<p>Recent studies show that populations of the freshwater isopod <em>Asellus aquaticus L. can rapidly become locally differentiated when submerged stonewort (<em>Chara spp.) vegetation expands in lakes. In the novel <em>Chara habitat, isopods become lighter pigmented and smaller than in the ancestral reed stands. In this study, I used laboratory experiments to investigate if selective predation by fish could be a possible explanation for these phenotypic changes. Predation from fish is generally considered to be a strong selective force on macroinvertebrate traits. In the first experiment I measured perch (<em>Perca fluviatilis L.) handling time for three size classes of <em>Asellus to see which size of those that would be the most profitable to feed upon. No difference in handling time was detected between prey sizes, hence the largest size would be the most beneficial to feed upon. In a second experiment I let perch feed on a mixture of <em>Asellus phenotypes in aquaria manipulated to mimic the substrates in either the <em>Chara or the reed habitats. Remaining isopods were significantly smaller and lighter pigmented in the fish aquaria than in the controls, showing that the perch preferred to feed on large and dark individuals. In the <em>Chara habitat, selection on isopod pigmentation was according to what could be expected from background matching, but in the reed habitat selection was quite the opposite. These results support the hypothesis that predation from fish is a strong selective force behind the rapid local adaptation seen in <em>Asellus populations in the novel <em>Chara habitat. </em></em></em></em></em></em></em></em></em></em></p>

Recent studies show that populations of the freshwater isopod Asellus aquaticus L. can rapidly become locally differentiated when submerged stonewort (Chara spp.) vegetation expands in lakes. In the novel Chara habitat, isopods become lighter pigmented and smaller than in the ancestral reed stands. In this study, I used laboratory experiments to investigate if selective predation by fish could be a possible explanation for these phenotypic changes. Predation from fish is generally considered to be a strong selective force on macroinvertebrate traits. In the first experiment I measured perch (Perca fluviatilis L.) handling time for three size classes of Asellus to see which size of those that would be the most profitable to feed upon. No difference in handling time was detected between prey sizes, hence the largest size would be the most beneficial to feed upon. In a second experiment I let perch feed on a mixture of Asellus phenotypes in aquaria manipulated to mimic the substrates in either the Chara or the reed habitats. Remaining isopods were significantly smaller and lighter pigmented in the fish aquaria than in the controls, showing that the perch preferred to feed on large and dark individuals. In the Chara habitat, selection on isopod pigmentation was according to what could be expected from background matching, but in the reed habitat selection was quite the opposite. These results support the hypothesis that predation from fish is a strong selective force behind the rapid local adaptation seen in Asellus populations in the novel Chara habitat.

The last century, humans have been altering almost all natural environments at an accelerating rate, including the Baltic Sea that has highly eutrophicated areas and many coastal industries such as Pulp-mills. For animals living in a habitat that changes there are basically two alternatives, either to cope with the change or become locally extinct. This thesis aims to investigate if recent anthropogenic disturbance in the Baltic Sea can affect natural populations on a genetic level through natural selection. First, we found a fine-scale genetic structure in three-spine sticklebacks (Gasterosteus aculeatus) populations along the Swedish coast (paper I), indicating limited gene-flow between populations in geographic proximity. Different genetic markers, specifically Amplified Fragment Lenght Polymorpism (AFLP, and microsatellites,  gave different results, highlighting the heterogeneous character of genomes which demonstrates that it is important to choose a genetic marker that is relevant for the question at hand. With a population genomic approach, and a multilocus genetic marker (AFLP), we detected convergent evolution in genotype composition in stickleback populations living in environments affected by pulp-mill effluent (paper II) and in highly eutrophicated environments (paper III), compared to adjacent reference populations. We found loci, in both studies (paper II, III), that were different from a neutral distribution and thus probably under divergent selection for the habitat differences investigated. The selective effect from pulp-mill effluents were more pronounced, but the two different habitats had mutual characters (AFLP loci). In paper IV, we converted five anonymous AFLP loci to sequenced markers and aligned them to the stickleback genome. Four out of five loci aligned within, or close to, coding regions on chromosome I, chromosome VIII, chromosome XIX and chromosome XX. One of the loci, located on chromosome VIII and identified as under divergent selection in both paper II and III, has been identified in other studies as to be under selection for fresh water adaptation, including Baltic Sea stickleback populations. In conclusion, anthropogenic alterations of natural environments can have evolutionary consequences, probably adaptive, for the animals living there and the evolutionary response exhibited by natural populations can be very fast. