Conservation genetics of pollinatorssituationof the Bumblebee Species Bombus terrestris in the Iberian Peninsula

Abstract

The common bumblebee Bombus terrestris is an insect species currently used worldwide for the pollination of crops such as tomatoes, melons and strawberries. However, despite having a positive effect on agriculture, it has become a threat to biodiversity in different regions due to the interactions (competition for floral resources or nesting sites, transmission of pathogens, etc.) it establishes with local populations. The endemic subspecies Bombus terrestris lusitanicus inhabits the Iberian Peninsula. Its distribution range overlaps with Bombus terrestris terrestris (one of the main subspecies used commercially) in the north in a natural way and in the south since the establishment of commercial populations since the 90s. The experiments developed in this Thesis aim to analyse the state of genetic diversity of bumblebee populations in the Iberian Peninsula and, specifically, the possible effects that commercial populations have on wild Iberian populations of B. terrestris, for which it was first necessary to elucidate the molecular differences existing between the two subspecies. The first chapter validated the use of a series of nuclear markers (microsatellites) commonly used for the study of the genetic diversity of populations, in different species of the genus Bombus that inhabit the Iberian Peninsula, confirming their usefulness for some very little studied species of the genus. In the second chapter, these microsatellites were used together with two other nuclear markers (ArgK and EF) and a mitochondrial one (16S) in search of a marker that could differentiate the subspecies molecularly. The haplotypes detected in the 16S fragment allowed for the differentiation of Iberian individuals from central Europe (France, Belgium), while no clear differences were found in the nuclear markers. In the third chapter, with the aim of exploiting the molecular differences found and designing an economical and rapid technique to differentiate the Iberian populations, the mitochondrial genomes of the two subspecies were sequenced. From the variable positions found when comparing the sequence of the two mitogenomes, four potentially sub-specific markers were designed, one of which (Bter_nad2) was validated to be used in restriction fragment length polymorphism tests (RFLP). The markers were tested in Iberian and Central European populations, revealing not only signs of hybridization and introgression in the south of the Iberian Peninsula, but also an expansion of B. t. lusitancius towards the north. In the last chapter the tools validated throughout this thesis were used to study the spatial and temporal patterns of genetic diversity in Iberian B. terrestris populations. The results obtained from the study of mitochondrial haplotypes and allelic frequencies of nuclear microsatellites in Iberian populations were compared with those obtained from populations in the Pyrenees, France and Belgium, as well as a temporal reference population, sampled prior to the introduction of the commercial bumblebee in the territory. The results show the expansion of the Central European haplotype not only in the south but also in the whole peninsula, evidencing the hybridization and introgression of the commercial and Iberian populations, although without clear effects on nuclear genetic diversity. This thesis confirms that the naturalization of commercial B. terrestris in the Iberian Peninsula is affecting wild populations, which may have unknown long-term consequences. Current legislation on the management and export of commercial bumblebees needs to be improved in order to preserve endemic Iberian bumblebee populations.