Contrary to traditional belief, the decisions that females make before, during and after mating shape its outcome and ultimately fitness. The aim of this thesis was to examine how females modify and adjust their mating decisions in line with social and environmental variability and how these directly and indirectly affect mating benefits. To address this aim I have formulated four main questions that correspond to chapters 2 to 6 in this thesis. Firstly I asked whether there was evidence for female choice being driven by mating benefits. More precisely, in chapter 2, I reviewed the literature in search of evidence for direct and indirect benefits in female choice among freshwater fish species. Direct mating benefits were defined as an increase in female’s reproductive success (number of offspring). Conversely, increases in offspring reproductive success were considered to be indirect benefits. The results showed that despite the multiple suggestions and the great amount of information available, to date there is still no evidence for both direct (increase of F1) or indirect (increase in F₂) mating benefits, nor their influence in female mating decisions if freshwater fishes species. Furthermore, although polyandry occurred in more than 60% of the species reviewed, I was unable to confirm that polyandry was maintained because of indirect benefits. These findings justified the need to experimentally investigate the drivers of female mating decisions in freshwater fish species and lead to the questions addressed on chapters 5 and 6. For the experimental chapters 3-6 I used the Trinidadian guppy, Poecilia reticulata, as a model species. Their unique and remarkable ecological and biological characteristics coupled with easy maintenance in laboratory conditions make guppies an ideal species for behavioural studies. But in particular, the fact that guppies live in a promiscuous mating system where females despite being constantly harassed by males may be able to control paternity, makes guppies the ideal species to address my aims. The second question was how much were females in control of their mating decisions, and how social environment could limit these decisions. In particular, in chapter 3 I looked at how females adapt and adjust their mating decisions in line with extreme differences in population sex ratio. In chapter 4, I examined to what extent male sexual harassment affects female reproductive behaviours. Results from these two chapters indicated that female guppies have a remarkable reproductive plasticity that enable them to control their mating decisions. When faced with extreme differences in population sex ratio, female mating decisions were made in an optimal way that maximized the ratio of female reproductive benefits per investment. This translated into producing bigger offspring when in a strong female biased environment, than when in a male biased environment. Further, chapter 4 illustrated that female guppies can, despite high levels of male sexual harassment, be in control of their mating decisions. These two chapters demonstrated and have reinforced previous findings of the remarkable reproductive adaptation of female guppies to differences in the social environment. The third question I addressed was: do multiply mated females have greater direct or indirect benefits than single mated females? To answer this question I followed for the first time reproductive success of females over two generations. I measured fitness directly (number of F₁ and F₂) and took as well as multiple indirect measures of fitness components for two generations in search for evidence of direct and indirect mating benefits in explaining the maintenance of female multiple mating. The results of chapter 5 revealed that female guppies do not have a higher number of F₁ and F₂ from polyandry or either from mating with males possessing allegedly good quality traits. I, therefore, stressed the idea that potential differences in sexual selection pressure between laboratory and wild populations may influence the expression and intensity of mating benefits between thus explaining the difficulty of finding mating benefits. In my fourth and last question, I used a novel statistical approach based on the analysis of the dispersion in phenotypes, to look for potential alternative explanations for the prevalence of polyandry. The results of this analysis show offspring from multiple mated mothers were phenotypically more diverse than offspring from single mated mothers. Given the direct relationship between phenotypic diversity and potential fitness gains in stochastic systems, female guppies are likely to get greater benefits from mating with males with different phenotypes than with males with a particular sexual trait. This result provides an alternative explanation for the maintenance of polyandry in resource free systems. Overall the results of this thesis reinforce previous suggestions that female guppies are active participants in the mating process, and not necessarily limited to post-copulatory mechanisms of selection of sperm. It also showed the remarkable ability of females to adjust their reproductive investment in line with changes in the social conditions. Interestingly, my results contradict the commonly accepted assertion that females’ mating preference converges towards unique male sexual traits. This result stresses the need to look at alternative explanations to justify female mating decisions.

The invasion of individuals into new habitats can pose a major threat to native species and to biodiversity itself. However, the consequences of invasions for native populations that are not fully reproductively isolated from their invaders are not yet well explored. Here I chose the Trinidadian guppy, Poecilia reticulata, to investigate how different population traits shaped the outcome of Haskins's introduction, a well-documented invasion of Guanapo river guppies into the Turure river. I especially concentrated on the importance of behaviour for invasive success. I investigated if the spread of Guanapo guppies is due to superiority in behaviour, life-history and/or genetics, or if the outcome of this translocation is due to chance. Despite the fact that by today the invasive front has passed the Turure's confluence with the River Quare many kilometres downstream of the introduction site, and the original genotype only survives in small percentages, as was revealed by genetic analysis in this and other studies, no obvious differences between invasive and native populations could be detected in any of the tested behavioural, life-history and genetic traits. When tested for mate choice, neither Guanapo nor Oropuche (Turure) males seemed to be able to distinguish between the population origin of females, but courted and mated at random. At the same time, females did not prefer to school with individuals of the same population over schooling with more distantly related females. The formation of mixed schools after an invasive event is therefore likely. Because female guppies showed a very low willingness to mate, even after having been separated from males for up to six months, sperm transfer through forced copulations will become more important. Taken together, these behaviours could increase the speed of population mixing after an invasion without the need for behavioural superiority of the invasive population. When tested for their schooling abilities, offspring of mixed parentage, in contrast to pure breds, displayed a large amount of variety in the time they spent schooling, a circumstance that can potentially influence survival rates and therefore the direction of gene pool mixing. Guanapo fish did not show reproductive superiority in a mesocosm experiment, where both populations were mixed in different proportions. On the contrary, in two out of three mixed treatments, the amount of Oropuche (Turure) alleles was significantly higher than expected from the proportion of initially stocked fish. The almost complete absence of distinguishable traits other than genetic variation between the examined populations that belong to different drainage systems, opposes the recent split of the guppy into two different species following drainage system borders, as is argued in this thesis. However, the successful invasion of the Turure by Guanapo guppies and the nearly entire disappearance of the original population can be explained in absence of differing population traits. Here I demonstrate how behavioural and genetic interactions between subspecies influence the outcome of biological invasions and second, how factors other than population traits, such as the geographic situation, can produce an advantageous situation for the invader even in the absence of population differences.

