Mechanisms underlying shoal composition in the Trinidadian guppy, Poecilia reticulata

Free-ranging groups are frequently assorted by phenotypic characters. However, very little is known about the underlying processes that determine this structuring. In this study, we investigate the mechanisms underlying the phenotypic composition of shoals of guppies ( Poecilia reticulata ) in a high-predation stream in Trinidad's Northern Mountain Range. We collected 57 entire wild shoals, which were strongly assorted by body length. Shoal encounters staged within an experimental arena showed shoal fission (but not fusion) events to be an important mechanism in generating phenotypic assortment. In the wild, fission and fusion between guppy shoals occurred extremely frequently and thus are unlikely to constrain the opportunities for shoal assortment. However, fission and fusion processes occur under the restrictions imposed by the distribution of individuals within the environment. We observed size specific segregation within the habitat in three dimensions, providing a passive mechanism that contributes to the maintenance of the observed homogeneity of group composition. Furthermore sex differences were found in social behaviour. Individual male guppies switched between shoals more frequently than females and left a shoal more often than females. We argue that shoal composition is determined by habitat segregation on a medium spatial scale and by fission/fusion processes on a small spatial scale (with sex-specific shoal dynamics adding a additional layer of complexity).     

Exocrine pancreatic carcinogenesis in the guppy Poecilia reticulata

Exocrine pancreatic neoplasms developed in the guppy Poecilia reticulata following exposure to the direct-acting carcinogen methylazoxymethanol acetate (MAM-Ac). Fish 6 to 10 d old were exposed to nominal, non-toxic concentrations of 4 and 10 mg MAM-Ac l(-1) for 2 h and then transferred to carcinogen-free water for grow-out. Whole specimens were sampled monthly up to 9 mo post-exposure to follow the histologic progression of the lesions. No neoplasms occurred in 119 control specimens examined. Pancreatic acinar cell adenomas and carcinomas occurred in 42 of 243 (17%) of the specimens exposed to MAM-Ac. As in earlier studies, specimens exposed to the low MAM-Ac concentration exhibited a higher pancreatic neoplasm incidence (27.8%) than those exposed to the high concentration (7.8%). Acinar cell adenomas accounted for 27 of the 42 neoplasms. Adenomas exhibited a high degree of acinar cell differentiation and some contained foci of atypical acinar cells that were less differentiated and more basophilic than were surrounding adenoma cells. Carcinomas occurred in 15 specimens and exhibited a range of cellular patterns. Although no distant metastases were found, carcinomas tended to invade neighboring tissues and organs. The occurrence of carcinogen-induced pancreatic neoplasms in guppies strengthens the usefulness of small fish species in carcinogen testing and provides an additional model for studying pancreatic neoplasia.     

Salinity tolerance of the guppy, Poecilia reticulata Peters

Experiments were conducted to determine the adaptability of the guppy to various salt concentrations. The guppy, Poecilia reticulata , (total length 11–40 mm) were subjected to abrupt and gradual changes from fresh water (salinity=0.1%) to various salinities (%). No mortality occurred when the fish were transferred from fresh water to 50% sea water (19.5%). Through gradual adaptation from 50% sea water to 80% of the fish were able to tolerate 100% (39%) sea water for 7 days. After a 7 day stay in sea water, fish were readapted to fresh water during a 3 h period. Through gradual adaptation fish were also able to tolerate salinities ranging from 39.0% (100% sea water) to 58.5%. After a 30 day stay in 150% sea water (58.5%), fish were readapted to fresh water over a 5 h period. The results indicate that they were well able to tolerate the abrupt change from 100 or 150% sea water to fresh water. Females that stayed in 150% sea water (58.5%) for 30 days had embryos in their gonads.     

Schooling preferences for familiar fish in the guppy, Poecilia reticulata

Although schooling behaviour is an important antipredator response of guppies ( Poecilia reticulata ) in Trinidad it is not known whether fish elect to school with particular individuals. Familiarity is a simple mechanism that could lead to non-random associations within schools. In this paper we test the idea that prior experience of particular individuals influences the choice of partners during schooling. We show (in a laboratory study) that female guppies prefer to associate with familiar individuals. However, we are unable to detect any schooling preference for unfamiliar females from the same population.     

