species, while genetic variances in reaction norm slope differed among populations. The slope of the ontogenetic trend in genetic variances of both reaction norm height and slope increased with latitude. We propose that differences in genetic variances reflect temporal and spatial variation in the strength and direction of natural selection on growth trajectories and age-dependent plasticity. Selection on age-dependent plasticity may depend on the interaction between temperature seasonality and time constraints associated with variation in life history traits such as generation length.

Faster is not always better: selection on growth rate fluctuates across life history and environments.

Growth rate is increasingly recognized as a key life-history trait that may affect fitness directly rather than evolve as a by-product of selection on size or age. An ongoing challenge is to explain the abundant levels of phenotypic and genetic variation in growth rates often seen in natural populations, despite what is expected to be consistently strong selection on this trait. Such a paradox suggests limits to how contemporary growth rates evolve. We explored limits arising from variation in selection, based on selection differentials for age-specific growth rates expressed under different ecological conditions. We present results from a field experiment that measured growth rates and reproductive output in wild individuals of a colonial marine invertebrate (Hippopodina iririkiensis), replicated within and across the natural range of succession in its local community. Colony growth rates varied phenotypically throughout this range, but not all such variation was available for selection, nor was it always targeted by selection as expected. While the maintenance of both phenotypic and genetic variation in growth rate is often attributed to costs of growing rapidly, our study highlights the potential for fluctuating selection pressures throughout the life history and across environments to play an important role in this process.

Spatial scale and divergent patterns of variation in adapted traits in the ocean

The geography of adaptive genetic variation is crucial to species conservation yet poorly understood in marine systems. We analyse the spatial scale of genetic variation in traits that broadly display adaptation throughout the range of a highly dispersive marine species. We conducted common garden experiments on the Atlantic silverside, Menidia menidia, from 39 locations along its 3000 km range thereby mapping genetic variation for growth rate, vertebral number and sex determination. Each trait displayed unique clinal patterns, with significant differences (adaptive or not) occurring over very small distances. Breakpoints in the cline differed among traits, corresponding only partially with presumed eco-geographical boundaries. Because clinal patterns are unique to each selected character, neutral genes or those coding for a single character cannot serve as proxies for the genetic structure as a whole. Conservation plans designed to protect essential genetic subunits of a species will need to account for such complex spatial structures.

Latitudinal and voltinism compensation shape thermal reaction norms for growth rate

Latitudinal variation in thermal reaction norms of key fitness traits may inform about the response of populations to climate warming, yet their adaptive nature and evolutionary potential are poorly known. We assessed the contribution of quantitative genetic, neutral genetic and environmental effects to thermal reaction norms of growth rate for populations of the damselfly Ischnura elegans. Among populations, reaction norms differed primarily in elevation, suggesting that time constraints associated with shorter growth seasons in univoltine, high-latitude as well as multivoltine, low-latitude populations selected for faster growth rates. Phenotypic divergence among populations is consistent with selection rather than drift as Q(ST) was greater than F(ST) in all cases. Q(ST) estimates increased with experimental temperature and were influenced by genotype by environment interactions. Substantial additive genetic variation for growth rate in all populations suggests that evolution of trait means in different environments is not constrained. Heritability of growth rates was higher at high temperature, driven by increased genetic rather than environmental variance. While environment-specific nonadditive effects also may contribute to heritability differences among temperatures, maternal effects did not play a significant role (where these could be accounted for). Genotype by environment interactions strongly influenced the adaptive potential of populations, and our results suggest the potential for microevolution of thermal reaction norms in each of the studied populations. In summary, the observed latitudinal pattern in growth rates is adaptive and results from a combination of latitudinal and voltinism compensation. Combined with the evolutionary potential of thermal reaction norms, this may affect populations' ability to respond to future climate warming.

