Asymmetric Isolation and the Evolution of Behaviors Influencing Dispersal: Rheotaxis of Guppies above Waterfalls

Spatial sorting caused by downstream dispersal: implication for morphological evolution in isolated populations of fat minnow inhabiting small streams flowing through terraced rice paddies

The evolutionary forces arising from differential dispersal are known as "spatial sorting," distinguishing them from natural selection arising from differential survival or differential reproductive success. Spatial sorting is often considered to be transient because it is offset by the return of dispersers in many cases. However, in riverine systems, spatial sorting by downstream dispersal can be cumulative in habitats upstream of migration barriers such as weirs or falls, which can block the return of the dispersers. Terraced rice paddies are often found on steep mountain slopes in Japan and often incorporate small streams with numerous migration barriers. This study investigated the morphological features of fat minnow, Rhynchocypris oxycephalus jouyi (Cyprinidae), inhabiting above-barrier habitats of the small streams flowing through flood-prone terraced rice paddies and examined their function via a mark-recapture experiment. Although this study did not reveal a consistent pattern across all local populations, some above-barrier populations were characterized by individuals with a thinner caudal peduncle, thinner body, and longer ventral caudal fin lobes than those in neighbouring mainstream populations. A mark-recapture experiment during minor flooding showed that a thinner caudal peduncle and deeper body helped fat minnow avoid downstream dispersal and ascend a small step, and suggested that a longer ventral caudal fin lobe was important for ascending. These results suggest that the caudal morphologies of some above-barrier populations avoid or reduce the risk of downstream dispersal, supporting the idea that spatial sorting shapes functional traits, enhancing the spatial persistence of individuals in upstream habitats.

Interpopulation variation in dispersal behavior of fat minnow Rhynchocypris oxycephalus jouyi juveniles inhabiting fragmented habitats

In many organisms, including fishes, downstream dispersal is often phenotype-dependent. Phenotype-dependent downstream dispersal can generate evolutionary pressure via spatial sorting, which non-randomly removes phenotypes enhancing downstream dispersal from upstream populations. Spatial sorting due to downstream dispersal could accumulate in fish populations in above-barrier habitats to which dispersed individuals cannot return, resulting in functional traits that reduce downstream dispersal. This evolutionary mechanism may be more important in smaller above-barrier habitats where downstream emigration occurs over shorter dispersal distances. This study observed the dispersal behavior of fat minnow Rhynchocypris oxycephalus jouyi juveniles in an experimental tank to examine whether juveniles from small above-barrier habitats show more behaviors favorable for reducing downstream dispersal than those from large above-barrier habitats. Juveniles from small above-barrier habitats avoided downstream dispersal for longer durations than those from large above-barrier habitats, but there was no difference in the frequency of ascending attempts. These results support the notion that behavioral traits of juveniles in small above-barrier populations have been refined by spatial sorting to reduce downstream dispersal. The finding that interpopulation variation in dispersal behavior occurred only for certain dispersal directions implies that the behavioral consequences of habitat fragmentation are more complex than previously assumed.

Parasite load, rather than parasite presence, decreases upstream movement in Trinidadian guppies Poecilia reticulata

Several factors influence whether an organism remains in its local habitat. Parasites can, for example, influence host movement by impacting their behavior, physiology, and morphology. In rivers, fish that swim efficiently against the current are able to maintain their position without being displaced downstream, a behavior referred to as positive rheotaxis. We hypothesized that both the presence and number of ectoparasites on a host would affect the ability of fish to avoid downstream displacement and thus prevent them from remaining in their habitat. We used the guppy-Gyrodactylus host-ectoparasite model to test whether parasite presence and parasite load had an effect on fish rheotaxis. We quantified rheotaxis of sham-infected and parasite-infected fish in a circular flow tank in the laboratory prior to infection and 5-6 days postinfection. Both parasite-infected and sham-infected individuals expressed similar levels of positive rheotaxis prior to infection and after infection. However, with increasing parasite numbers, guppies covered less distance in the upstream direction and spent more time in slower flow zones. These results suggest that higher numbers of Gyrodactylus ectoparasites negatively influence rheotactic movements. Further research is needed to understand the ecological and evolutionary implications of this ectoparasite on fish movement.