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript.</p>

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Tesis por compendio / [ES] La selección divergente puede cambiar las frecuencias de los marcadores genéticos en direcciones opuestas, produciéndose frecuencias alélicas intermedias en estos marcadores cuando ambas líneas divergentes son consideradas conjuntamente en los análisis genéticos. Por lo tanto, los experimentos de selección divergente aumentan el poder de detección para estudios de asociación de genoma completo (GWAS) y para estudios de escaneo genómico por medio de métodos de huellas de selección. GWASs bayesianos, utilizando el modelo Bayes B, se implementaron para analizar datos genómicos de los caracteres de tamaño de camada del experimento de capacidad uterina con 181 hembras. Las asociaciones fueron evaluadas calculando los factores de Bayes para cada SNP, y calculando los porcentajes de la varianza genómica para cada ventana no solapada de 1-Mb. Los GWASs descubrieron SNPs asociados con el número total de gazapos al parto y los embriones implantados. Además, se revelaron regiones genómicas relevantes para el número total de gazapos al parto (1 región), el número de nacidos vivos (1 región), los embriones implantados (3 regiones) y la tasa de ovulación (5 regiones). Los porcentajes de varianza genómica que explicaban los anteriores caracteres de tamaño de camada fueron 39,48%, 10,36%, 37,21% y 3,95%, respectivamente, en un modelo que excluye el efecto línea; y 7.36%, 1.27%, 15.87% y 3.95%, respectivamente, en un modelo con el efecto línea. La región genómica localizada en el cromosoma del conejo (OCU) 17 en 70.0 - 73.3 Mb se consideró como un nuevo locus de carácter cuantitativo (QTL) asociado a caracteres reproductivos en conejos, ya que esta región fue encontrada solapada para el número total de gazapos al parto, el número de nacidos vivos y los embriones implantados. El gen de la proteína morfogenética ósea 4, BMP4, es el principal gen candidato prometedor dentro del nuevo QTL. Una combinación de GWASs fueron implementados para analizar los datos genómicos del experimento de la grasa intramuscular con 480 conejos. Los métodos de GWASs incluyeron un método bayesiano, modelo Bayes B; y un método frecuentista, regresiones de marcadores únicos con los datos ajustados por el parentesco genómico. Este estudio reveló cuatro regiones genómicas relevantes en OCU1 (1 región), OCU8 (2 regiones) y OCU13 (1 región) asociadas con la grasa intramuscular. La región asociada más importante estaba en OCU8 en 24.59 - 26.95 Mb, y explicó el 7.34% de la varianza genómica. El bajo porcentaje explicado por las principales regiones genómicas relevantes indica un gran componente poligénico para la grasa intramuscular. Los análisis funcionales recuperaron genes vinculados con las rutas y funciones de los metabolismos de energía, carbohidratos y lípidos. Además, se realizó un estudio de escaneo genómico usando conejos del experimento de selección divergente para grasa intramuscular, y usando tres métodos de firmas de selección: índice de fijación de Wright (Fst), coeficiente de verosimilitud compuesto entre poblaciones (XP-CLR) y extensión de homocigosidad de los haplotipos entre poblaciones (XP-EHH). Los resultados mostraron múltiples huellas de selección en todo el genoma del conejo. Ninguna de estas huellas de selección concuerda con las regiones genómicas asociadas con la grasa intramuscular, provenientes de los resultados de los GWASs. En síntesis, los resultados de ambos experimentos, GWASs y el estudio de escaneo genómico, sugieren que la arquitectura genómica de la grasa intramuscular en el conejo parece ser altamente poligénica y sus variantes causales serían apenas detectables. Este estudio demuestra que la detección de variantes causales y marcadores genéticos asociados depende de las hipotéticas arquitecturas genómicas de los caracteres, independientemente de las exitosas respuestas logradas en los dos experimentos de selección divergente. Hasta la fecha, estos hallaz / [CA] La selecció divergent pot alterar les freqüències dels marcadors genètics en direccions oposades, donant lloc a freqüències al·lèliques intermèdies quan