Genetic analysis of growth rate and Kleiber ratio in Zandi sheep

Genetic parameters for average daily gain from birth to weaning (ADGa), birth to 6 months (ADGb), weaning to 6 months (ADGc), weaning to yearling age (ADGd), and corresponding Kleiber ratios (KRa, KRb, KRc, and KRd) were estimated by using records of 3,533 Zandi lambs, descendent of 163 sires and 1265 dams, born between 1991 and 2005 at the Zandi Sheep Breeding Station at Khojir National Park, Tehran, Iran. A derivative-free algorithm combined with a series of six single-trait linear animal models was used to estimate phenotypic variance and its direct, maternal, and residual components. In addition, bivariate analyses were done to estimate (co)variance components between traits. Estimates of direct heritability (h(2)) were 0.11, 0.15, 0.09, 0.10, 0.10, 0.10, 0.06, and 0.07 for ADGa, ADGb, ADGc, ADGd, KRa, KRb, KRc, and KRd, respectively, thereby indicating the presence of low additive genetic variation for growth rate and Kleiber ratio in this population of Zandi sheep. Maternal genetic component was found to be significant on ADGa and KRa and contributed 3% and 5%, respectively, in total phenotypic variance of ADGa and KRa. A widespread range of genetic correlations among traits studied was observed. Except for negative genetic correlations between ADGa and KRc, ADGa and KRd, and between KRa and KRc, in other cases, genetic correlations were positive and moderate to very high. Phenotypic correlations ranged from -0.49 (ADGa/KRd) to 0.94 (ADGc/KRc). These results indicate that selecting for improved growth rate or Kleiber ratio in Zandi sheep would generate a relatively slow genetic progress.

Swiss Stone Pine Provenance Experiment in Romania: II Variation in Growth and Branching Traits to Age 14

Summary After the nursery testing, twelve Swiss stone pine (Pinus cembra L.) provenances from the Alps and Carpathian Mountains were planted out at two sites located at high elevation in the Southern and Northern Carpathians. Total height growth (H), annual height growth (h), root collar diameter (RCD), branches per whorl (BW) and survival (SV) were measured and analyzed. Analysis of variance showed highly significant (p < 0.01; p < 0.001) differences between provenances for all traits, except survival, suggesting that selection at the provenance level could be possible. Also, over locations analysis revealed significant genotype x environment interaction, demonstrating that some provenances react differently to environmental conditions and, selection should take this into account. The phenotypic coefficient of variation was moderate for growth and high for number of branches per whorl suggesting that selection within provenance can also be applied. Finding of significant and highly significant age-age and trait-trait phenotypic correlations indicated that early and indirect selection in Swiss stone pine species is possible. According to DUNCAN’s multiple range test the best performing provenances of the two mountain ranges were selected for operational planting and breeding programmes. The results of this study validate that a very slow growing species, such as Swiss stone pine may still possess very high genetic variation in growth rate; consequently, this trait can be improved. Finally, an attempt has been made to develop a seed transfer guidelines for the species by using the pattern of geographic variation as a basis.

Growth rate of growing pigs is weakly correlated genetically with litter size, while the amount of genetic variation for growth rate changes with litter size

We reasoned that if we assessed pigs for litter size and growth rate during the grow-out stage of production (25–100 kg body weight) we would find: 1) an unfavourable genetic correlation between litter size and growth rate; and 2) that the amount of additive genetic variation expressed for growth rate varies with litter size. We tested these premises by assessing 2212 litters from the Yorkshire breed for litter size, while 3038 growing pigs from 550 of these litters were assessed for growth rate during the grow-out stage of production. Genetic correlations were estimated using the additive genetic (co)variances obtained from a bivariate linear animal model fitted to litter size and growth rate. The amount of additive genetic variation expressed for growth rate as a function of litter size was estimated by fitting a univariate linear animal model with random regression on litter size. Our findings did not support the first of our premises as we found that the genetic correlation between litter size and growth rate was favourable (0.28±0.27), albeit not significantly different from zero. However, we were able to support our second premise as we found that the relationship between amount of additive genetic variation for growth rate and litter size was quadratic; the amount of additive genetic variation was highest in small and large litters (h2=0.60 and 0.65), and lowest in intermediate litter sizes (h2=0.29). These findings indicate that: 1) breeding for litter size would not reduce the growth rate of growing pigs; and 2) the amount of genetic variation for growth rate changes with litter size.

