Rapid evolution of escape ability in Trinidadian guppies (Poecilia reticulata)

Predators are widely assumed to create selection that shapes the evolution of prey escape abilities. However, this assumption is difficult to test directly due to the challenge of recording both predation and its evolutionary consequences in the wild. We examined these events by studying natural and experimental populations of Trinidadian guppies, Poecilia reticulata, which occur in distinct high-predation and low-predation environments within streams. Importantly, in the last two decades several populations of guppies have been experimentally introduced from one type of predatory environment into the other, allowing measurements of the consequences of change. We used this system to test two hypotheses: First, that changes in predatory environments create phenotypic selection favoring changes in escape ability of guppies, and second, that this selection can result in rapid evolution. For the first test we compared escape ability of wild caught guppies from high- versus low-predation environments by measuring survival rates during staged encounters with a major predator, the pike cichlid Crenicichla alta. We used guppies from three streams, comparing two within-stream pairs of natural populations and three within-stream pairs of an introduced population versus its natural source population. In every comparison, guppies from the high-predation population showed higher survival. These multiple, parallel divergences in guppy survival phenotype suggest that predatory environment does create selection of escape ability. We tested our second hypothesis by rearing guppies in common garden conditions in the laboratory, then repeating the earlier experiments using the F2 generation. As before, each comparison resulted in higher survival of guppies descended from the high-predation populations, demonstrating that population differences in escape ability have a genetic basis. These results also show that escape ability can evolve very rapidly in nature, that is, within 26-36 generations in the introduced populations. Interestingly, we found rapid evolutionary loss of escape ability in populations introduced into low-predation environments, suggesting that steep fitness trade-offs may influence the evolution of escape traits.     

Intrinsic reproductive isolation between Trinidadian populations of the guppy, Poecilia reticulata

Although Trinidadian populations of the guppy, Poecilia reticulata, show considerable adaptive genetic differentiation, they have been assumed to show little or no reproductive isolation. We tested this assumption by crossing Caroni (Tacarigua River) and Oropuche (Oropuche R.) drainage populations from Trinidad's Northern Range, and by examining multiple aspects of reproductive compatibility in the F1, F2 and BC1 generations. In open-aquarium experiments, F1 males performed fewer numbers of mating behaviours relative to parental population controls. This is the first documentation of hybrid behavioural sterility within a species, and it suggests that such sterility may feasibly be involved in causing speciation. The crosses also uncovered hybrid breakdown for embryo viability, brood size and sperm counts. In contrast, no reductions in female fertility were detected, indicating that guppies obey Haldane's rule for sterility. Intrinsic isolation currently presents a much stronger obstacle to gene flow than behavioural isolation, and our results indicate that Trinidadian populations constitute a useful model for investigating incipient speciation.     

Characterization of tetranucleotide microsatellite markers in guppy (Poecilia reticulata)

Ten tetranucleotide microsatellite loci are characterized for guppy, Poecilia reticulata, an important model species in the study of adaptation and mating systems. Loci were isolated following a microsatellite enrichment procedure using probe‐labelled magnetic beads. These microsatellites were designed for use in examining gene flow, reproductive isolation, and parentage within natural guppy populations.     

Evolutionary implications of large-scale patterns in the ecology of Trinidadian guppies, Poecilia reticulata

Previous investigations of the guppy, Poecilia reticulata , in Trinidad have demonstrated rapid population differentiation at small geographic scales. However, most studies to date have focused on localities in Trinidad's Northern Range where barrier water falls mark sharp discontinuities in the selection regime and limit scope for gene flow. There is little information on the ecology of the guppy in the rest of the island even though its distribution amongst fish communities and habitats has important evolutionary implications. To determine how large-scale distribution patterns might affect the evolutionary potential of the guppy we surveyed 80 sites representative of a broad range of freshwater environments. We found guppies, which occurred in 80% of our samples, to be the most widely distributed freshwater fish in Trinidad. Guppies are common in predator-rich and turbid habitats, precisely those localities where female preferences are likely to be undermined. Moreover, the widespread distribution of this adaptable species, combined with its promiscuous mating system, may promote gene flow across geographical scales that transcend local selection regimes. These factors are likely to impede the evolution of reproductive isolation.     

