The relationship between maternal phenotype and offspring quality: Do older mothers really produce the best offspring?

Maternal effects are increasingly recognized as important drivers of population dynamics and determinants of evolutionary trajectories. Recently, there has been a proliferation of studies finding or citing a positive relationship between maternal size/age and offspring size or offspring quality. The relationship between maternal phenotype and offspring size is intriguing in that it is unclear why young mothers should produce offspring of inferior quality or fitness. Here we evaluate the underlying evolutionary pressures that may lead to a maternal size/age-offspring size correlation and consider the likelihood that such a correlation results in a positive relationship between the age or size of mothers and the fitness of their offspring. We find that, while there are a number of reasons why selection may favor the production of larger offspring by larger mothers, this change in size is more likely due to associated changes in the maternal phenotype that affect the offspring size-performance relationship. We did not find evidence that the offspring of older females should have intrinsically higher fitness. When we explored this issue theoretically, the only instance in which smaller mothers produce suboptimal offspring sizes is when a (largely unsupported) constraint on maximum offspring size is introduced into the model. It is clear that larger offspring fare better than smaller offspring when reared in the same environment, but this misses a critical point: different environments elicit selection for different optimal sizes of young. We suggest that caution should be exercised when interpreting the outcome of offspring-size experiments when offspring from different mothers are reared in a common environment, because this approach may remove the source of selection (e.g., reproducing in different context) that induced a shift in offspring size in the first place. It has been suggested that fish stocks should be managed to preserve these older age classes because larger mothers produce offspring with a greater chance of survival and subsequent recruitment. Overall, we suggest that, while there are clear and compelling reasons for preserving older females in exploited populations, there is little theoretical justification or evidence that older mothers produce offspring with higher per capita fitness than do younger mothers

An investigation of genetic and reproductive differences between Faroe Plateau and Faroe Bank cod (Gadus morhua L.)

The Atlantic cod (Gadus morhua L.) fishery is of great economic importance to the Faroese economy. There are two separately managed cod stocks around the Faroe Islands, the Faroe Plateau and the Faroe Bank cod. Both have experienced dramatic decreases in size and informed management decisions are vital for both stock viability and exploitation. The stocks are geographically isolated by an 800 m deep channel and water temperatures are on average 1 – 2 ºC higher on the Faroe Bank than on the Faroe Plateau. There are clear phenotypic differences between the stocks; in particular, the markedly higher growth rate for the Faroe Bank cod has caught public and scientific attention. There is continuing debate regarding the relative importance of genetics and environmental contributions to the contrasting phenotypes. Analyses of reproductive parameters (field data and experimental captive spawnings) as well as analyses of microsatellite and single nucleotide polymorphism (SNP) markers were undertaken to better resolve the issue. Field data as well as data from experimental captive spawnings provided evidence of reproductive differences between Faroe Plateau and Faroe Bank cod. Peak spawning occurred earlier on the Faroe Plateau than on the Faroe Bank and this difference in timing of spawning was maintained in captivity. In particular, differences in sizes of eggs (average diameters of 1.40 and 1.30 mm for Faroe Plateau and Faroe Bank cod eggs, respectively) and indirect evidence of greater volumes spawned by the Faroe Bank females suggested stock differences with respect to egg size – egg number trade-off. It was hypothesised that the strategy adopted by cod on the Faroe Bank, with a higher number of smaller eggs, evolved in response to a more hostile environment (bare seabed and higher exposure to predators) experienced by early life stages in this area. Experimental captive spawnings with Faroe Bank cod showed a large interfamily skew in survival rates of cod eggs and fry. Egg size was identified as a useful indicator of survival rates in the egg stage, but egg survival rates could not be used to predict viability in later developmental stages, thus highlighting the importance of employing some sort of genetic monitoring of cod fry to ensure sufficient family representation in the progeny. While no tank effect was evident concerning fry survival, a significant tank effect was identified concerning body sizes of fry. Microsatellite data were analysed using large sample sizes of Faroe Plateau and Faroe Bank cod with the Faroe Plateau divided into two locations, Faroe Plateau North-East and Faroe Plateau West (cod from each of the two were known to belong to separate spawning grounds). Two Norwegian coastal cod samples were included as outlier populations. While no genetic differentiation was detected between the two Faroe Plateau locations, these analyses revealed a detectable, albeit relatively modest, degree of genetic differentiation between cod from the Faroe Plateau and the Faroe Bank (FST = 0.0014 and 0.0018; DJost_EST = 0.0027 and 0.0048; P < 0.0001 and P < 0.001 for the Faroe Plateau North-East – Faroe Bank and the Faroe Plateau West – Faroe Bank comparisons). These values were several times smaller than those between Faroese and Norwegian coastal cod (pairwise FST and DJost_EST values in the range of 0.0061 – 0.0137 and 0.0158 – 0.0386, respectively). Despite recent reductions in census population sizes for Faroe Plateau and, particularly, Faroe Bank cod, genetic diversity estimates were comparable to the ones observed for Norwegian coastal cod and there was no evidence of significant genetic bottlenecks. Lastly, data for one of the markers (Gmo132) indicated genotype-dependent vertical distribution of cod (as investigated for Faroe Plateau North-East cod). Contrary to some previously published studies, analysis of SNPs of two candidate genes for adaptive divergence, the hemoglobin gene Hb-ß1 and the transferrin gene Tf1, failed to detect differentiation between samples of Faroe Plateau and Faroe Bank cod analysed in this thesis. Of 3533 novel SNPs simultaneously discovered and genotyped by restriction-site associated DNA (RAD) sequencing, 58 showed evidence of genetic differentiation between Faroe Plateau North-East and Faroe Bank cod (P 0.25; P < 0.0005) were selected for validation in larger samples, that included cod from both Faroe Plateau areas and the Faroe Bank as well as Norwegian coastal and White Sea cod. Six out of the eight loci amplified successfully with a PCR-based method and there was 100 % concordance between genotypes of individuals screened by both techniques. Due to ascertainment bias, the SNPs should only be applied with caution in a broader geographical context. Nonetheless, these SNPs did confirm the genetic substructure suggested for Faroese cod by microsatellite analyses. While no genetic differentiation was evident between the two Faroe Plateau locations, significant genetic differentiation was evident between Faroe Plateau and Faroe Bank cod at five of the SNPs (FST values in the range of 0.0383 – 0.1914). This panel of five SNPs could confidently be used to trace groups of Faroe Plateau and Faroe Bank cod to their population of origin. In conclusion, multiple lines of evidence demonstrate that Faroe Plateau and Faroe Bank cod are truly two genetically distinct populations. While the findings contribute to a broader understanding of the biology and the genetics of Faroe Plateau and Faroe Bank cod, the novel SNPs developed may provide a valuable resource for potential future demands of i.e. genetic stock identification methods