Phenotypic plasticity and evolutionary potential in somatic cells of Armillaria gallica

Somatic cells of Armillaria gallica fruit bodies have been shown to possess different genotypes for molecular-marker and mating-type loci. Here we report experiments on six quantitative traits and demonstrate that somatic cells of fruit bodies possess almost as much genetic variation for growth rate and phenotypic plasticity as do spores, the products of meiosis. Genetically distinct somatic cells therefore have the potential to grow at different rates relative to one another during primordial fruit body formation. This may confer an advantage on all cell lines within a fruit body, not just those that happen to grow better under a particular set of conditions. To our knowledge, genetic variation for fitness-related traits that make up a single genetic individual has not been reported before.

Genetic variation for growth rate, feed conversion efficiency, and disease resistance exists within a farmed population of rainbow trout

The objective of this study was to test that additive genetic (co)variation for survival, growth rate, feed conversion efficiency, and resistance to viral haemorrhagic septicaemia (VHS) exists within a farmed population of rainbow trout. Thirty sires and 30 dams were mated by a partly factorial mating design. Each sire was mated to two dams, and each dam was mated to two sires, producing 50 viable full-sib families (29 sires, 25 dams). The fish from these families were reared for a 215-day growout period, and were assessed for survival between days 52 and 215, growth rate (i.e., body weight on days 52, 76, 96, 123, 157, 185, and 215, and body length on days 52 and 215); feed conversion efficiency between days 52–215, 52–76, 77–96, 97–123, 124–157, 158–185, and 186–215, and VHS resistance. REML estimates of additive genetic variation for the body weights, body lengths, and feed conversion efficiencies were obtained by fitting univariate linear (reduced) animal models. Additive genetic variation for VHS resistance was estimated by fitting a Weibull, sire–dam frailty model to time until death of fish challenged with VHS. Genetic correlations were estimated among the body weights, body length, and feed conversion efficiencies that expressed additive genetic variation, while genetic correlations between VHS resistance and the body weights, body length, and feed conversion efficiencies were approximated as product–moment correlations among predicted breeding values of the sires and dams. Additive genetic variation was found to be very low for survival, body weight on days 52 and 76, body length on day 52, and feed conversion efficiency between days 185 and 215. However, additive genetic variation was detected for body weight on days 96, 123, 157, 185, and 215 [coefficient of additive genetic variation (CV)=8.4–28.4%, heritability ( h 2)=0.35 for body weight on day 215], body length on day 215 (CV=6.9%, h 2=0.53), feed conversion efficiency between days 52–215, 52–76, 77–96, 97–123, 124–157, and 158–185 (CV=4.0–13.9%), and VHS resistance (additive genetic variance for log-frailty=0.24, h 2 on the logarithmic-time scale=0.13). Genetic correlations among the body weights, body length, and feed conversion efficiencies that expressed additive genetic variation were generally favourable and moderate-to-very strong (0.55–0.99), though there were unfavourable correlations (−0.01 to −0.33) between the predicted breeding values for VHS resistance and the predicted breeding values for the body weights, body length, and feed conversion efficiencies. These results demonstrate that additive genetic (co)variation for growth rate, feed conversion efficiency, and VHS resistance does exist within the farmed population of rainbow trout, and indicates that selective breeding for these traits can be successful.

The quantitative genetics of growth in a field cricket

AbstractBecause of its relationship with both development time and adult size, the rate of growth in determinately growing organisms is an important aspect of their life histories. We reared sixty‐nine families of Gryllus pennsylvanicus derived from a natural population and found significant genetic variation in growth rate as estimated by the slope of linearized growth trajectories. We found no evidence for a genetic tradeoff between rate of growth and survival, nor rate of growth and fecundity. In principle, adult size may be determined both by the rate of growth and the time taken by the nymphs to develop. Our data indicate that variation in adult size is explained by variation in growth rate, not by variation in development time. We conclude with a discussion of the plausible explanations for the presence of genetic variation in growth rate in this natural population.

