Density–dependent selection on horn phenotype in Soay sheep

Clutton‐Brock, Tim; Wilson, Kenneth; Stevenson, Ian R.

doi:10.1098/rstb.1997.0064

Cited by 51 publications

(33 citation statements)

References 23 publications

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“…Dark animals are significantly heavier than light ones, providing a possible mechanism for their better survival (CluttonBrock et al 1997). There is no difference in female fecundity between dark and light sheep (Clutton-Brock et al 1997). At present, there is no explanation for why the light-color morph is maintained in the population; clearly, being able to distinguish the three genotypes may shed light on this puzzle.…”

Section: Discussionmentioning

confidence: 97%

Development of a Linkage Map and Mapping of Phenotypic Polymorphisms in a Free-Living Population of Soay Sheep (Ovis aries)

et al. 2006

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An understanding of the determinants of trait variation and the selective forces acting on it in natural populations would give insights into the process of evolution. The combination of long-term studies of individuals living in the wild and better genomic resources for nonmodel organisms makes achieving this goal feasible. This article reports the development of a complete linkage map in a pedigree of free-living Soay sheep on St. Kilda and its application to mapping the loci responsible for three morphological polymorphisms for which the maintenance of variation demands explanation. The map was derived from 251 microsatellite and four allozyme markers and covers 3350 cM ($90% of the sheep genome) at $15-cM intervals. Marker order was consistent with the published sheep map with the exception of one region on chromosome 1 and one on chromosome 12. Coat color maps to chromosome 2 where a strong candidate gene, tyrosinase-related protein 1 (TYRP1), has also been mapped. Coat pattern maps to chromosome 13, close to the candidate locus Agouti. Horn type maps to chromosome 10, a location similar to that previously identified in domestic sheep. These findings represent an advance in the dissection of the genetic diversity in the wild and provide the foundation for QTL analyses in the study population.

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Section: Discussionmentioning

confidence: 97%

Development of a Linkage Map and Mapping of Phenotypic Polymorphisms in a Free-Living Population of Soay Sheep (Ovis aries)

et al. 2006

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“…The weight of Soay sheep in August has been shown to be important in virtually all analyses of their potential over-winter survival and reproductive success (Clutton-Brock et al 1992, 1997Illius et al 1995 ;Forchhammer et al 2001). In lambs and yearlings, of the various parasite taxa, only strongyle egg count explained variation in August weight.…”

Section: Associations Between Parasite Taxa and Host Weightmentioning

confidence: 99%

“…First, we provide a basic description of associations between helminth, protozoa and arthropod species across hosts. Second, we investigate whether the intensities of different parasite taxa have independent associations with host weight, which is itself a predictor of over-winter survival and reproductive success in the study population (Clutton-Brock et al 1992, 1997Illius et al 1995 ;Forchhammer et al 2001).…”

Section: Introductionmentioning

confidence: 99%

Metazoan-protozoan parasite co-infections and host body weight in St Kilda Soay sheep

et al. 2008

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S U M M A R YFor hundreds of years, the unmanaged Soay sheep population on St Kilda has survived despite enduring presumably deleterious co-infections of helminth, protozoan and arthropod parasites and intermittent periods of starvation. Important parasite taxa in young Soay sheep are strongyles (Trichostrongylus axei, Trichostrongylus vitrinus and Teladorsagia circumcincta), coccidia (11 Eimeria species) and keds (Melophagus ovinus) and in older animals, Teladorsagia circumcincta. In this research, associations between the intensity of different parasite taxa were investigated. Secondly, the intensities of different parasite taxa were tested for associations with variation in host weight, which is itself a determinant of over-winter survival in the host population. In lambs, the intensity of strongyle eggs was positively correlated with that of Nematodirus spp. eggs, while in yearlings and adults strongyle eggs and coccidia oocysts were positively correlated. In lambs and yearlings, of the parasite taxa tested, only strongyle eggs were significantly and negatively associated with host weight. However, in adult hosts, strongyles and coccidia were independently and negatively associated with host weight. These results are consistent with the idea that strongyles and coccidia are exerting independent selection on Soay sheep.

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“…Therefore, as scurred males appear to gain a lower number of paternities per year, this could potentially be balanced by greater longevity generating equal fitness in both groups. Furthermore, scurred females have been shown to have on average higher conception, weaning rates, and overwinter survival, relative to a combined class of normal and polled females (Clutton-Brock et al 1997). This suggests that antagonistic selection may also be maintaining the phenotypic polymorphism, with the advantage of scurring in females opposing the selective forces acting against scurring in males (Moorcroft et al 1996;Clutton-Brock et al 1997;Milner et al 2004).…”

mentioning

confidence: 99%

“…Furthermore, scurred females have been shown to have on average higher conception, weaning rates, and overwinter survival, relative to a combined class of normal and polled females (Clutton-Brock et al 1997). This suggests that antagonistic selection may also be maintaining the phenotypic polymorphism, with the advantage of scurring in females opposing the selective forces acting against scurring in males (Moorcroft et al 1996;Clutton-Brock et al 1997;Milner et al 2004). However, although we therefore have evidence of several associations between horn phenotype and different components of fitness in either sex, a full comparison of selection on horn phenotype acting through lifetime breeding success in this long-lived species has not previously been conducted.…”

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confidence: 99%

Live Fast, Die Young: Trade-Offs Between Fitness Components and Sexually Antagonistic Selection on Weaponry in Soay Sheep

et al. 2006

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Abstract. Males are predicted to compete for reproductive opportunities, with sexual selection driving the evolution of large body size and weaponry through the advantage they confer for access to females. Few studies have explored potential trade-offs of investment in secondary sexual traits between different components of fitness or tested for sexually antagonistic selection pressures. These factors may provide explanations for observed polymorphisms in both form and quality of secondary sexual traits. We report here an analysis of selection on horn phenotype in a feral population of Soay sheep (Ovis aries) on the island of Hirta, St. Kilda, Scotland. Soay sheep display a phenotypic polymorphism for horn type with males growing either normal or reduced (scurred) horns, and females growing either normal, scurred, or no (polled) horns; further variation in size exists within horn morphs. We show that horn phenotype and the size of the trait displayed is subject to different selection pressures in males and females, generating sexually antagonistic selection. Furthermore, there was evidence of a trade-off between breeding success and longevity in normal-horned males, with both the normal horn type and larger horn size being associated with greater annual breeding success but reduced longevity. Therefore, selection through lifetime breeding success was not found to act upon horn phenotype in males. In females, a negative association of annual breeding success within the normal-horned phenotype did not result in a significant difference in lifetime fitness when compared to scurred individuals, as no significant difference in longevity was found. However, increased horn size within this group was negatively associated with breeding success and longevity. Females without horns (polled) suffered reduced longevity and thus reduced lifetime breeding success relative the other horn morphs. Our results therefore suggest that trade-offs between different components of fitness and antagonistic selection between the sexes may maintain genetic variation for secondary sexual traits within a population.

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Density–dependent selection on horn phenotype in Soay sheep

Cited by 51 publications

References 23 publications

Development of a Linkage Map and Mapping of Phenotypic Polymorphisms in a Free-Living Population of Soay Sheep (Ovis aries)

Development of a Linkage Map and Mapping of Phenotypic Polymorphisms in a Free-Living Population of Soay Sheep (Ovis aries)

Metazoan-protozoan parasite co-infections and host body weight in St Kilda Soay sheep

Live Fast, Die Young: Trade-Offs Between Fitness Components and Sexually Antagonistic Selection on Weaponry in Soay Sheep

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