Generations Of Relaxed Selection Research Articles

Artificial selection on sexual aggression: Correlated traits and possible trade-offs.

Forced copulation is an extreme form of sexual aggression that can affect the evolution of sex-specific anatomy, morphology, and behavior. To characterize mechanistic and evolutionary aspects of forced copulation, we artificially selected male fruit flies based on their ability to succeed in the naturally prevalent behavior of forced matings with newly eclosed (teneral) females. The low and high forced copulation lineages showed rapid divergence, with the high lineages ultimately showing twice the rates of forced copulation as the low lineages. While males from the high lineages spent more time aggressively pursuing and mounting teneral females, their behavior toward non-teneral and heterospecific females was similar to that of males from the low lineages. Males from the low and high lineages also showed similar levels of male-male aggression. This suggests little or no genetic correlations between sexual aggression and non-aggressive pursuit of females, and between male aggression toward females and males. Surprisingly however, males from the high lineages had twice as high mating success than males from the low lineages when allowed to compete for consensual mating with mature females. In further experiments, we found no evidence for trade-offs associated with high forced mating rates: males from the high lineages did not have lower longevity than males from the low lineages when housed with females, and four generations of relaxed selection did not lead to convergence in forced mating rates. Our data indicate complex interactions among forced copulation success and consensual mating behavior, which we hope to clarify in future genomic work.

Artificial selection for increased dispersal results in lower fitness.

Dispersal often covaries with other traits, and this covariation was shown to have a genetic basis. Here, we wanted to explore to what extent genetic constraints and correlational selection can explain patterns of covariation between dispersal and key life-history traits-lifespan and reproduction. A prediction from the fitness-associated dispersal hypothesis was that lower genetic quality is associated with higher dispersal propensity as driven by the benefits of genetic mixing. We wanted to contrast it with a prediction from a different model that individuals putting more emphasis on current rather than future reproduction disperse more, as they are expected to be more risk-prone and exploratory. However, if dispersal has inherent costs, this will also result in a negative genetic correlation between higher rates of dispersal and some aspects of performance. To explore this issue, we used the dioecious nematode Caenorhabditis remanei and selected for increased and decreased dispersal propensity for 10 generations, followed by five generations of relaxed selection. Dispersal propensity responded to selection, and females from high-dispersal lines dispersed more than females from low-dispersal lines. Females selected for increased dispersal propensity produced fewer offspring and were more likely to die from matricide, which is associated with a low physiological condition in Caenorhabditis nematodes. There was no evidence for differences in age-specific reproductive effort between high- and low-dispersal females. Rather, reproductive output of high-dispersal females was consistently reduced. We argue that our data provide support for the fitness-associated dispersal hypothesis.

Evolutionary Biology of Diet, Aging, and Mismatch

Author(s): Rutledge, Grant Allen | Advisor(s): Rose, Michael R | Abstract: The search for a diet or supplement that improves healthspan in humans has been daunting. This dissertation applies theory on the evolution of aging to understand what diets are best for extending healthspan. Chapter 1 reviews strategies for healthspan extension using diet and supplementation from the standpoint of evolutionary biology. Then it introduces a new evolutionary strategy for dietary enhancement of healthspan. Our intuitive understanding of adaptation by natural selection is dominated by the power of selection at early ages in large populations. Yet, as the forces of natural selection fall with adult age, we expect adaptation to be attenuated with age. Explicit simulations of age-dependent adaptation suggest that populations adapt to a novel environment quickly at early ages, but only slowly and incompletely at later adult ages. Chapter 2 tests for age-dependent adaptation in laboratory populations of Drosophila melanogaster. The results show clear age-specificity of adaptation in two examples of evolution in response to dietary transition. In the first example, populations perform better on an ancestral, long-abandoned, fruit diet compared to an evolutionarily recent fruit diet, only at later ages. In the second example, the gain and loss of urea adaptation is strongest at early ages and weakest at later ages. Chapter 3 evaluates the effects of combining both diet and botanical supplementation on Drosophila healthspan. One botanical extract, derived from Rosa damascena, decreases survivorship when added to an ancestral diet, but increases lifespan when added to an evolutionarily recent diet. Another botanical, derived from Rhodiola rosea, extends life-span by approximately 20% with a 20% decrease in average fecundity. In addition, there is evidence of an antagonistic effect when these botanicals are combined, supporting the “Poisoned Chalice” hypothesis that novel combinations of substances may produce adverse physiological responses. Chapter 4 studies the effects of evolutionary history on phenotypic convergence in D. melanogaster populations selected for desiccation resistance and larval urea tolerance. We find that extreme selection can have long-lasting impacts on phenotypic differentiation, particularly for longevity, even after a few hundred generations of relaxed selection.

