(2009) Grillenberger, Bernd Klaus
To gain a thorough understanding of the evolutionary processes that lead to the enormous biodiversity in nature, evolutionary biology tries to develop models that describe the essence of the underlying process. These models are getting increasingly complex and are relying on a growing set of assumptions. To be able to gather enough knowledge on a biological system to accommodate and validate a model, evolutionary biologists rely on the study of model organisms. The jewel wasps of the genus Nasonia has become a model organism in evolutionary biology for a variety of behavioural, developmental and genetic studies. A particular fruitful field of research has been sex allocation, which is one of the best understood adaptive behaviours.
The genus Nasonia consists of three closely related species: N. vitripennis which is cosmopolitan, N. giraulti which has only been found in north-eastern North America and N. longicornis which is endemic to north-western North America. As N. vitripennis can be found in sympatry as well as in allopatry with both its sister species and its distribution encompasses multiple environmental conditions, the system offers unique opportunities to study the validity of models on speciation and adaptation. The ability of Nasonia to influence its offspring’s sex ratio and to induce diapause in its offspring under unfavourable conditions, are two adaptive traits that are in particular being considered in this study. (Chapter 1)
The goal of the research described in this thesis was to evaluate the underlying assumptions of models describing adaptive behaviour (sex allocation and diapause). I aimed to acquire information on the adequacy of adaptation in a natural environment and to gain a better understanding of the multiple selective forces that shape life history traits. The results of this study will help to place the many results of theoretical and laboratory studies into the biological reality of a natural environment to identify remaining questions about parasitoid life history evolution.
Nasonia has been found to follow quite closely the predictions of local mate competition (LMC) theory in the laboratory. LMC theory predicts that an organism should produce a strongly female biased sex ratio, when its offspring is competing for mates among each other. However, LMC theory depends on the validity of many assumptions about the population genetic structure, and it is unknown to what extent these are met in nature. In Chapter 2 I evaluated some essential assumptions made in sex ratio theory on data from two European N. vitripennis field populations. In particular I investigated the genetic population structure, foundress number per patch, parasitation sequence and clutch sizes. I found that most assumptions made in recent LMC theory are fulfilled by N. vitripennis: local mating, random dispersal of females and asynchronous parasitism. My research showed that other assumptions of more basic models, such as equal clutch sizes, random mating among offspring within patches and synchronous parasitism are clearly violated and therefore rightfully adjusted in the recent theory.
Subsequently, I investigated in Chapter 3 to what extent the predictions made by LMC theory are matching the sex ratios observed in the European populations and what the strengths and weaknesses of present models are. I showed that some factors included in these models are less relevant than thought before (e.g. the total number of females parasitizing a patch), whereas other factors play a more important role (e.g. the relative clutch size of a parasitizing female compared to earlier females). The general message of this chapter is that females are limited by the cues they can obtain from their environment, and these can be different from what the researcher expects. A female might not have total information on what has happened and will happen in a patch that she is going to parasitize. The limited information poses boundaries to the adaptive response of the individual.
Whereas in Europe N. vitripennis has no closely related competitors, the situation in North America is more complex. Given that N. vitripennis and N. giraulti are living in close sympatry in parts of North America, the question arises whether traits have evolved to avoid hybridization between the species. While it has been found that there are clear differences in courtship behaviour, it is still unknown whether there are also adaptations with respect to LMC. In Chapter 4 I investigated the reproductive strategies of Nasonia in a two species situation, regarding the sex ratio adjustment as well as diapause production, focusing on how well N. vitripennis is adapted to the competition with its close relative N. giraulti. I found that N. vitripennis does not adjust its sex ratio to conspecifics only, but responds to host parasitation by N. giraulti as if encountering conspecific clutches. This indicates that the two species have not diverged far enough yet in so far that a female is able to recognize the parasitation event of the competitor as different from conspecific competitors. Given that N. vitripennis has been found to recognize further diverged species’ eggs as different, a foundress may in principle be capable of differentiating between hetero- and conspecific clutches. Furthermore, I found a higher level of diapause production of N. vitripennis in North America compared to European populations, and to N. giraulti in the same location. This can either indicate that there are species specific differences in the factors, which play a role in a foundress’ decision to produce diapause, or it can be interpreted as an imprecise adaptation to the local environment.
In order to evaluate how far adaptation of N. vitripennis to the competitive situation in North America might have progressed, information is required on the population history of Nasonia in North America. So far, it has been assumed that the cosmopolitan species N. vitripennis has its origin in North America, as that seems to be the hot spot of diversity within the genus Nasonia. In Chapter 5 I tested the hypotheses whether N. vitripennis originates from North America, or from outside the New World. Using a combination of mtDNA sequences, nuclear microsatellites and Wolbachia sequences, I compared the genetic variability among North American and European samples. I found evidence that the North American N. vitripennis population is much younger than the European population, which places the species origin into the New World. This raises the question whether there was sympatric speciation of all three species in North America, or whether only N. longicornis and N. giraulti evolved in North America and N. vitripennis is of Eurasian origin and invaded the New World more recently. So far, the differences in Wolbachia infections among the three Nasonia species were thought to be the driving force for the speciation process. However, as the distribution ranges of N. longicornis and N. giraulti do not overlap, and given that N. vitripennis has its origin outside of North America, it is conceivable that the differences in Wolbachia infection are not cause, but consequence of the speciation.
A prerequisite of local adaptation is that there is only limited gene flow between areas with and without selection pressure on the adaptive trait. N. vitripennis might have evolved adaptations to the presence of its sister species N. giraulti and N. longicornis in North America. However, as there are large areas in North America where N. vitripennis occurs allopatrically, without selection for competition with a close relative, the question arises whether gene flow can prevent local adaptation in the sympatric areas. In this context I investigated in Chapter 6 the dispersal capabilities of N. vitripennis on a local scale with a mark release recapture experiment as well as on a larger scale with molecular markers. I found that N. vitripennis is a long distance disperser that can easily cover more than 2 km, and that populations as far apart as 100 km are still hardly differentiated. This provides an explanation for why previous studies did not find differentiation on a smaller scale, and helps to determine what a population is in Nasonia. The high level of admixture of a large population might counteract selection for local adaptation, as selected and unselected sub-populations constantly exchange genetic material.
In the final Chapter 7 I merged the results of the previous chapters and sketched the current knowledge of the population structure and history of N. vitripennis in Europe and North America. The results of the previous chapters made me believe that the most probable hypothesis of speciation in the genus Nasonia is the following: the two sister species N. giraulti and N. longicornis developed independent from N. vitripennis in North America, while the latter is an Eurasian species that spread to the New World more recently. The consequence of this rather recent event is the imprecise adaptation towards the local climate (diapause production) and towards the presence of a close relative in the habitat (sex ratio adaptation). The high gene flow over large distances and the presumably reduced adaptive potential after the founder event prevented a rapid adaptation.
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