Marked genetic divergence revealed by allozymes among populations of the guppy Poecilia reticulata (Poeciliidae), in Trinidad

Populations of the guppy Poecilia reticulata from six locations in N. Trinidad were examined by starch gel electrophoresis to estimate their degree of genetic divergence. Variability at seven enzyme-coding loci demonstrated that populations differed markedly in allele frequencies with some allelic substitution between sites (for 23 loci, Nei's mean genetic identity, Ī ranged from 0.869–1.00; the coefficient of gene differentiation, G _ST= 0.086; and the absolute differentiation between populations, _m= 0.044). There was a good correspondence between degree of physical isolation and extent of genetic differentiation, as exemplified by the partitioning of gene diversity (Nei's gene diversity analysis = 66% between river basins; 32% within river basins; 2% within rivers), though there was considerable variability in the contribution of individual loci. Most populations were in Hardy-Weinberg equilibrium, and any significant deviations were due exclusively to heterozygote deficiencies. The patterns of population differentiation are discussed in relation to probable past geological and historical events, and present-day evolutionary forces. Notable among these are previous continental land connections, periodic immigration from N.E. South America, and post-colonization events including differential predation, sexual selection, apparent restricted vagility and small effective population sizes.     

The influence of water velocity on the display behavior of male guppies, Poecilia reticulata

The objective of this experiment was to determine if differentwater velocities during ontogeny affect male physical condition,male signal intensity, and female mating preferences in theguppy (Poecilia reticulata). Guppies were raised in aquariain either high or low water velocities and fed in excess. Analysisof 72 males from four replicates indicated that high velocitymales had longer mean displays and spent more total time displayingthan low velocity males. These males also had significantlyfaster swimming speeds, wider caudal peduncles, and were moreattractive to females than low velocity males. There were nodifferences in display rates, body widths, standard lengths,or copulation attempts between high and low velocity males.These results indicate that water velocity conditions duringontogeny act as a proximal factor that influences the displaybehavior of guppies. Water velocity caused an indirect increasein the power of the male display to produce a female sexualresponse. The morpho-metric measurements suggest that the proximalmechanism behind the increased display intensity was the increasedmuscle development of males raised in high water velocities,which resulted in longer prolonged swimming speeds and moreintense displays. Therefore, display behavior of guppies maybe a general indicator of overall male physical condition.[BehavEcol07: 272–278 (1996)]     

The genetic and environmental basis of adaptive differences in shoaling behaviour among populations of Trinidadian guppies,Poecilia reticulata

The degree of plasticity an individual expresses when moving into a new environment is likely to influence the probability of colonization and potential for subsequent evolution. Yet few empirical examples exist where the ancestral and derived conditions suggest a role for plasticity in adaptive genetic divergence of populations. Here we explore the genetic and plastic components of shoaling behaviour in two pairs of populations of Poecilia reticulata (Trinidadian guppies). We contrast shoaling behaviour of guppies derived from high‐ and low‐predation populations from two separate drainages by measuring the shoaling response of second generation laboratory‐reared individuals in the presence and absence of predator induced alarm pheromones. We find persistent differences in mean shoaling cohesion that suggest a genetic basis; when measured under the same conditions high‐predation guppies form more cohesive shoals than low‐predation guppies. Both high and low‐predation guppies also exhibit plasticity in the response to alarm pheromones, by forming tighter, more cohesive shoals. These patterns suggest a conserved capacity for adaptive behavioural plasticity when moving between variable predation communities that are consistent with models of genetic accommodation.     

Invasion success and genetic diversity of introduced populations of guppies Poecilia reticulata in Australia

High genetic diversity is thought to characterize successful invasive species, as the potential to adapt to new environments is enhanced and inbreeding is reduced. In the last century, guppies, Poecilia reticulata, repeatedly invaded streams in Australia and elsewhere. Quantitative genetic studies of one Australian guppy population have demonstrated high additive genetic variation for autosomal and Y-linked morphological traits. The combination of colonization success, high heritability of morphological traits, and the possibility of multiple introductions to Australia raised the prediction that neutral genetic diversity is high in introduced populations of guppies. In this study we examine genetic diversity at nine microsatellite and one mitochondrial locus for seven Australian populations. We used mtDNA haplotypes from the natural range of guppies and from domesticated varieties to identify source populations. There were a minimum of two introductions, but there was no haplotype diversity within Australian populations, suggesting a founder effect. This was supported by microsatellite markers, as allelic diversity and heterozygosity were severely reduced compared to one wild source population, and evidence of recent bottlenecks was found. Between Australian populations little differentiation of microsatellite allele frequencies was detected, suggesting that population admixture has occurred historically, perhaps due to male-biased gene flow followed by bottlenecks. Thus success of invasion of Australia and high additive genetic variance in Australian guppies are not associated with high levels of diversity at molecular loci. This finding is consistent with the release of additive genetic variation by dominance and epistasis following inbreeding, and with disruptive and negative frequency-dependent selection on fitness traits.     

