Causes and consequences of intra-specific variation in vertebral number

Drivers of neutral and adaptive differentiation in pike (Esox lucius) populations from contrasting environments

Understanding how eco-evolutionary processes and environmental factors drive population differentiation and adaptation are key challenges in evolutionary biology of relevance for biodiversity protection. Differentiation requires at least partial reproductive separation, which may result from different modes of isolation such as geographic isolation (allopatry) or isolation by distance (IBD), resistance (IBR), and environment (IBE). Despite that multiple modes might jointly influence differentiation, studies that compare the relative contributions are scarce. Using RADseq, we analyse neutral and adaptive genetic diversity and structure in 11 pike (Esox lucius) populations from contrasting environments along a latitudinal gradient (54.9-63.6°N), to investigate the relative effects of IBD, IBE and IBR, and to assess whether the effects differ between neutral and adaptive variation, or across structural levels. Patterns of neutral and adaptive variation differed, probably reflecting that they have been differently affected by stochastic and deterministic processes. The importance of the different modes of isolation differed between neutral and adaptive diversity, yet were consistent across structural levels. Neutral variation was influenced by interactions among all three modes of isolation, with IBR (seascape features) playing a central role, wheares adaptive variation was mainly influenced by IBE (environmental conditions). Taken together, this and previous studies suggest that it is common that multiple modes of isolation interactively shape patterns of genetic variation, and that their relative contributions differ among systems. To enable identification of general patterns and understand how various factors influence the relative contributions, it is important that several modes are simultaneously investigated in additional populations, species and environmental settings.

How well do genetic markers inform about responses to intraspecific admixture? A comparative analysis of microsatellites and RADseq

Background

Fitness consequences of intraspecific genetic admixture can vary from positive to negative depending on the genetic composition of the populations and environmental conditions. Because admixture has potential to influence the success of management and conservation efforts, genetic similarity has been suggested to be used as a proxy to predict the outcome. Studies utilizing microsatellites (a neutral marker) to investigate associations between genetic distance and admixture effects show conflicting results. Marker types that yield information on genome-wide and/or adaptive variation might be more useful for predicting responses to inter-population hybridization. In this study we utilized published data for three populations of pike (Esox lucius) to investigate associations between offspring performance (hatching success) and parental genetic similarity in experimentally purebred and admixed families, based on neutral (microsatellites), genome-wide neutral (RADseq SNPs), and adaptive (SNPs under selection) markers.

Results

Estimated similarity varied among the markers, likely reflecting differences in their inherent properties, but was consistently higher in purebred than admixed families. A significant interaction between marker type and admixture treatment reflected that neutral SNPs yielded higher estimates than adaptive SNPs for admixed families whereas no difference was found for purebred families, which indicates that neutral similarity was not reflective of adaptive similarity. When all samples were pooled, no association between similarity and performance was found for any marker. For microsatellites, similarity was positively correlated with hatching success in purebred families, whereas no association was found in admixed families; however, the direction of the effect differed between the population combinations.

Conclusions

The results strengthen the notion that, as of today, there is no proxy that can reliably predicted the outcome of admixture. This emphasizes the need of further studies to advance knowledge that can shed light on how to safeguard against negative consequences of admixture, and thereby inform management and promote conservation of biological diversity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12863-021-00974-3.

Comparing the Performance of Microsatellites and RADseq in Population Genetic Studies: Analysis of Data for Pike (Esox lucius) and a Synthesis of Previous Studies