Gas exchange of 20-year-old black spruce families displaying a genotype × environment interaction in growth rate

Gas exchange and xylem water potential were measured on 20-year-old black spruce (Piceamariana (Mill.) B.S.P) trees from four full-sib families from a 7 × 7 diallel. These families constitute a 2 parent × 2 parent factorial breeding structure. One female parent (59) produced families that have displayed high productivity on three plantation sites, whereas the other female parent (63) produced families that had high growth rates on two of the sites but not on the third, least productive, site. In 1991 (a hot, dry year) measurements were obtained from trees at two sites; in 1992 (a cool, wet year) measurements were obtained from only one site. Gas exchange was measured on detached branches cut from the upper third of the crown; measurements were initiated within 2 min from cutting. In 1991, daytime xylem water potential, predawn xylem water potential, net photosynthesis (Pn), and needle conductance (gwv) were all lower in trees growing on the least productive site (2) than in trees growing on the most productive site (3), indicating that trees on site 2 were under greater water stress than those on site 3. Progeny of female 59 displayed higher Pn than progeny of female 63 over both years of contrasting rainfall and on both the wetter and drier sites during the first year. However, progeny of the two female parents did not differ with respect to gwv in either year nor on either site during the first year. The genetic variation in Pn/gwv, observed over sites and years, is positively related to growth rate only on the dry site. Thus, the genotype × environment interaction in growth appears to be due to relatively stable genetic differences in Pn. We hypothesize that water stress on the dry site reduces Pn below a threshold value at which point genetic variation in Pn impacts on productivity and results in genetic variation in growth rate. Furthermore, we hypothesize that on the wet site carbon gain per unit leaf area is not as limiting to growth rate; thus, the observed genetic differences in Pn do not influence genetic rankings in growth rate.

Adaptive variation in growth rate: life history costs and consequences in the speckled wood butterfly,Pararge aegeria.

An important assumption made in most lifehistory theory is that there is a trade-off between age and size at reproduction. This trade-off may, however, disappear if growth rate varies adaptively. The fact that individuals do not always grow at the maximum rate can only be understood if high growth rates carry a cost. This study investigates the presence and nature of such costs inPararge aegeria. Five females from two populations with known differences in life history (south Sweden and Maderia) were allowed to oviposit in the laboratory and their offspring were reared in environmental chambers under conditions leading to direct development. We measured several aspects of life history, including development times, pupal and adult weights, growth rate, female fecundity, longevity and larval starvation endurance. In both populations there seemed to be genetic variation in growth rate. There was no evidence for a trade-off between age and size at pupation. As predicted, larvae with high growth rates also lost weight at a relatively higher rate during starvation. High weight-loss rates were furthermore associated with a lower probability of surviving when food became available again. This is apparently the first physiological trade-off with growth rate that has been experimentally demonstrated. In both populations there were significant differences in growth rate between the sexes, but the populations differed in which sex was growing at the highest rate. In Sweden males had higher growth rates than females, whereas the reverse was true for Madeira. These patterns most likely reflect differences in selection for protandry, in turn caused by differences in seasonality between Sweden and Madeira. Together with the finding that males had shorter average longevity than females in the Swedish, but not in the Maderiran, population, this indicates that a lower adult quality also may be a cost of high growth rate. We argue that for the understanding of life history variation it is necessary to consider not only the two dimensions of age and size, but also to take into full account the triangular nature of the relationship between size, time and growth rate.

Genetic variation in growth rate of Atlantic salmon with different trypsin-like isozyme patterns

The growth rates of three different year-classes (1984, 1985 and 1987) of Atlantic salmon ( Salmo salar) were studied using trypsin-like isozyme pattern in the pyloric caeca as a biological genetic marker. The fish with the homozygote variant TRP-2(92/92) and with the heterozygote variant TRP-2(92/100) had higher average weight than those with the genotypes which did not possess the variant TRP-2(92). The difference in weight between the two groups (with and without TRP-2(92) variant) was significant from smolt until the end of the experiment in autumn of the first sea-year (1984 year-class) and until spring in the second sea-year (1985 year-class). The difference was about 30%, and decreased to 10% at maturation. The specific growth rate was critical during the first sea-year. The fish with the allele TRP-2(92) showed the advantage of faster growth in the first sea-year, especially during winter time. With the covariation of significant difference in weight (until the autumn in the second sea-year) among different families, higher production of food fish of at least 20% could be possible at the time of slaughter during the second sea-year with the use of a selective breeding programme based either on trypsin-like isozyme pattern and family or on the isozyme pattern of individuals. The effects of selection by using the variant TRP-2(92) could be observed within the period of the generation selected.