Experimental removal of sexual selection leads to decreased investment in an immune component in female Tribolium castaneum

Because of divergent selection acting on males and females arising from different life-history strategies, polyandry can be expected to promote sexual dimorphism of investment into immune function. In previous work we have established the existence of such divergence within populations where males and females are exposed to varying degrees of polyandry. We here test whether the removal of sexual selection via enforced monogamy generates males and females that have similar levels of investment in immune function. To test this prediction experimentally, we measured differences between the sexes in a key immune measurement (phenoloxidase (PO) activity) and resistance to the microsporidian Paranosema whitei in Tribolium castaneum lines that evolved under monogamous (sexual selection absent) vs polyandrous (sexual selection present) mating systems. At generation 49, all selected lines were simultaneously assessed for PO activity and resistance to their natural parasite P. whitei after two generations of relaxed selection. We found that the polyandrous regime was associated with a clear dimorphism in immune function: females had significantly higher PO activities than males in these lines. In contrast, there was no such difference between the sexes in the lines evolving under the monogamous regime. Survival in the infection experiment did not differ between mating systems or sexes. Removing sexual selection via enforced monogamy thus seems to erase intersexual differences in immunity investment. We suggest that higher PO activities in females that have evolved under sexual selection might be driven by the increased risk of infections and/or injuries associated with exposure to multiple males.

Are there genetic trade-offs between immune and reproductive investments in Tribolium castaneum?

Parasites impose strong selection on hosts to defend themselves, which is expected to result in trade-offs with other fitness traits such as reproduction. Here we test for genetic trade-offs between reproductive traits and immunity using Tribolium castaneum lines that were subject to experimental evolution. The lines have been exposed to contrasting sexual selection intensities via different sex ratios (female-biased, equal and male-biased). After 56 generations, the lines have significantly diverged and those experiencing high sexual selection have evolved males who are superior competitors for reproductive success, and females who are more resistant to multiple mating. All selected lines were assessed for both an immune measurement (phenoloxidase (PO) activity) and host resistance to the microsporidian Nosema whitei after two generations of relaxed selection. In contrast to our expectations we did not find any evidence for a genetic trade-off between investment in reproduction and immunity. Both PO and Nosema resistance did not differ between lines, despite their divergences in reproductive investment due to variation in sexual selection and conflict. Nevertheless, overall we found that females had higher PO activities and in the Nosema free control treatment survived longer than males, suggesting that females generally invest more in PO and survival under control conditions than males. This result fits the Bateman’s principle, which states that females gain fitness through increased immunity and longevity, while males gain fitness through increased mating success.

Alternative methods of selection for litter size in mice: III. Response to 21 generations of selection.

Alternative methods of selection to increase litter size in mice have been practiced for 21 generations followed by six generations of relaxed selection. Three replicates were used with four selection criteria: index of components (IX:I = 1.21 x total ovulation rate + 9.05 x ova success), uterine capacity (UT), litter size (LS), and an unselected control (LC). In IX, ovulation rate and ova success were measured by number of corpora lutea and number of pups born/number of corpora lutea, respectively. In UT, uterine capacity was measured and defined as number of pups born to unilaterally ovariectomized (right ovary excised) females. Selection in LS was based on number born to unaltered dams. In all cases, number born was fully formed, live or dead pups. Pups from 16 randomly chosen LC dams and from the top 16 dams in IX, UT, and LS were selected to produce the next generation in each criterion-replicate line. Response in number born, selected criteria deviated from control, was regressed on generation number over the 21 generations of selection. Responses for the IX and LS criteria were quite similar (.14 +/- .01 and .16 +/- .01 pups per generation, respectively), whereas response in UT, with only one functional horn, was slightly lower (.09 +/- .01). The average cumulative selection differentials for IX, LS, and UT at Generation 21 were 32.78 index units, 36.38 pups, and 28.53 pups, respectively. The LC criterion had an unintentional cumulative selection differential of 3.3 pups.(ABSTRACT TRUNCATED AT 250 WORDS)