Phenotypic correlates of spermatozoon quality in the guppy, Poecilia reticulata

The phenotype-linked fertility hypothesis suggests that a femalecan benefit directly from mate choice when the cues she usesindicate the quantity and/or quality of his spermatozoa. Wetested the link between sperm quality and male body size andcoloration in the resource-free mating system of the guppy,a tropical fish characterized by strong female choice. Largermales possessed larger testes and are therefore predicted toproduce larger numbers of spermatozoa than smaller males. Largermale guppies also produced longer spermatozoa than smaller males.Degree of carotenoid coloration did not predict either the quantityor the quality of a male's spermatozoa. These results are consistentwith a previous study that showed that female guppies in thestudy population prefer larger males to brightly colored males.The male-size directed increase in spermatozoon size may bethe result of interplay between sperm competition and the coevolutionof spermatozoon traits with the female reproductive tract.     

Inheritance of the snakeskin color pattern in the guppy, Poecilia reticulata

We made single-pair reciprocal crosses between the Green Snakeskin and Yellow Snakeskin domesticated strains of the guppy, Poecilia reticulata. The two snakeskin strains differ by a single autosomal gene, with the Green Snakeskin strain having the wild-type background coloration caused by the dominant gene (B), whereas the Yellow Snakeskin is homozygous for the recessive blond allele (bb). The snakeskin body and tail patterns characterizing males of these two strains are determined by two genes--Ssb and Sst--that are closely linked on the Y chromosome. The greenish-yellow tail color of the Green Snakeskin strain is mediated by an X-linked dominant gene, Grt. The recessive wild-type allele, Grt+, gives the hyaline tail color. In the Yellow Snakeskin strain, the Grt gene is expressed as a golden-yellow color as a result of the presence of the bb homozygous condition. The putative genotypes of the males and females of the Green Snakeskin strain are BB XGrt YSsb,Sst and BB XGrt XGrt, respectively. Males and females of the Yellow Snakeskin strain have the putative genotypes bb XGrt YSsb,Sst and bb XGrtXGrt, respectively. As a result of crossing over between the X and Y chromosomes, a few males and females of these two snakeskin strains may carry one or both snakeskin pattern genes (Ssb and Sst) on the X chromosome.     

Identification of major histocompatibility complex genes in the guppy,Poecilia reticulata

The guppy, Poecilia reticulata, a teleostean fish of the order Cyprinodontiformes, has been used extensively in studies of host-parasite interactions, courtship behavior, and mating preference, as well as in ecological and evolutionary genetics. A related species was among the first poikilotherm vertebrates to be used in the study of histocompatibility genes. All these studies could benefit from the identification and characterization of the guppy major histocompatibility complex (Mhc) genes. Here, both class I and class II genes of the guppy are described. The number of expressed loci, as determined by representation of clones in a cDNA library, sequencing, and Southern blot analysis, may be low in both Mhc classes: combined evidence suggests that there may be one expressed class II locus only and one or two expressed class I loci. The variability of aquaristic guppy stocks is very low: only three and two genes have been detected at the class I and class II loci, respectively, in the stocks examined. This genetic paucity is most likely the consequence of breeding practices employed by aquarists and commercial establishments. Limited sampling of wild guppy populations revealed extensive Mhc polymorphism at loci of both classes in nature. Comparison of guppy Mhc sequences with those of other vertebrates has revealed the existence of a set of insertions/deletions which can be used as characters in cladistic analysis to infer phylogenetic relationships among vertebrate taxa and the Mhc genes themselves. These indels are particularly frequent in the regions coding for the loops of 1 and 2 domains of class I proteins.The nucleotide sequence data reported in this publication have been submitted to the EMBL nucleotide sequence database and have been assigned the accession numbers Z54076-Z54095     

Two-phase exploration of a novel environment in the guppy, Poecilia reticulata

Swimming speed and percentage of time fish swam along the boundary of an aquarium gradually decreased with the time following release of juvenile guppies into unfamiliar surroundings. When swimming speed was plotted against time on a log scale, two distinct phases were identified. The high activity phase lasted c, 3 h in 30-day-old guppies, 6 h in 10-day-olds and not less than S h in 3-day-olds. Observed trends in swimming in guppies are interpreted as a component of exploratory behaviour. Alternating the surroundings confirmed the exploratory role of such swimming and suggested that a period of general exploration was followed by a period of more detailed investigation of habitat. An interaction between exploratory and avoidance (fear) responses is discussed.     