Artificial introductions, evolutionary change and population differentiation in Trinidadian guppies (Poecilia reticulata; Poeciliidae)

The evolutionary consequences of three artificial introductions of the guppy, Poecilia reticulata , in Trinidad were examined by comparing the allozymic structure (observed heterozygosity ( H _o), and mean number of alleles ( N _a) of each corresponding source (S) and transplant (T) population. In 'Haskins' (H) and EndlerY (E) introduction, 200 guppies (half female) were transferred to guppy-free habitats in 1957 and 1976 respectively. 'Kenny's' (K) introduction in 1981 involved the release of a single pregnant female into an isolated ornamental pond. Analysis of allozyme frequencies at 25 enzyme-coding loci revealed reductions in observed heterozygosity at some loci in all three transplant samples, and a marked decline in the mean number of alleles in Kenny's pond sample. Significant genetic differentiation occurred between (S) and (T) samples at some loci in all introductions, but was most marked in H(T) and K(T). Despite previous studies on rapid evolutionary changes in the life histories and morphology of Endler's transplant guppies, there was little support for any major effects of stochastic forces on allozymic diversity arising from the introduction. Selection arising from changes in predation pressure appeared to be the predominant factor causing the remarkably rapid adaptation of guppies to their new environments. Genetic divergence in some marginal or isolated natural populations was similar to, or greater than, Kenny's pond guppies (Reynolds' genetic distance, R = 0.496), indicating that chance colonization and founder effects may have contributed to the observed geographic patterns of genetic differentiation in Trinidad.     

Caudal eyespots on fish predators influence the inspection behaviour of Trinidadian guppies, Poecilia reticulata

The effect of the false caudal eyespots (ocelli) of fish predators on guppy inspection behaviour was tested by use of predator ( Crenicichla ) models with and without a caudal ocellus. When a false eyespot was present, inspecting guppies spent significantly less time near the tail compared to a control model without a tail eyespot.     

Evidence for sperm transfer during sneaky mating in wild Trinidadian guppies

Among Trinidadian guppies Poecilia reticulata , 15% of non-receptive females had recoverable sperm that must have come from successful sneaky matings. These results provide the first evidence that sneaky mating is a successful method of sperm transfer in wild populations of the guppy.     

Primers for 12 polymorphic microsatellite DNA loci from the guppy (Poecilia reticulata)

Twelve polymorphic microsatellite DNA loci were isolated from genomic guppy DNA. We assessed the level of genetic diversity for these loci using individuals from five native Trinidadian populations. All the loci were polymorphic although there were considerable differences among loci and populations in allele frequencies. Within populations, the number of alleles ranged from 1 to 22 and observed heterozygosities ranged from 0.000 to 0.990. Allele frequencies differed substantially between populations suggesting divergence.     

Isolation and characterization of 43 microsatellite DNA markers for guppy (Poecilia reticulata)

Thirty‐nine polymorphic microsatellite DNA markers were isolated from the guppy (Poecilia reticulata) genomic library. All of the loci showed moderate allelic variation ranging from two to seven alleles, with observed heterozygosities from 0.000 to 0.938. The microsatellite DNA markers isolated will be available for use in analysis of quantitative trait loci in breeding programmes and for population genetic studies on experimental fish.     

Development of 51 genomic microsatellite DNA markers of guppy (Poecilia reticulata) and their application in closely related species

Fifty‐one microsatellite DNA markers of guppy (Poecilia reticulata) were developed. All of the markers detected moderate allelic variation. The number of alleles for each locus ranged from two to 10. Observed and expected heterozygosities varied from 0.10 to 0.63 and from 0.23 to 0.77, respectively. Three additional fish species of family Poeciliidae were used for the cross‐species amplification of these markers. It was found that only four to 16 markers detected polymorphism in these three fish species.     

Brain size affects the behavioural response to predators in female guppies (Poecilia reticulata)

Large brains are thought to result from selection for cognitive benefits, but how enhanced cognition leads to increased fitness remains poorly understood. One explanation is that increased cognitive ability results in improved monitoring and assessment of predator threats. Here, we use male and female guppies (Poecilia reticulata), artificially selected for large and small brain size, to provide an experimental evaluation of this hypothesis. We examined their behavioural response as singletons, pairs or shoals of four towards a model predator. Large-brained females, but not males, spent less time performing predator inspections, an inherently risky behaviour. Video analysis revealed that large-brained females were further away from the model predator when in pairs but that they habituated quickly towards the model when in shoals of four. Males stayed further away from the predator model than females but again we found no brain size effect in males. We conclude that differences in brain size affect the female predator response. Large-brained females might be able to assess risk better or need less sensory information to reach an accurate conclusion. Our results provide experimental support for the general idea that predation pressure is likely to be important for the evolution of brain size in prey species.     