Population genetic studies reveal biodiversity patterns and inform about drivers of evolutionary differentiation and adaptation, including gene flow, drift and selection. This can advance our understanding and aid decision making regarding management and conservation efforts. Microsatellites have long been used in population genetic studies. Thanks to the development of newer techniques, sequencing approaches such as restriction site associated DNA sequencing (RADseq) are on their way to replace microsatellites for some applications. However, the performance of these two marker types in population genetics have rarely been systematically compared. We utilized three neutrally and adaptively differentiated populations of anadromous pike (Esox lucius) to assess the relative performance of microsatellites and RADseq with respect to resolution and conclusiveness of estimates of population differentiation and genetic structure. To this end, the same set of individuals (N = 64) were genotyped with both RADseq and microsatellite markers. To assess effects of sample size, the same subset of 10 randomly chosen individuals from each population (N = 30 in total) were also genotyped with both methods. Comparisons of estimated genetic diversity and structure showed that both markers were able to uncover genetic structuring. The full RADseq dataset provided the clearest detection of the finer scaled genetic structuring, and the other three datasets (full and subset microsatellite, and subset RADseq) provided comparable results. A search for outlier loci performed on the full SNP dataset pointed to signs of selection potentially associated with salinity and temperature, exemplifying the utility of RADseq to inform about the importance of different environmental factors. To evaluate whether performance differences between the markers are general or context specific, the results of previous studies that have investigated population structure using both marker types were synthesized. The synthesis revealed that RADseq performed as well as, or better than microsatellites in detecting genetic structuring in the included studies. The differences in the ability to detect population structure, both in the present and the previous studies, are likely explained by the higher number of loci typically utilized in RADseq compared to microsatellite analysis, as increasing the number of markers will (regardless of the marker type) increase power and allow for clearer detection and higher resolution of genetic structure.

Adaptations of early development to local spawning temperature in anadromous populations of pike (Esox lucius)

BACKGROUND

In the wake of climate change many environments will be exposed to increased and more variable temperatures. Knowledge about how species and populations respond to altered temperature regimes is therefore important to improve projections of how ecosystems will be affected by global warming, and to aid management. We conducted a common garden, split-brood temperature gradient (4.5 °C, 9.7 °C and 12.3 °C) experiment to study the effects of temperature in two populations (10 families from each population) of anadromous pike (Esox lucius) that normally experience different temperatures during spawning. Four offspring performance measures (hatching success, day degrees until hatching, fry survival, and fry body length) were compared between populations and among families.

RESULTS

Temperature affected all performance measures in a population-specific manner. Low temperature had a positive effect on the Harfjärden population and a negative effect on the Lervik population. Further, the effects of temperature differed among families within populations.

CONCLUSIONS

The population-specific responses to temperature indicate genetic differentiation in developmental plasticity between populations, and may reflect an adaptation to low temperature during early fry development in Harfjärden, where the stream leading up to the wetland dries out relatively early in the spring, forcing individuals to spawn early. The family-specific responses to temperature treatment indicate presence of genetic variation for developmental plasticity (G x E) within both populations. Protecting between- and within-population genetic variation for developmental plasticity and high temperature-related adaptive potential of early life history traits will be key to long-term viability and persistence in the face of continued climate change.

Predator-prey role reversal may impair the recovery of declining pike populations

Many fish populations have experienced declines in recent decades due to anthropogenic disturbances, such as overfishing and habitat exploitation. Despite management actions, many populations show a limited capacity to recover. This may be attributed to reversal of predator-prey roles, yet empirical evidence to that effect remains scarce. Here, we combine field and laboratory studies to investigate the interaction between pike (Esox lucius), a large keystone top predatory fish, and the small-bodied mesopredatory threespine stickleback (Gasterosteus aculeatus) in the Baltic Sea where pike populations have declined. Our data suggest that stickleback predation on pike larvae depletes a large proportion of the recruitment and influences the size distribution through size-selective predation, which is corroborated by a gape-limitation experiment and diet analysis of wild-captured sticklebacks. The effects of stickleback predation are present across several populations and years, and our data suggest that early arrival of sticklebacks has stronger effects on juvenile pike survival. Finally, we use data on pike gape-limitation and the size distribution of sticklebacks to illustrate the process of role reversal. These findings suggest that mesopredator behaviour can reduce recruitment of a top predator species and impair the capacity of populations to recover. This emphasizes predator-prey role reversal as an important ecological and evolutionary driver that influences the outcome of restoration and management actions.