Interpopulation variation of behavioural and morphological traits that affect downstream displacement of the juvenile white-spotted charr Salvelinus leucomaenis

Downstream displacement, the passive downstream dispersal of riverine organisms, can generate evolutionary pressures that selectively remove susceptible individuals from upstream habitats. These evolutionary pressures may accumulate over time in fish populations situated upstream of a tall check dam that displaced fish are unable to swim over and can be diluted by the homing of displaced individuals in the absence of such barriers. Here, we conducted interpopulation comparisons between above-dam and unrestricted open-stream populations of the juvenile white-spotted charr Salvelinus leucomaenis to test the hypothesis that above-dam juveniles possess more advantageous traits that reduce downstream displacement than open-stream juveniles. We focused on sedentary behaviour and body depth, both of which are known to affect downstream displacement. Interpopulation comparisons revealed that juveniles from above-dam populations were consistently more sedentary than those from open-stream populations. On the other hand, there were no systematic differences in body depth between above-dam and open-stream populations. These results are consistent with the evolution of behaviours in above-dam populations that inhibit downstream displacement. However, several other factors could explain the results obtained and further studies will be needed to confirm the presence of behavioural evolution in our study system.

Compiling forty years of guppy research to investigate the factors contributing to (non)parallel evolution

Examples of parallel evolution have been crucial for our understanding of adaptation via natural selection. However, strong parallelism is not always observed even in seemingly similar environments where natural selection is expected to favour similar phenotypes. Leveraging this variation in parallelism within well-researched study systems can provide insight into the factors that contribute to variation in adaptive responses. Here we analyse the results of 36 studies reporting 446 average trait values in Trinidadian guppies, Poecilia reticulata, from different predation regimes. We examine how the extent of predator-driven phenotypic parallelism is influenced by six factors: sex, trait type, rearing environment, ecological complexity, evolutionary history, and time since colonization. Analyses show that parallel evolution in guppies is highly variable and weak on average, with only 24.7% of the variation among populations being explained by predation regime. Levels of parallelism appeared to be especially weak for colour traits, and parallelism decreased with increasing complexity of evolutionary history (i.e., when estimates of parallelism from populations within a single drainage were compared to estimates of parallelism from populations pooled between two major drainages). Suggestive - but not significant - trends that warrant further research include interactions between the sexes and different trait categories. Quantifying and accounting for these and other sources of variation among evolutionary 'replicates' can be leveraged to better understand the extent to which seemingly similar environments drive parallel and nonparallel aspects of phenotypic divergence.

Reproductive benefits associated with dispersal in headwater populations of Trinidadian guppies (Poecilia reticulata)

Theory suggests that the evolution of dispersal is balanced by its fitness costs and benefits, yet empirical evidence is sparse due to the difficulties of measuring dispersal and fitness in natural populations. Here, we use spatially explicit data from a multi-generational capture-mark-recapture study of two populations of Trinidadian guppies (Poecilia reticulata) along with pedigrees to test whether there are fitness benefits correlated with dispersal. Combining these ecological and molecular data sets allows us to directly measure the relationship between movement and reproduction. Individual dispersal was measured as the total distance moved by a fish during its lifetime. We analysed the effects of dispersal propensity and distance on a variety of reproductive metrics. We found that number of mates and number of offspring were positively correlated to dispersal, especially for males. Our results also reveal individual and environmental variation in dispersal, with sex, size, season, and stream acting as determining factors.

Resistance and resilience of genetic and phenotypic diversity to "black swan" flood events: A retrospective analysis with historical samples of guppies

Rare extreme "black swan" disturbances can impact ecosystems in many ways, such as destroying habitats, depleting resources, and causing high mortality. In rivers, for instance, exceptional floods that occur infrequently (e.g., so-called "50-year floods") can strongly impact the abundance of fishes and other aquatic organisms. Beyond such ecological effects, these floods could also impact intraspecific diversity by elevating genetic drift or dispersal and by imposing strong selection, which could then influence the population's ability to recover from disturbance. And yet, natural systems might be resistant (show little change) or resilient (show rapid recovery) even to rare extreme events - perhaps as a result of selection due to past events. We considered these possibilities in two rivers where native guppies experienced two extreme floods - one in 2005 and another in 2016. For each river, we selected four sites and used archived "historical" samples to compare levels of genetic and phenotypic diversity before vs. after floods. Genetic diversity was represented by 33 neutral microsatellite markers, and phenotypic diversity was represented by body length and male melanic (black) colour. We found that genetic diversity and population structure was mostly "resistant" to even these extreme floods; whereas the larger impacts on phenotypic diversity were short-lived, suggesting additional "resilience". We discuss the determinants of these two outcomes for guppies facing floods, and then consider the general implications for the resistance and resilience of intraspecific variation to black swan disturbances.