Investigating life-history polymorphism: modelling mites

The thesis presents research on the life-history polymorphism in the mite Sancassania berlesei. Males of this species are andropolymorphic: there are two distinct male phenotypes. One, the fighter, develops a third thickened leg pair, with which it kills off other fighters and males which do not exhibit a third thickened leg pair, the non-fighters. A review of the life-history of S. berlesei is given, focussing on its general biology, diet, dispersal and mating behaviour. This is followed by a review of the andropolymorphism, and the current understanding of the mechanisms underlying it. The major conclusions from the experimental work presented in this thesis are that fighters primarily develop at low population densities; though the proportion of males becoming fighters at any given density may change over time. This change is likely to be due to condition-dependence. Data is presented to illuminate these matters and a model is developed linking fighter development to the costs of being a fighter (in terms of survival) and the benefits of being a fighter (in terms of fecundity). The sex ratio in S. berlesei is 1:1, and there is no evidence of density or frequency-dependent deviations from this. A delay in food supply at maturation delays the time of maximum fecundity of females for about seven days and lowers their overall egg output. Density-dependent effects reduce the overall daily fecundity of females in higher densities. Female survival is affected by density, food present and rearing conditions. Nearly all eggs laid by S. berlesei hatch regardless of the conditions. Eggs laid in very poor conditions hatched even earlier than the average time of between day three and four. At density two, animals do synchronise their frequency, when isolated together from egg stage. Poor conditions reverse female density-dependence from convex to concave with the lowest life expectancy at intermediate densities. The trade-off between survival and fecundity is the likely cause. Amalgamating the results from the previous experiments, the influence of stochastic population dynamics on male strategy was then modelled. The results indicate that the fighter morph development rule is sensitive to the probability of low population densities arising. When low densities occur, there is a selective advantage to being a fighter. With increasing probability of lower densities, becoming a fighter is more feasible. The ESS rule changes, while in a stable high density environment a density-dependent fighter rule is never selected for. This indicates an influence of stochastic population dynamics on life-history evolution. Modelling demographic stochasticity in the fighter rule shows some buffering effect of this form of stochasticity. The fighter morph determination rule is less sensitive to environmental stochasticity with a high frequency of low densities. Using an agent based model with diploid genetics, I show that under high densities a fighter male is less successful at passing on his genes than a non-fighter. At a density of one male, the fighter gains no advantage to developing the fighter phenotype (as he is not competing with other males). In this case, the advantage may arise through future increases in density (such as through immigration or maturation of offspring). The density-dependent fighter development rule is then switched within the model from density-dependent to frequency-dependent, and the model indicates, that even under the frequency-dependent rule a possible ratio of fighters to non-fighters could exist. The system does not reach this state due to condition-dependence in reality. Following on from the findings discussed above, that morph determination has a condition-dependent component, I develop an argument that relates the observed forms of morph determination (density-dependent and frequency-dependent) in three closely related species of mites via an underlying condition-dependence. It is shown that condition-dependence is likely the linking factor between frequency and density-dependence. This is shown to be possibly a rule for all species displaying polymorphism which includes physical alterations of their bodies

Local adaptation to cold temperatures by larval coho salmon (Oncorhynchus kisutch) from different populations throughout British Columbia

