Determinación de las relaciones filogenéticas y tróficas de la acarofauna en cítricos

Abstract

Tetranychus urticae Koch and Panonychus citri (McGregor) (Trombidiformes: Tetranychidae) are important mite pests in the Mediterranean citrus agroecosystem in clementine mandarins and oranges, respectively. These species are part of a mite complex including also Phytoseiidae predatory mites (Mesostigmata). Phylogeny of Acari is controversial and ambiguous. Classical identification and taxonomy are limited by the number of available characters that difficult the establishment of their phylogeny. Molecular systematics provides a greater number of characters, mostly based on the sequence analysis of nuclear and mitochondrial DNA, which may be key to determine these phylogenetic relationships. The non-coding intergenic regions of the nuclear ribosomal DNA (ITS, internal transcribed spacers) and mitochondrial gene cytochrome oxidase I (COI) have been successfully used for this purpose. Thus, our first study has been carried out to clarify the phylogeny of tetranychid and phytoseiid mites occurring in Spanish citrus orchards and their relationship to other mite clades using these markers. Our results show that both ITS and COI markers are useful to clarify the mite phylogeny and confirm their monophyletic origin. The taxa Ixodida, Mesostigmata, Trombidiformes and Sarcoptiformes are also monophyletic. The first taxon splitting from Acari was Ixodida, followed by Sarcoptiformes and Trombidiformes, and finally Mesostigmata. The COI gene enabled the separation of mite species at the nucleotide level. ITS regions had a greater divergence in size and composition of the sequences, which made them useful for species differentiation, but not for phylogeographic relationships in this study. The comparison of Acari phylogenetic differentiation and lifestyle suggests that the ancestral feeding system was detritivorous and subsequently led to herbivory and entomophagy. The feeding preferences of an arthropod predatory guild are diverse and determine their trophic relationships. Phytoseiidae species occurring in Spanish citrus orchards, exhibit the different lifestyles described for this family. However, the regulatory role that these species may play on citrus spider mite pests remains unknown. The small size of these mites precludes the direct observation of predation events in the field. Thus, gut prey DNA analysis is presented as a valuable tool to investigate their trophic relationships. In this thesis, we have developed a multiplex PCR for identification of six phytoseiid species (Euseius stipulatus (Athias-Henriot), Phytoseiulus persimilis Athias-Henriot, Neoseiulus californicus (McGregor), Neoseiulus barkeri Hughes, Typhlodromus (Typhlodromus) phialatus Athias- Henriot and Amblyseius swirskii Athias-Henriot), and the identification and detection of two of their potential prey (T. urticae and P. citri). Prey DNA degradation rate in the gut of three of the most important species of phytoseiids (E. stipulatus, N. californicus and P. persimilis) were predator and prey dependent. These results, together with the actual abundance of each phytoseiid species, allowed us to estimate their relative impact on the two tetranychid species. This molecular technique highlighted the importance of the tetranychid specialist predator P. persimilis in field conditions, although it was the less abundant phytoseiid species. Contrarily, the importance of the generalist E. stipulatus should be attributed to its abundance rather than to its predatory capacity. Additionally, the molecular technique developed in this thesis represents a valid alternative to the classical taxonomy to identify Acari to species level. The predatory behavior of arthropods varies throughout the day and season. There is abundant information about predatory patterns of different Arachnida but those of Phytoseiidae are poorly known. In this thesis the daily predatory pattern of the Phytoseiidae E. stipulatus, N. californicus and P. persimilis when exposed to the prey T. urticae was studied in summer and winter under laboratory conditions. This is a prerequisite for maximizing the effectiveness of any field sampling aimed at quantifying mite predation, such as the molecular one described above. Our results indicated that the predatory behavior of phytoseiids specialized in tetranychids (P. persimilis and N. californicus) varies depending on the season and daylength. These phytoseiids showed non-overlapping maximum predation peaks that can contribute to their coexistence in the citrus agroecosystem. Contrarily, the predatory behavior of the generalist E. stipulatus did not show any clear diel and seasonal pattern. Therefore this phytoseiid could exert an increased intraguild pressure on both P. persimilis and N. californicus. These different predatory patterns can have serious implications for the design of effective laboratory assays and field sampling programs. Furthermore, our results provide new information on phytoseiid behavior.