les dos línies divergents es consideren conjuntament en els anàlisis genètics. Per tant, els experiments de selecció divergents augmenten el poder de detecció en estudis d'associació de genoma ampli (GWAS) i en estudis d'exploració genòmica a través de mètodes de signatures de selecció. GWASs bayesians, utilitzant el model Bayes B, es van implementar per a analitzar dades genòmiques de caràcters de grandària de ventrada de l'experiment de capacitat uterina amb 181 conilles femelles. Les associacions es van provar calculant els factors de Bayes per a cada SNP, i calculant els percentatges de la variància genòmica per a cada finestra no superposada d'1-Mb. Els GWASs van descobrir SNPs associats amb el número total de llorigons al part i els embrions implantats. A més, es van revelar regions genòmiques rellevants per al número total de llorigons al part (1 regió), el número de nascuts vius (1 regió), els embrions implantats (3 regions) i la taxa d'ovulació (5 regions). Els percentatges de variància genòmica que explicaven els anteriors caràcters de grandària de ventrada van ser 39,48%, 10,36%, 37,21% i 3,95%, respectivament, sota un model que exclou l'efecte de línia; i 7.36%, 1.27%, 15.87% i 3.95%, respectivament, sota un model amb efecte de línia. La regió genòmica situada en el cromosoma del conill (OCU) 17 en 70.0 - 73.3 Mb es va considerar com un nou locus de caràcters quantitatius (QTL) associat a caràcters reproductius en conills, ja que aquesta regió es va superposar per al número total de llorigons al part, el número de nascuts vius i els embrions implantats. El gen de la proteïna morfogenètica òssia 4, BMP4, és el principal gen candidat prometedor dins del nou QTL. Una combinació de GWASs es van implementar per a analitzar les dades genòmiques de l'experiment del greix intramuscular amb 480 conills. Els mètodes GWASs van incloure un mètode bayesià, model Bayes B; i un mètode frecuentista, regressions de marcadors únics amb les dades ajustades pel parentiu genòmico. Aquest estudi va revelar quatre regions genòmiques rellevants en OCU1 (1 regió), OCU8 (2 regions) i OCU13 (1 regió) associades amb el greix intramuscular. La regió associada més important estava en OCU8 en 24.59 - 26.95 Mb, i va explicar el 7.34% de la variància genòmica. El baix percentatge explicat per les principals regions genòmiques rellevants indica un gran component poligènic per al greix intramuscular. Els anàlisis funcionals van recuperar gens relacionats amb les rutes i la funció d'energia, metabolismes de carbohidrats i lípids. A més, es va realitzar un estudi d'exploració del genoma usant conills de l'experiment de selecció divergent per a greix intramuscular, i usant tres mètodes de signatures de selecció: índex de fixació de Wright (Fst), coeficient de versemblança compost entre poblacions (XP-CLR) i extensió de homocigosidad dels haplotipos entre poblacions (XP-EHH). Els resultats van mostrar múltiples petjades de selecció en tot el genoma del conill. Cap d'aquestes petjades de selecció concorda amb les regions genòmiques associades a partir dels resultats dels GWASs. En síntesi, els resultats dels dos experiments, GWASs i estudi d'exploració del genoma, suggereixen que l'arquitectura genòmica del greix intramuscular en el conill sembla ser altament poligènica i les seues variants causals serien a penes detectables. Aquest estudi demostra que la detecció de variants causals i marcadors genètics associats depèn de les hipotètiques arquitectures genòmiques dels caràcters, independentment de les respostes reeixides en els dos experiments de selecció divergents. Fins ara, aquestes troballes no tindrien implicacions valuoses per als programes de cria de conills. / [EN] Divergent selection can alter frequencies of genetic markers in opposite directions, leading to intermediate allelic frequencies when both divergent lines are jointly considered in the genetic analyses. Therefore, divergent selection experiments increase the detection power for genome wide association studies (GWAS) and for genomic scan studies through methods of selection signatures. Bayesian GWASs using Bayes B model was used to analyse genomic data of litter size traits of the uterine capacity experiment with 181 does. The associations were tested by computing Bayes factors for each SNP, and by computing percentages of the genomic variance for each 1-Mb non-overlapping window. The GWASs uncovered SNPs associated with total number born and implanted embryos. Moreover, relevant genomic regions were revealed for total number born (1 region), number born alive (1 region), implanted embryos (3 regions), and ovulation rate (5 regions). The percentages of genomic variance that accounted for these litter size traits were 39,48%, 10.36%, 37.21%, and 3.95%, respectively, under a model excluding line effect; and 7.36%, 1.27%, 15.87%, and 3.95%, respectively, under a model with line effect. The genomic region