Evolution and speciation : insects as model organisms
Charles Darwin is the father of evolution as we know it today. In his book “The origin of species” he states that new species originate from ancestral species that change over time, and that the mechanism of the change is natural selection. How the variation natural selection need is generated and passed from generation to generation was solved by Gregor Mendel and Thomas Hunt Morgans (and his group). Based on experiments with pea plants, Mendel formed laws about segregation and assortment of traits and Morgans group demonstrated that Mendels hypothetical factors are specific points on the chromosome. Evolution mostly deals with how populations become adapted to their environment, but not how this adaption leads to speciation. For speciation to occur, barriers for the gene flow between populations have to evolve. There are two general modes of speciation defined by how the gene flow between populations is interrupted. In allopatric speciation a physically barrier isolates a population, whereas sympatric speciation occur within a single geographical area and reproductive isolation arises between individuals that always have the opportunity to interbreed. Insects are good models when the mechanisms underling evolution and speciation are studied, there are more than one million species and their diversity and distribution is amazing. Olfaction is the primary sense by which the environment is interpreted by insects, and olfactory cues can be important for separation of population evolving in sympatry. That was demonstrated by Löfstedt and co-workers who studied nine species of sympatric ermine moth Yponomeuta. All species had a mixture of (E)-11 and (Z)-11 tetradecenyl acetate as primary pheromone compounds, however, the females produced the compounds in a specific ratios that never overlapped if the species were not isolated by other barriers. Pheromones are well studied compared to the plant-produced odors. However, insects can detect relevant plant odors with the same selectivity and sensitivity as they detect pheromones. The number of volatiles emitted from fruit and plants is much higher than the number of components in the female pheromone, yet, Stensmyr et al. (2003) demonstrated that Drosophila melanogaster only needs a few key components to locate and detect a food source. The Rhagoletis pomonella sibling species complex is a model system for sympatric host race formation and speciation (e.g. Forbes et al., 2005; Linn et al., 2003; Linn et al., 2005b). The complex consists of several strains with different host preference – a preference based on olfactory cues. Host choice is of evolutionary significance for Rhagoletis as they mate on or near the fruit of their respective host plant. Adult flies tend to mate on or near the same species of host fruit as the one they infested as larvae. Thus, differences in host preferences can translate into mate choice and can act as pre-mating barriers to gene flow. As for other insects, gall midge behavior has been shown to be guided by olfactory cues; they use pheromones when locating a suitable mate and plant volatiles for host plant recognition. Thus, host plant volatiles might be important when gall midges shift between hosts and subsequently in the formation of new gall midge species. In my thesis I will study possible evolutionary mechanisms behind the great diversity of the gall midges. The two main questions I will address in my thesis are: do gall midges associated with the same host plant use the same or a similar set of odors to identify it? And, conversely, do closely related species that have different host plant requirements respond to odors common for the different plants?
Introductory Paper at the Faculty of Landscape Planning, Horticulture and Agricultural Science
Publisher: Faculty of Landscape Planning, Horticulture and Agricultural Science, Swedish University of Agricultural Sciences
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