Population genomics of natural and experimental populations of guppies (Poecilia reticulata)

Convergent evolution represents one of the best lines of evidence for adaptation, but few cases of phenotypic convergence are understood at the genetic level. Guppies inhabiting the Northern Mountain Range of Trinidad provide a classic example of phenotypic convergent evolution, where adaptation to low or high predation environments has been found for a variety of traits. A major advantage of this system is the possibility of long‐term experimental studies in nature, including transplantation from high to low predation sites. We used genome scans of guppies from three natural high and low predation populations and from two experimentally established populations and their sources to examine whether phenotypic convergent evolution leaves footprints at the genome level. We used population‐genetic modelling approaches to reconstruct the demographic history and migration among sampled populations. Naturally colonized low predation populations had signatures of increased effective population size since colonization, while introduction populations had signatures of decreased effective population size. Only a small number of regions across the genome had signatures of selection in all natural populations. However, the two experimental populations shared many genomic regions under apparent selection, more than expected by chance. This overlap coupled with a population decrease since introduction provides evidence for convergent selection occurring in the two introduced populations. The lack of genetic convergence in the natural populations suggests that convergent evolution is lacking in these populations or that the effects of selection become difficult to detect after a long‐time period.     

Multiple selection pressures influence Trinidadian guppy (Poecilia reticulata) antipredator behavior

Animals alter their behavior to avoid a variety of differenttypes of predators. Trinidadian guppies (Poecilia reticulata)have been an important system for examining the evolution ofantipredator behavior because geographically isolated populationsexperience different amounts of aquatic predation. Althoughthe influence of aquatic predators has been well documented,selective pressures from other types of predators are less wellunderstood. We examined the response of wild-caught individualsfrom upstream and downstream populations to an aquatic predatorand a simulated aerial predator. As previously documented, fishfrom the downstream population responded more strongly to theaquatic predator than did fish from the upstream population,inspecting for longer periods of time. Guppies also exhibiteda strong behavioral response to the potential aerial predator.Although both populations displayed a similar magnitude of response,they differed in the specific behaviors used. Upstream fishtended to freeze on the tank bottom, whereas downstream fishtended to hide under shelter. Field observations suggest thatthese strategies are related to habitat features specific toeach site. The behaviors used against aerial predators differedsubstantially from the behaviors used against aquatic predators,suggesting that different types of predators exert conflictingselection pressures. This research demonstrates the importanceof considering multiple selection pressures acting on an organismwhen trying to understand the evolutionary history of behavioraland morphological traits.     

Genome‐wide DNA methylation signatures of infection status in Trinidadian guppies (Poecilia reticulata)

Epigenetic modification, especially DNA methylation, can play an important role in mediating gene regulatory response to environmental stressors and may be a key process affecting phenotypic plasticity and adaptation. Parasites are potent stressors with profound physiological and ecological effects on their hosts, yet it remains unclear how parasites influence host methylation patterns. Here, we used a well‐studied host–parasite system, the guppy Poecilia reticulata and its ectoparasitic monogenean Gyrodactylus turnbulli to gain mechanistic insight into the dynamics of DNA methylation in host–parasite interactions. To explore this, we quantitatively measured genome‐wide DNA methylation in guppy skin tissue using reduced representation bisulphite sequencing and characterized differential methylation patterns in guppies during distinct phases of infection. We identified 365, 313, and 741 differentially methylated regions (DMRs) between infected and control fish in early infection, peak infection and recovery phases, respectively. The magnitude of the methylation difference was moderate in DMRs, with an average of 29% (early infection), 27% (peak infection) and 30% (recovery) differential methylation per DMR. Approximately 50% of DMRs overlapped with CpG islands, and over half of the DMRs overlapped with gene bodies, several of which encode proteins relevant to immune response. These findings provide the first evidence of an epigenetic signature of infection by ectoparasites and demonstrate the changing relationship between epigenetic variation and immune response in distinct phases of infection.     

Multiple mating influences offspring size in guppies

Multiply mated female guppies Poecilia reticulata descended from the Tacarigua population in Trinidad produced individually larger offspring than their singly mated counterparts.