located on the rabbit chromosome (OCU) 17 in 70.0 - 73.3 Mb was deemed as a novel quantitative trait locus (QTL) of reproductive traits in rabbits, since this region was found overlapped for total number born, number born alive and implanted embryos. Bone morphogenetic protein 4 gene, BMP4, is the main promising candidate gene within the novel QTL. A combination of GWASs were performed for analysing the genomic data of the intramuscular fat experiment with 480 rabbits. The GWAS methods included a Bayesian method, Bayes B model; and a frequentist method, single marker regressions with the data adjusted by genomic relatedness. This study revealed four relevant genomic regions in OCU1 (1 region), OCU8 (2 regions) and OCU13 (1 region) associated with intramuscular fat. The most important associated region was on OCU8 in 24.59 - 26.95 Mb, and accounted for 7.34% of the genomic variance. The low percentage explained by the main relevant genomic regions indicates a large polygenic component for intramuscular fat. Functional analyses retrieved genes linked to pathways and function of energy, carbohydrate and lipid metabolisms. In addition, a genome scan study was performed using rabbits from the divergent selection experiment for intramuscular fat, and using three methods of selection signatures: Wright's fixation index (Fst), cross population composite likelihood ratio (XP-CLR) and cross population extended haplotype homozygosity (XP-EHH). The results showed multiple selection signatures across the rabbit genome. None of these selection signatures agreed with the associated genomic regions from GWAS findings. In synthesis, the results of both experiments, GWAS and genome scan study, suggest that the genomic architecture of intramuscular fat in rabbit seems to be highly polygenic and their causative variants would be hardly detectable. This study demonstrates that detection of causative variants and associated genetic markers depends on the hypothetical genomic architectures of traits, regardless of the successful responses attained in the two divergent selection experiments. Hitherto, these findings would not have worthwhile implications for the rabbit breeding programs. / Sosa Madrid, BS. (2020). Genomic analysis of divergently selected experimental lines in rabbit [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/141376 / Compendio

In dioecious plants, selection due to sex function differences has produced sex-specific life histories, morphologies, and physiologies. In many dioecious seed plants, dimorphisms and population sex ratios have been plausibly linked, but similar links are not yet apparent in dioecious bryophytes. Population sex ratio bias is often expected to favor the sex with lower investment in sexual reproduction, especially in resource-poor environments. Unlike in seed plants, bryophyte males may have higher average reproductive investment than females, which typically have low offspring production rates due to sperm limitation. However, traits aside from reproductive investment such as shoot and leaf arrangement may be differentially selected and could influence life history and sex ratio, but these are rarely tested. My questions concentrated on the dimorphic traits responsible for sex ratio bias and their links to sex function. My studies, using the moss Bryum argenteum, included field and greenhouse experiments investigating sex ratio bias and morphological plasticity along a light/canopy openness (exposure) gradient, a greenhouse comparison of clump morphology and water-holding capacity, and a field and growth chamber study on sex-specific responses to stress (high temperature and desiccation). The sex ratio of urban Lexington, KY was highly female-biased, did not correlate with exposure, and was not linked with pre-zygotic reproductive investment. Leaf characteristics of B. argenteum plastically responded to exposure but were not sex-specific. However, juvenile females produced shoots at a faster rate and grew taller in high light. Juvenile male shoots held more external water than female shoots, but this did not predict mature clump water-holding capacity. Male clumps were shorter, denser, and held less water than females likely to shed sperm-laden water for sexual reproduction. Clump height did not trade off with reproductive investment, adding evidence that sex-specific size is linked with other aspects of sex function. Although chlorophyll fluorescence data (a measure of the status of photosystem II) from both field and growth chamber experiments indicated subtle sex-specific stress recovery responses among sexually immature and mature plants, differences were weaker than predicted and sexually mature shoots did not fare worse than vegetative shoots. The sex differences in size, clump morphology, and clump water-holding capacity very likely affect survival, growth, competitive ability, and ultimately adult sex ratio bias.