The influence of environmental variables on larval development of coho salmon (Oncorhynchus kisutch), with specific focus on the influence of near-freezing incubation temperatures, was examined across populations within British Columbia. A survey across the geographical distribution within British Columbia was conducted to determine the range and variability of incubation temperatures experience by incubating coho salmon. Temperatures throughout incubation differed significantly among locations, averaging approximately 1 °C in colder interior locations and approximately 5 °C in warmer coastal locations. Environmental variables influenced egg size, fecundity, female size and gonadal somatic index, such that higher latitude of spawning grounds increased, larger systems decreased, and increased temperatures experienced by a population increased the four life-history traits. Suggesting significant effects of latitude of spawning grounds, size of spawning system and temperatures experienced by a population on shaping patterns of reproductive investment. ...coho salmoncoldbritish columbi

Consequences of Competitive Asymmetry in Broods of the Black Guillemot Cepphus grylle

I examined natural variation in the reproductive strategy of the black guillemot, with particular emphasis on the consequences for nestlings. Data were collected from a population of c. 65 breeding pairs on the Holm of Papa Westray, northern Scotland. Temporal differences in reproductive strategy are likely to reveal the reproductive constraints most pertinent to a species, exhibit how reproductive costs are manifest, and highlight the potential trade-offs selected to maximise reproductive success in the face of such costs. Constraints appeared to operate both at the egg production and chick rearing stages, with annual and seasonal effects influencing egg size, chick growth and survival. However, females did not alter the allocation of resources between eggs, nor the degree of hatching asynchrony, suggesting that females maintained the level of competitive asymmetry within the brood, despite these constraints. The relationship between egg size and breeding success was investigated. Chick hatching size and quality were positively related to egg size, but I found no effect of egg size on chick growth or survival. Within clutches, first-laid eggs (a-eggs) were larger than second- laid (b-eggs), but egg-size disparity decreased with decreasing a-egg size. Thus females producing large eggs (i.e. potentially higher quality females) appeared to be striving for within- clutch egg size disparity, inferring that such a difference might be adaptive in the context of sibling competitive asymmetry. Paradoxically, however, egg-size disparity exhibited no relationship with hatching asynchrony. Egg size was a significant predictor of hatching success in the b-egg, suggesting that egg viability decreases once a certain minimum size was attained. I also examined breeding success in relation to natural variation in hatching asynchrony. At all levels of hatching asynchrony, a-chicks attacked their siblings more frequently than vice versa. Consequently, a-chicks in asynchronous broods grew faster, reached higher asymptotic weights and were more likely to survive to fledging than b-chicks. No such differences were evident between siblings in S3aichronous broods, despite a-chick aggressive dominance in these broods. Overall, broods hatching with a two-day hatching interval achieved the highest breeding success. I investigated food amount and competitive asymmetry as potential proximate cues for sibling aggression. Parental provisioning rates were experimentally manipulated in broods comprising a range of hatching intervals over a twelve-hour period. Aggression became evident only after parental provisioning rates were experimentally reduced. When parental provisioning resumed, adults did not increase their feeding rate to compensate for the induced food deficit and the result of sibling rivalry was a change in the allocation of parental deliveries from one of equality to one in favor of the dominant chick. Food deprived chicks from synchronous broods were more aggressive than those from asynchronous broods, suggesting that one benefit of hatching asynchrony in the black guillemot is to establish an efficient competitive hierarchy among siblings which minimizes the need for costly aggressive interactions. Nonetheless, dominance was always established by the chick hatching from the first-laid egg, suggesting that factors in addition to size disparity are important in establishing competitive hierarchies. These results provide the first evidence that short-term food shortage per se acts as an initial trigger for aggression, yet also reveal that the aggressive response is complicated by factors associated with hatching and laying order. Before the functional significance of hatching asynchrony within any avian species can be resolved, it is first necessary to determine the proximate mechanisms controlling the pattern of hatching. To date, most studies have tacitly assumed that hatching patterns are effected predominantly by parental incubation behaviour. I compared incubation periods of male and female black guillemot (Cepphus grylle) embryos to ascertain whether development rates are a function of embryo sex and, if so, the effects of clutch sex-composition on hatching pattern. Chick sex was determined using a molecular DNA technique based on the CHD gene. Laying date and egg mass had no significant effect on incubation period, but eggs containing male embryos developed significantly faster than those containing females. The onset of incubation in relation to clutch completion is variable in black guillemots. Thus, in mixed-sexed clutches where the first-laid embryo is male, hatching asynchrony was attained regardless of the incubation regime employed. These results clearly show that mechanisms in addition to incubation behavior are important in establishing avian hatching patterns. I also demonstrated that pre-laying maternal allocation varied according to progeny sex and brood sex composition. Furthermore, survival probability was found to depend not only on the sex of the individual and its position in the laying/hatching sequence, but also on the sex of its sibling. Females appeared to respond to this gender related mortality by dramatically skewing the frequency of brood sex compositions in favour of those less prone to mortality