Phylogeny and evolutionary biology of Platycheirus (Diptera: Syrphidae)

The goal of this project is to address the phylogeny and evolutionary biology for one of the most diverse and species-rich genera of flower flies (Diptera: Syrphidae), Platycheirus LePeletier & Serville. Although studied taxonomically for a long time, the phylogenetic relationships of its species groups, subgroups and species are not well understood; this is especially the case for large genera of hoverflies. The COI gene is widely used to clarify phylogenetic relationships among taxa. A fragment of 658 base pairs of COI was analysed for 74 species, 72 Platycheirus and two outgroups (both Melanostoma spp.). The monophyly of the genus Platycheirus was well supported. However, phylogenetic relationships of the Platycheirus species groups and subgroups was still not completely clarified. We used phylogenetic comparative methods to study the evolution of quantitative traits of Platycheirus. First, we estimate the level of phylogenetic signal in both reproductive and morphological traits using Pagel's λ and Blomberg's K. We found that male reproductive traits tend to evolve rapidly and divergently with low phylogenetic signal relative to morphological traits, while most female reproductive traits do not. Second, we quantify the rate of evolution of six morphological and seven internal reproductive traits of species within seven other genera of Syrphidae using their estimated phylogenetic trees. Using a likelihood-based approach, the evolutionary rate estimates show clearly that the reproductive traits evolve about 110 times more rapidly than all morphological traits except proboscis length; the latter exhibits a faster rate of evolution than other morphological traits, faster than some reproductive traits. These results provide quantitative evidence that male reproductive traits are less constrained phylogenetically, perhaps because they are under strong sexual selection. Third, the activity rhythms and foraging behaviours of Platycheirus species in relation to environmental conditions was studied in an evolutionary context. The foraging behaviour for some of the Platycheirus and Melanostoma species seen visiting flowers for nectar or pollen was recorded. Although tested with too few data, the results for both Pagel's and Blomberg's methods suggest that this behavioural trait is not constrained phylogenetically. Further, there was detectable phylogenetic signal in neither Pagel's λ nor Blomberg's K in indices of the responses of species to temperature. This indicates the greater lability of behavioural traits in contrast to morphological and reproductive characters. Finally, we used phylogenetic multivariate techniques to elucidate the main patterns of morphological evolution and their association with the evolution of reproductive traits among species of Platycheirus. Major evolved differences between species appear to lie primarily in relative proboscis length, and secondarily in relative wing size. The number of mature eggs stored in one ovary was positively correlated with relative proboscis length. There was a trade-off between ovariole number and egg size in females, but this was not associated with morphology except the width of the abdomen. Male traits showed a trade-off between testis length and the size of the accessory glands, but again this was not associated with morphology.