There is considerable evidence that pollinator shifts can explain many differences in flower morphology between closely related plant species, but the extent to which pollinator shifts can explain the maintenance of among-population variation in floral traits within species is poorly known. In this thesis, I combined comparative and experimental approaches to examine the evolution of floral traits in the moth-pollinated orchid Platanthera bifolia. More specifically, I investigated (1) the relationship between flower and pollinator morphology, (2) phenotypic selection on morphology and phenology in populations in contrasting environments, (3) components of prezygotic reproductive isolation among divergent populations, and (4) the adaptive and functional significance of two correlated floral traits. A study of Scandinavian of P. bifolia populations revealed that spur length was positively correlated with proboscis length of local pollinators, which suggests that variation in spur length reflects adaptive evolution in response to geographically variable pollinator-mediated selection. A phenotypic selection study on Öland, SE Sweden, suggested that disruptive selection on spur length contributes to the maintenance of a bimodal distribution of spur length in mixed habitats, but provided very limited evidence of divergent selection on plant morphology and flowering phenology in grassland and woodland habitats. Field experiments revealed strong reproductive isolation between divergent populations on Öland, due to differences in spatial distribution, flowering phenology, and pollinators, and among-population incompatibility. The results suggest that prezygotic reproductive isolation contributes to the maintenance of population differentiation in floral traits in P. bifolia. A field manipulation experiment demonstrated that spur length but not perianth size affects pollination success and seed production. This suggests that among-population differentiation in perianth size may be the result of a genetic correlation with spur length. Taken together, the results of this thesis suggest that pollinator-mediated selection can shape the evolution of intraspecific floral variation.

As individuals and populations diverge ecologically, they become exposed to new parasites and pathogens with potentially harmful fitness consequences. Populations are therefore expected to evolve resistance, possibly at a cost of less resistance to parasites rarely encountered parasites. This trade-off in resistance should generate local adaptation to parasites in different habitats. In chapter one, I show how local adaptation can potentially evolve in response to variation in parasite exposure among eighteen ecologically variable populations of threespine stickleback (Gasterosteus aculeatus). Within populations infection appeared to reflect morphology/diet based exposure differences among individuals. Among populations, however, these patterns were absent or reversed, consistent with the evolution of local adaptation. In chapters two and three I set out to test whether variation major histocompatibility (MHC) genes can underly such local adaptation in stickleback. MHC genes are important components of vertebrate immunity; however, there is little direct empirical support for spatially divergent selection driving local adaptation on MHC loci in the wild. In chapter two I tested for the action of parasite mediated balancing and divergent selection on on MHC loci using naturally infected stickleback in three replicate lake-stream pairs. Despite consistent divergence in parasites and MHC alleles, lakes tended to show decreased parasite burdens with increased allelic richness (consistent with balancing selection), while streams showed some support for divergent selection between lake and stream types. In chapter three I use the same lake-stream pairs to investigate how divergent selection could instead be reflected in variation in the effects of individual MHC alleles among populations. When comparing parapatric populations experiencing gene flow, MHC alleles maintained at relatively high frequency in one population were more likely to be associated with reduced, rather than increased, parasite abundances in that population. Allopatric populations experiencing no gene flow showed no such general relationship between allele frequency and resistance. These results are only consistent with spatially divergent selection, and imply that gene flow and environmental heterogeneity can be important for maintaining MHC diversity. / text

Responses in growth rate, food conversion efficiency, carcass composition, residual food intake, meat quality and reproduction traits to selection in pigs for four years, for high or low post-weaning growth rate on a fixed but restricted ration were measured in progeny grown on either restricted, ad libitum individual or ad libitum group feeding regimens. Genetic and phenotypic correlations among the traits were also examined on animals whose full pedigrees comprised a total of 5324 animals. Substantial divergent response was achieved from selection for high vs. low growth rate on restricted feeding, with the genetic means of daily gain diverging by 81.25 g or 2.78 genetic standard deviations (SigmaG). The high and low growth rate lines also differed in ultrasonic backfat and food conversion efficiency by 1.12 SigmaG and 0.84 SigmaG, and by 0.88 SigmaG, 1.10 SigmaG, and 1.14 SigmaG for carcass weight daily gain, carcass fat and lean percentage, respectively. These responses were consistent with the moderately to highly, favourable genetic correlations among and between performance and carcass composition traits. Genetic changes in growth rate, ultrasonic backfat, food conversion ratio, carcass daily gain, carcass fat and lean percentage when measured on ad libitum individual feeding were all economically beneficial. The high growth line had a significantly lower voluntary daily food intake and residual food intake than the contemporaneous low line, indicating a reduced energy requirement for maintenance in the high relative to the low line. Genetic correlations between either growth