Sex-specific effects of outbreeding on offspring quality in pike (Esox lucius)

Intraspecific genetic admixture occurs when previously separated populations within a species start interbreeding, and it can have either positive, negative, or neutral effects on reproductive performance. As there currently is no reliable predictor for the outcome of admixture, an increased knowledge about admixture effects in different species and populations is important to increase the understanding about what determines the response to admixture. We tested for effects of admixture on F1 offspring quality in three subpopulations of pike (Esox lucius). Gametes were collected in the field, and eggs from each female were experimentally fertilized with milt from a male from each population (one "pure" and two "admixed" treatments). Three offspring quality measures (hatching success, fry survival, and fry length) were determined and compared between (a) pure and admixed population combinations and (b) the sex-specific treatments within each admixed population combination (based on the origin of the male and female, respectively). The results suggested that although there were no overall effects of admixture on offspring quality, the consequences for a given population combination could be sex-specific and thus differ depending on which of the parents originated from one or the other population. All offspring quality traits were influenced by both maternal ID and paternal ID. Sex- and individual-specific effects can have implications for dispersal behavior and gene flow between natural populations, and are important to consider in conservation efforts.

Variation in salinity tolerance between and within anadromous subpopulations of pike (Esox 1ucius)

Environmental heterogeneity is a key determinant of genetic and phenotypic diversity. Stable and homogenous environments tends to result in evolution of specialism and local adaptations, while temporally unpredictable environments may maintain a diversity of specialists, promote generalist strategies, or favour diversified bet hedging strategies. We compared salinity tolerance between two anadromous subpopulations of pike (Esox Lucius) that utilize freshwater spawning sites with different salinity regimes. Eggs from each population were artificially fertilized and incubated in a salinity gradient (0, 3, 5, 7, and 9 psu) using a split-brood design. Effects on embryonic development, hatching success, survival of larvae, and fry body length were compared between populations and families. The population naturally spawning in the stable freshwater habitat showed signs of specialization for freshwater spawning. The population exposed to fluctuating selective pressure in a spawning area with occasional brackish water intrusions tolerated higher salinities and displayed considerable variation in reaction norms. Genetic differences and plasticity of salinity tolerance may enable populations to cope with changes in salinity regimes associated with future climate change. That geographically adjacent subpopulations can constitute separate units with different genetic characteristics must be considered in management and conservation efforts to avoid potentially negative effects of genetic admixture on population fitness and persistence.

Proximate cause, anatomical correlates, and obstetrical implication of a supernumerary lumbar vertebra in humans

OBJECTIVES

Three issues are considered on variation in number of presacral vertebrae (PSV) in humans: (1) sexual difference in number of PSV, (2) inactivation of Hoxd-11 gene as etiology for a supernumerary lumbar vertebra, and (3) anatomical correlates of a supernumerary lumbar vertebra, including lumbar-sacral nearthrosis, and pelvic size.

MATERIALS AND METHODS

Sample was 407 skeletonized females and 1,318 males from United States; ages at death were 20 to 49 years. Two subsamples of males were used: (1) 98 with modal numbers of cervical, thoracic, lumbar, and sacral vertebrae (PSV = 24) and (2) 45 with a supernumerary lumbar vertebra but modal numbers for other vertebral segments (PSV = 25). Measurements were taken of ulna, second metacarpal, vertebrae, femur, and pelvis; presence of lumbar-sacral nearthrosis was observed.

RESULTS

Although 90% of females and males have 24 PSV, females have higher frequency of 23 PSV and males have higher frequency of 25 PSV. Compared to males with 24 PSV, males with 25 PSV and supernumerary lumbar vertebra show (1) no difference in anatomies associated with inactivation of Hoxd-11, and (2) higher frequency of lumbar-sacral nearthrosis and smaller pelvic inlet circumference.

DISCUSSION

Sexual difference in number of PSV may be due to tempo of somite formation and Hox gene activation. Hypothesis is not supported that a supernumerary lumbar vertebra is due to inactivation of Hoxd-11. The presence of a supernumerary lumbar vertebra is associated with small pelvic inlet circumference, which can be obstetrically disadvantageous.