Research Note: Genetic Adaptation to Multiple-Bird Cage Environment is Less Evident with Effective Beak Trimming ,

Selection of hens with intact beaks for high performance in multiple-bird cages has produced a stock that clearly exceeds its unselected control when hens have intact beaks but shows less advantage when comparisons involve effectively beak-trimmed birds. The selected stock performed at about the same level as a stock recently derived from a commercial source (but with two generations of relaxed selection) when birds had intact beaks but at a lower level when birds had their beaks trimmed. Relative incidences of deaths from cannibalistic pecking were partially responsible for the results obtained.

Plasma and Yolk Cholesterol Levels in Japanese Quail Divergently Selected for Plasma Cholesterol Response to Adrenocorticotropin

Studies were conducted to determine the effect of divergent selection for plasma cholesterol response to adrenocorticotropin (ACTH) on the levels and relationships between plasma and yolk cholesterol in Japanese quail. Cholesterol data were obtained in Generation 25, following seven generations of relaxed selection, from birds maintained under a normal environment with no exposure to exogenous ACTH. Levels of plasma and yolk cholesterol were determined at 22 and 28 wk. Plasma cholesterol levels of quail in the low cholesterol line were significantly (P<.01) lower than levels in the high line at both ages (224 versus 383 and 209 versus 326 mg/100 mL, respectively). In contrast, yolk cholesterol levels were significantly (P<.01) higher in the low line than in the high line (24.1 versus 21.5 and 21.1 versus 16.9 mg cholesterol/g yolk at 22 and 28 wk, respectively). A significant line by sex interaction was present at both ages for plasma cholesterol with females having higher cholesterol values than males in the low line and males having higher values than females in the high line. A negative relationship was observed between changes in plasma and yolk cholesterol in the selected lines. Greater deposition of cholesterol in the yolk of the line with lower plasma cholesterol indicates that excretion rate may play a role in explaining genetic differences in plasma cholesterol.

Maternal performance over three parities in rats selected for rate or efficiency of protein gain.

Direct, maternal and heterosis effects were estimated for preweaning traits over three parities, with unstandardized litters, in rats selected for 14 generations for rate (LG) or efficiency (LE) of postweaning protein gain. In generation 25, after 11 generations of relaxed selection, 60 males and females from each of the selected lines and the control (C) line were mated in a 3 X 3 diallel. Two more parities were produced by remating the females at each weaning. Loss of female breeders due to infertility and death was higher in the selected lines (LG 43%, LE 40%, C 27%). Rebreeding intervals in parities 2 and 3 were 3.1 (P less than .01) and 3.0 (P less than .10) d longer in LG compared with C, while LE was similar to C. Heterosis was significant only for litter size born and it was negative. Because the estimate included all line interactions, negative heterosis could arise from incompatibilities between sperm and ova or embryo and uterus. Both selected lines, but especially LE, had positive direct effects and negative maternal effects on litter size born and weaned. Maternal effects of LG on average pup weight were similar or slightly better than C, while lactational performance was lower in LE.

A computer model of speciation by founder effects

SummaryA computer model of a two-locus genetic system with epistatic selection was used to investigate factors influencing the probability of the origin of reproductive isolation, due to a genetic revolution following a founder event (Mayr, 1954; Carson, 1975). Restricted population size can sometimes cause such a system to drift from one equilibrium to another, which can result in loss of fitness to hybrids with the ancestral population. The chance of such an event was found to be low unless the bottleneck in population size associated with the founder event was preceded by many generations of relaxed selection. It is highest when the bottleneck is not prolonged and when the population size during the bottleneck is not too small. It seems to be difficult to achieve a high degree of reproductive isolation in one step by this method, and it is concluded that it is unlikely to be a major cause of rapid speciation, although it could be a contributory factor.