rate or backfat and carcass traits were favourable but unfavourable between daily food intake and carcass fat or lean percentage, implying that selection strategies which place emphasis on increased leanness and decreased fatness would result in a decline in voluntary food intake of the animals. The descendants of breeding pigs selected for high post-weaning daily gain on restricted feeding showed significantly increased growth rates, slaughter weights and carcase lean percentages and reduced fatness when grown in group housing with ad libitum feeding. This suggests that restricted feeding is an effective performance testing approach for seedstock lines supplying commercial regimens with group housing and ad libitum feeding. Carcase lean pH values measured 24 hours post-mortem (pH24) were not statistically different between the selected lines, suggesting that an increase in basal metabolic rate due to increased rate of lean deposition in the high relative to the low line may have been compensated for by a reduction in heat production from other sources such as physical activity. Such a reduction was evidenced by a lower residual food intake in the high than the low growth lines. This supports the hypothesis that selection for high growth rate on restricted feeding does not have detrimental effects on pork quality. The sows of the high growth line produced more piglets with significantly heavier weights at birth, giving a potential for better post-natal growth and higher yield of pork weaned per sow per year than the low growth sows. Body weights of the high line sows prior to mating were also significantly higher than those of low line sows suggesting earlier age of sexual maturity. A non-significant tendency for high line sows to have a lower voluntary food intake than low line sows pointed to the possibility that their lower maintenance requirement as growers was carried over to the lactational phase of life. It is concluded that selection for high growth rate on a restricted ration of fixed amount will produce pig strains with a high genetic potential for lean growth, reduced energy requirement for maintenance and improved reproductive performance, and which will be suitable for growing under a wide range of commercial feeding environments.

Responses in growth rate, food conversion efficiency, carcass composition, residual food intake, meat quality and reproduction traits to selection in pigs for four years, for high or low post-weaning growth rate on a fixed but restricted ration were measured in progeny grown on either restricted, ad libitum individual or ad libitum group feeding regimens. Genetic and phenotypic correlations among the traits were also examined on animals whose full pedigrees comprised a total of 5324 animals. Substantial divergent response was achieved from selection for high vs. low growth rate on restricted feeding, with the genetic means of daily gain diverging by 81.25 g or 2.78 genetic standard deviations (SigmaG). The high and low growth rate lines also differed in ultrasonic backfat and food conversion efficiency by 1.12 SigmaG and 0.84 SigmaG, and by 0.88 SigmaG, 1.10 SigmaG, and 1.14 SigmaG for carcass weight daily gain, carcass fat and lean percentage, respectively. These responses were consistent with the moderately to highly, favourable genetic correlations among and between performance and carcass composition traits. Genetic changes in growth rate, ultrasonic backfat, food conversion ratio, carcass daily gain, carcass fat and lean percentage when measured on ad libitum individual feeding were all economically beneficial. The high growth line had a significantly lower voluntary daily food intake and residual food intake than the contemporaneous low line, indicating a reduced energy requirement for maintenance in the high relative to the low line. Genetic correlations between either growth rate or backfat and carcass traits were favourable but unfavourable between daily food intake and carcass fat or lean percentage, implying that selection strategies which place emphasis on increased leanness and decreased fatness would result in a decline in voluntary food intake of the animals. The descendants of breeding pigs selected for high post-weaning daily gain on restricted feeding showed significantly increased growth rates, slaughter weights and carcase lean percentages and reduced fatness when grown in group housing with ad libitum feeding. This suggests that restricted feeding is an effective performance testing approach for seedstock lines supplying commercial regimens with group housing and ad libitum feeding. Carcase lean pH values measured 24 hours post-mortem (pH24) were not statistically different between the selected lines, suggesting that an increase in basal metabolic rate due to increased rate of lean deposition in the high relative to the low line may have been compensated for by a reduction in heat production from other sources such as physical activity. Such a reduction was evidenced by a lower residual food intake in the high than the low growth lines. This supports the hypothesis that selection for high growth rate on restricted feeding does not have detrimental effects on pork quality. The sows of the high growth line produced more piglets with significantly heavier weights at birth, giving a potential for better post-natal growth and higher yield of pork weaned per sow per year than the low growth sows. Body weights of the high line sows prior to mating were also significantly higher than those of low line sows suggesting earlier age of sexual maturity. A non-significant tendency for high line sows to have a lower voluntary food intake than low line sows pointed to the possibility that their lower maintenance requirement as growers was carried over to the lactational phase of life. It is concluded that selection for high growth rate on a restricted ration of fixed amount will produce pig strains with a high genetic potential for lean growth, reduced energy requirement for maintenance and improved reproductive performance, and which will be suitable for growing under a wide range of commercial feeding environments.