Recognition of chemical stimuli by guppy, Poecilia reticulata Peters

An alimentary conditioned reflex to beta-phenylethanol and a subsequent differentiation of cumarine were elaborated in conditions of simultaneous paired choice in guppies. It was shown that beta-phenylethanol in a concentration not over 7 X 10(-6) M is perceived through olfaction. The conditioned reflex persists for a long time: twenty five to fifty days after section of the olfactory nerves (the conditioned reflex is restored after their regeneration) and at least seventy days in intact fishes.     

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.     

Factors affecting the distribution of genetic variability in the guppy, Poecilia reticulata

Previous studies on Trinidadian guppies have shown an apparent association of genetic variability, expressed as mean heterozygosity (Ho) at allozyme loci, with river order: fish from lowland sites exhibit higher mean Ho than their upland counterparts. Detailed genetic and behavioural observations were undertaken in the present study by repeated sampling throughout two river courses to examine population heterogeneity. Results show that the predation regime mediated mating behaviour of upland and lowland populations in the wild is not as divergent as previous laboratory observations have suggested. Furthermore, genetic variation at the eight loci variable in Trinidadian populations (p₁₀₀=0·13–O·88, H₀=O·03–0·23) appears to bear little association with predation regime, but is related to sub-structuring of populations by habitat features and geographic isolation. We therefore conclude that the observed patterns of genetic variability arise primarily as a consequence of stochastic as opposed to deterministic factors.     

Geographical variation in allometry in the guppy (Poecilia reticulata)

Variation in static allometry, the power relationship between character size and body size among individuals at similar developmental stages, remains poorly understood. We tested whether predation or other ecological factors could affect static allometry by comparing the allometry between the caudal fin length and the body length in adult male guppies (Poecilia reticulata) among populations from different geographical areas, exposed to different predation pressures. Neither the allometric slopes nor the allometric elevations (intercept at constant slope) changed with predation pressure. However, populations from the Northern Range in Trinidad showed allometry with similar slopes but lower intercepts than populations from the Caroni and the Oropouche drainages. Because most of these populations are exposed to predation by the prawn Macrobrachium crenulatum, we speculated that the specific selection pressures exerted by this predator generated this change in relative caudal fin size, although effects of other environmental factors could not be ruled out. This study further suggests that the allometric elevation is more variable than the allometric slope.     

Ultraviolet vision and mate choice in the guppy (Poecilia reticulata)

Ultraviolet (UV) vision is well documented for many speciesof vertebrates. UV cues are known to be used in foraging, navigationand in mate choice. We conducted a series of behavioral experimentsto investigate the role of UV perception in mate choice inboth female and male guppies (Poecilia reticulata). In ourexperiments the visual appearance of potential mates was manipulatedusing either UV transmitting (UV+) or UV blocking (UV-) filters.Female guppies significantly preferred UV+ males. Male guppiestended to prefer UV- females, but their preferences were marginallynonsignificant. Further experiments investigating the roleof luminance, indicate that UV wavelengths are probably beingused for color discrimination rather than for detecting differencesin brightness. These experiments raise the possibility thatUV is used in mate assessment in different ways by male andfemale guppies. This may reflect the different strategies thatthe two sexes have in order to maximize reproductive success.To our knowledge, these are the first data showing that UVis used by any fish species in mate selection.     