Exploring Jordan's rule in Pacific three-spined stickleback Gasterosteus aculeatus

Coastal marine Gasterosteus aculeatus were captured from seven locations along the Pacific coast of North America, ranging across 21·8° latitude to test Jordan's rule, i.e. that vertebral number should increase with increasing latitude for related populations of fish. Vertebral number significantly increased with increasing latitude for both total and caudal vertebral number. Increasing length with latitude (sensu Bergmann's rule) was also supported, but the predictions for Jordan's rule held when controlling for standard length. Pleomerism was weakly evidenced. Gasterosteus aculeatus exhibited sexual dimorphism for Jordan's rule, with both sexes having more vertebrae at higher latitudes, but only males showing a positive association between latitude and the ratio of caudal to abdominal vertebrae. The number of dorsal- and anal-fin rays and basals increased with increasing latitude, while pectoral-fin ray number decreased. This study reinforces the association between phenotypic variation and environmental variation in marine populations of G. aculeatus.

Can spatial sorting associated with spawning migration explain evolution of body size and vertebral number in Anguilla eels?

Spatial sorting is a process that can contribute to microevolutionary change by assembling phenotypes through space, owing to nonrandom dispersal. Here we first build upon and develop the "neutral" version of the spatial sorting hypothesis by arguing that in systems that are not characterized by repeated range expansions, the evolutionary effects of variation in dispersal capacity and assortative mating might not be independent of but interact with natural selection. In addition to generating assortative mating, variation in dispersal capacity together with spatial and temporal variation in quality of spawning area is likely to influence both reproductive success and survival of spawning migrating individuals, and this will contribute to the evolution of dispersal-enhancing traits. Next, we use a comparative approach to examine whether differences in spawning migration distance among 18 species of freshwater Anguilla eels have evolved in tandem with two dispersal-favoring traits. In our analyses, we use information on spawning migration distance, body length, and vertebral number that was obtained from the literature, and a published whole mitochondrial DNA-based phylogeny. Results from comparative analysis of independent contrasts showed that macroevolutionary shifts in body length throughout the phylogeny have been associated with concomitant shifts in spawning migration. Shifts in migration distance were not associated with shifts in number of vertebrae. These findings are consistent with the hypothesis that spatial sorting has contributed to the evolution of more elongated bodies in species with longer spawning migration distances, or resulted in evolution of longer migration distances in species with larger body size. This novel demonstration is important in that it expands the list of ecological settings and hierarchical levels of biological organization for which the spatial sorting hypothesis seems to have predictive power.

Testing for Local Adaptation to Spawning Habitat in Sympatric Subpopulations of Pike by Reciprocal Translocation of Embryos

We tested for local adaption in early life-history traits by performing a reciprocal translocation experiment with approximately 2,500 embryos of pike (Esox lucius) divided in paired split-family batches. The experiment indicated local adaptation in one of the two subpopulations manifested as enhanced hatching success of eggs in the native habitat, both when compared to siblings transferred to a non-native habitat, and when compared to immigrant genotypes from the other subpopulation. Gene-by-environment effects on viability of eggs and larvae were evident in both subpopulations, showing that there existed genetic variation allowing for evolutionary responses to divergent selection, and indicating a capacity for plastic responses to environmental change. Next, we tested for differences in female life-history traits. Results uncovered that females from one population invested more resources into reproduction and also produced more (but smaller) eggs in relation to their body size compared to females from the other population. We suggest that these females have adjusted their reproductive strategies as a counter-adaptation because a high amount of sedimentation on the eggs in that subpopulations spawning habitat might benefit smaller eggs. Collectively, our findings point to adaptive divergence among sympatric subpopulations that are physically separated only for a short period during reproduction and early development-which is rare. These results illustrate how combinations of translocation experiments and field studies of life-history traits might infer about local adaptation and evolutionary divergence among populations. Local adaptations in subdivided populations are important to consider in management and conservation of biodiversity, because they may otherwise be negatively affected by harvesting, supplementation, and reintroduction efforts targeted at endangered populations.