This thesis focuses on the guppy, Poecilia reticulata, as an invasive species. Its non-native distribution, the biology behind its success and the reasons for its introduction are examined. A worldwide email survey revealed that the guppy is established in at least 73 countries outside of its native range and that mosquito control schemes and the release of unwanted aquarium fish are the two primary routes of introduction. Knowledge gaps were identified; primarily the scarcity of scientific evidence for negative impacts of guppy introductions and similarly for mosquito control efficacy. Replicated mesocosm experiments demonstrated that female guppies are capable of routinely establishing populations, and that these retain behavioural viability over several generations. The first mesocosm study suggested that founders with very different evolutionary histories were equally good at establishing populations. The second mesocosm study suggested that monandrous females were extremely successful at establishing behaviourally viable populations, with no decline in behavioural variation. The effectiveness of guppies as mosquito control agents was examined in two related foraging experiments. The first study found little evidence for the presence of ‘prey switching’ in guppies, questioning the validity of previous work advocating their introduction to stabilise prey populations. The second study revealed a preference for non-vector mosquito larvae in a two-prey system. However, both mosquito species were consumed equally readily when habitat complexity increased. The presence of conspecifics affected female foraging behaviour. The presence of males reduced the strength of prey preference in the first study, and the presence of conspecifics of either sex removed prey preference in the second. Both demonstrate that multi-prey systems have important implications for the efficacy of poeciliids in biological control. Despite severe demographic bottlenecks, their adaptability and ability to rapidly increase in numbers enable guppies to establish and persist when introduced. Such bottlenecks are typical of introduction scenarios, warning that particular caution should be exerted when introducing this species, or other livebearing fish, to natural water bodies.

Variegated patterns on the caudal fin are a common and popular trait in guppy strains commercially cultured in Singapore. Gene control of this highly variable mosaic pattern of black spots and patches of different shapes and sizes on a brightly colored tail fin was elucidated by reciprocal crosses between the Green Variegated (GV) strain and wild-type (WT) stock. F₁ progenies were produced by single-pair crossing between GV and WT, while the F₂ generation was obtained from full-sib mating between F₁ males and F₁ females. Data for the F₁ and F₂ generations were segregated according to phenotypes and sex, and tested by chi-square analyses. Inheritance of variegated tail patterns appears to be determined by a single locus on the X- and Y-chromosomes. Genotypes of males and females of the GV strain are proposed to be X_VarY_Var and X_VarX_Var, respectively. The allele for variegated tail patterning, Var, is dominant over that of the wild-type, Var+, which does not exhibit these patterns. Recombination frequency between the Var locus and sexdetermining region (SdR) in male guppies was estimated to be about 1.9% (map distance ≈1.9 map units). The segregation and mode of inheritance of the Var gene are illustrated by genetic models

The present letter is a response to the paper „Interallelic interaction between the autosomal Blond and the sex-linked Nigrocaudatus gene in the guppy (Poecilia reticulata)” (Petrescu-Mag et al 2007).
Key Words: Guppy, Poecilia reticulata, Gold, Blond, Nigrocaudatus

The guppy has the major classes of chromatophores found in teleosts: xantho-erythrophores, iridophores and melanophores. These color cells render the adult body essentially opaque. A transparent guppy was developed, the See-thru guppy, combining alleles that genetically remove the major classes of guppy color cells: xantho-erythrophores, iridophores, melanophores and leucophores. The See-thru guppy may have interesting applications for scientists already using the guppy as a model organism. In our own research into the inheritance of color patterns in the guppy, the See-thru guppy may be useful for exploring the specificity of mutations to color cell types.
Key Words: transparent guppy, Albino Blau, Glass Belly Panda.

Poecilia obscura, new species, is described from the Oropuche system, Trinidad. A mitochondrial DNA-sequence based molecular phylogenetic analysis revealed the status of the new species as a separate taxon. It is most closely related to the Common guppy, P. reticulata and to the recently described species, P. wingei. It can also be distinguished by morphometrics and gonopodial characteristics from these two species, although the ranges for all values overlap. A definition of the new species on morphology criteria alone is thus impossible. Therefore, P. obscura forms a cryptic species complex with the two other species. P. winge i is now unequivocally defined by the molecular phylogeny as a valid species. The three guppy species are included in the subgenus Acanthophacelus Eigenmann (1907), which is considered as generically different from all other taxa of the Poeciliinae sensu Parenti (1981).
Key words: molecular phylogeny, sexual dimorphism, freshwater fish, artificial introduction, hybridization, cryptic species complex