Selection for Increased Rate or Efficiency of Lean Growth in Rats: Correlated Responses in Reproductive Performance

Two lines of rats selected for either rate or efficiency of postweaning lean growth and a control line were evaluated to determine whether any differences in reproductive performance had accompanied the genetic improvement in rate or efficiency of lean growth after 14 generations of selection (followed by six generations of relaxed selection). All progeny were reared in litters of nine until weaning at 21 days of age, and in like-sex or unlike-sex pairs thereafter. Like-sex paired females were placed with unrelated males of similar age for mating at approximately 70 days of age, while unlike-sex paired females remained with the original unrelated male until mated or autopsied at 85 days of age. Selection for either rate or efficiency of lean growth increased age at vaginal opening (38.9 and 38.6 vs 36.9 days for controls; P<.01) and weight at vaginal opening (133.1 and 126.6 vs 119.8 g for controls; P<.01) in like-sex paired females; similar findings were made for unlike-sex paired females. Ovulation rate, number of fetuses and pre- and postimplantation losses were not altered by selection. Age at first spontaneous mating did not differ among lines of male rats. Testes weight at 85 days of age were lower (P< .01) for males selected for efficiency of lean growth. The results indicate that there are alterations in reproductive characteristics as a consequence of selection for either rate or efficiency of lean growth in rats.

Two Generations of Relaxed Selection in Commercial Crosses of Poultry

A study of two generations of relaxed selection in three commercial crosses of chickens confirmed a previous report of an 11% to 15% decline in hen-day and hen-housed number of eggs in the first generation. No additional loss in these traits occurred as a result of the second generation of relaxation.No significant change due to relaxation of selection was found for egg weight, body weight, age at 50% production, adult mortality, or hatchability.

Long-term selection for a quantitative character in large replicate populations ofDrosophila melanogaster: 1. Response to selection

SUMMARYThe response to long-term selection for increased abdominal bristle number was studied in six replicate lines ofDrosophila melanogasterderived from thescCanberra outbred strain. Each line was continued for 86–89 generations with 50 pairs of parents selected at an intensity of 20%, and subsequently for 32–35 generations without selection. Response continued for at least 75 generations and average total response was in excess of 36 additive genetic standard deviations of the base population (σA) or 51 times the response in the first generation. The pattern of longterm response was diverse and unpredictable typically with one or more accelerated responses in later generations. At termination of the selection, most of the replicate lines were extremely unstable with high phenotypic variability, and lost much of their genetic gains rapidly upon relaxation of selection.The variation in response among replicates rose in the early phase of selection to level off at approximately 7·6around generation 25. As some lines plateaued, it increased further to a level higher than would be accommodated by most genetic models. The replicate variation was even higher after many generations of relaxed selection. The genetic diversity among replicates, as revealed in total response, the individuality of response patterns and variation of the sex-dimorphism ratio, suggests that abdominal bristle number is influenced potentially by a large number of genes, but a smaller subset of them was responsible for selection response in any one line.

GENETIC MODIFICATION OF RECOMBINATION RATE IN TRIBOLIUM CASTANEUM

Asymmetrical responses were obtained in a replicated study of 15 generations of two-way selection for recombination rate between the ruby (rb) and jet (j) loci in Tribolium castaneum. Recombination rates in the two replicate high lines increased from an average of 0.22 in the base populations to an average of 0.42 at generation 15. Recombination rate pooled over the 15 generations of selection in each low line was significantly less than the control but there was no clear downward trend in response to selection for decreased recombination rate. The realized heritabilities were 0.16 +/- 0.03 and 0.17 +/- 0.02 in the two high lines, and were not significantly different from zero in the two low lines. Selection was based on crossing over in cis females only; however, rates measured in cis males after 12 generations showed the same response patterns as female rates. Similar response patterns were also determined for recombination measured in trans males and females at generation 18 following three generations of relaxed selection. The distribution of recombination rates measured in backcross beetles [(H X L) X H and (H X L) X L] at generation 12 indicated polygenic control with those genes decreasing recombination rate being dominant. Detailed analysis of recombination rates in F1's produced by interline crosses at generation 15 confirmed the directional dominance findings. Under a polygenic model of recombination modifiers in which low recombination is dominant to high, average recombination rates will increase as inbreeding progresses, thus providing a mechanism for the production of new gene combinations in small populations.