Judith Mank

The mechanisms driving colour variation in organisms have long fascinated scientists. The occurrence of novel phenotypes through random mutations and other factors has always opened new avenues of research to understand the genetic basis and inheritance patterns of these phenotypes. Studying colour asymmetry is an additional way to account for the diverse colour patterns present in nature. In this thesis, I present an investigation into the inheritance pattern and genetic basis of pigmentation in two related Poeciliid species: (Poecilia wingei and Poecilia picta). Additionally, I conducted an analysis of asymmetry in a third related Poeciliid species, the guppy (Poecilia reticulata). The study on P. wingei aimed to understand the heritability and genetic basis of a mutation affecting melanin pigment production. For P. picta, the focus was on understanding the inheritance pattern and genetic basis of a red phenotype referred to as 'blush'. Breeding experiments were conducted, for both P. picta and P. wingei. Subsequently, genotyping of all offspring was performed to determine the inheritance pattern of the mutation in P. wingei and the inheritance pattern of the blush phenotype in P. picta. The findings revealed that the mutation in P. wingei was autosomal recessive, and the blush phenotype in P. picta was not Y-linked, similar to a previous study. Next, an analysis of colour asymmetry was conducted in P. reticulata artificially selected for a high and low proportion of orange, aiming to better understand colour asymmetry present in orange and black guppy ornaments. Fluctuating asymmetry was found to be the main form of asymmetry present in the selection lines, and orange ornaments were more symmetric than black. Furthermore, I figured out that FA in orange ornaments were weakly correlated with FA in black ornaments, indicating that the traits are differentially sensitive to developmental instability. Moreover, the investigation determined the presence of a sizable, albeit insignificant, effect of FA in orange. Finally, heritability calculations for FA in orange and black ornaments were revealed to be quite low. Overall, this thesis contributes valuable insights into the genetic basis, inheritance patterns, and asymmetry in colour pigmentation in some Poeciliid species.

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Selection for increased fitness often results in changes to an organism’s behavioural repertoire, and sexual behaviour can be modified in a plethora of ways. Here, I used the Trinidadian guppy, Poecilia reticulata, as a model study species to investigate two different aspects of male sexual behaviour. First, I examined how social environment affects male mate choice. I evaluated whether male guppies with previous social experience of female receptivity cues learn to prefer and adjust their behavioural repertoire towards females with higher receptiveness levels, as this represents an optimal use of time and energy and is more likely to result in insemination. While males that previously experienced receptivity cues did not change the strength of preference towards receptive females (results from dichotomous choice test), these males adapted their mating tactic compared to naïve males (results from no choice tests). This change in mating tactics but lack of preference towards receptive females suggests that although learning from previous experience is important, it might be weaker than predicted in this species. Next, I tested the association between two crucial factors driving sexual selection in this species, male colouration and male mating behaviour. To experimentally test the causal link between these two factors, I used guppies artificially selected for a high or low proportion of orange and quantified their mating tactics. I found that male guppies with greater orange colour increased their mating effort in sexual behaviours directed to court females for insemination. Additionally, males with greater orange colour increased their mating effort in coercive mating tactics. My results suggest that this sexually-selected signal is directly correlated with overall sexual effort. These results support the idea that orange is associated with greater male vigour through higher overall levels of sexual behaviour.

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