Fine structure of the storage micropocket of spermatozoa in the ovary of the guppy Poecilia reticulata

To investigate the internal fertilization of the guppy Poecilia reticulata, the present electron microscopic observations were focused on the morphology of the sperm storage site in females. In the ovary of the mature female guppy, many spermatozoa were found in a synaptic knob-shaped micropocket (SSP) as the sperm storage (probably sperm entry) site on the follicle surface which was the expanded blind alley of a small tract extending from the ovarian cavity. Oocytes in the developmental stage of oil droplet formation already showed the attachment of the terminal end of the small tract opening into the ovarian cavity. The lateral wall of the tract attaching to the follicle surface consisted of epithelial cells fast jointed with tight junctions and desmosomes. The thick lateral wall of SSP was constructed with complex epithelial cell layers, and the terminal bottom was comprised of a single layer of epithelial cells on the surface of the follicular layer, which consisted of a very thin thecal cell layer, basement membrane, and granulosa cell layer. The vitellus was enclosed by the follicular layer and thin chorion, in which the micropyle was absent. In fully-grown oocytes, the germinal vesicle containing comparative short chromosomes did not always locate in the vicinity of the storage SSP of spermatozoa.     

Dominant lethal effects of triethylenemelamine in the guppy Poecilia reticulata.

The need to screen potential chemical pollutants for mutagenicity has increased with the increasing volume of such materials being introduced into natural water. The present study demonstrates the utility of a fish, the guppy (Poecilia reticulata), as a model test system in which water-borne chemical mutagens may be assayed for dominant lethal effects. Mature male guppies were injected with three doses of triethylenemelamine (0.1, 0.2 and 0.4 mg/kg) in addition to a control sham treatment. Each male was subsequently mated to virgin females. In an alternative test, male fish were allowed to swim in triethylenemelamine solutions of known concentration for a period of 24 h prior to being mated to virgin female guppies. 10 days following matings, females were dissected, and numbers of live and dead embryos were recorded. Significant dose effects were demonstrated by analysis of variance techniques in both the injection and the emersion tests with the results showing higher percentages of dead embryos and lower total number of embryos with increasing doses of TEM.     

Age and experience affect female choice in the guppy (Poecilia reticulata)

Female choices of males, and how these choices are influenced by ecological and social factors, have been studied extensively. However, little is known about the effects of age and breeding experience on female mating decisions. We used video techniques to examine female mate choice in guppies based on the area of carotenoid (orange) pigmentation on the body. Females were presented with paired images of males, one ornamented and the other plain. Visual preference for each male was measured. Age-related changes in the criteria of choice were examined by comparing the responses of the same mature but sexually inexperienced 6-mo-old and 12-mo-old females. Effects of breeding experience on female choice were examined by comparing mate preferences of 12-mo-old female virgins with their preferences after they had mated and produced a brood. Female preferences for ornamented males with large areas of carotenoid pigment changed with age but not with mating experience. Six-month-old virgin females preferred ornamented males, whereas 12-mo-old virgin and postpartum females did not differentiate between males based on orange coloration. The results are discussed in light of life-history theory and have important implications for studies of sexual selection as well as for the design of mate-choice studies.     

Predator inspection,shoaling and foraging under predation hazard in the Trinidadian guppy,Poecilia reticulata

Synopsis Guppies,Poecilia reticulata, living in stream pools in Trinidad, West Indies, approached a potential fish predator (a cichlid fish model) in a tentative, saltatory manner, mainly as singletons or in pairs. Such behavior is referred to as predator inspection behavior. Inspectors approached the trunk and tail of the predator model more frequently, more closely and in larger groups than they approached the predator's head, which is presumably the most dangerous area around the predator. However, guppies were not observed in significantly larger shoals in the stream when the predator model was present. In a stream enclosure, guppies inspected the predator model more frequently when it was stationary compared to when it was moving, and made closer inspections to the posterior regions of the predator than to its head. Therefore, the guppies apparently regarded the predator model as a potential threat and modified their behavior accordingly when inspecting it. Guppies exhibited a lower feeding rate in the presence of the predator, suggesting a trade-off between foraging gains and safety against predation. Our results further suggest that predator inspection behavior may account for some of this reduction in foraging. These findings are discussed in the context of the benefits and costs of predator inspection behavior.