Temporal and spatial patterns of orchid mycorrhizal fungi in forest and grassland ecosystems

Abstract

Üks liigirikkamaid õistaimede sugukondi on orhideelised ehk käpalised, mille liigid moodustavad tihedaid seoseid nii putuktolmeldajate kui ka seenjuurt moodustavate seentega. Orhideede mükoriisa erineb teistest mükoriisatüüpidest nii anatoomiliselt, seentaksonite poolest kui ka funktsionaalselt. Käpalised on seotud mükoriisa seentega alates seemnete idanemise faasist, kuna nende seemned on tolmpeened ning ilma idanemiseks piisavalt vajalike toitaineteta. Samas jäävad mükoriisa seened mängima olulist rolli ka hiljem, kui orhideedel arenevad rohelised lehed ja fotosünteesivõime. Arvatavasti aga muutub seensümbiotide esinemine ning kooslus täiskasvanud orhideede juurtes vegetatsiooniperioodi vältel. Tavaliselt on rohelised, s.o fotosünteesivad orhideed seotud mullas leiduvate saproobidega sugukondadest Tulasnellaceae ja Ceratobasidiaceae ning seltsist Sebacinales. Nimetatud saproobide ökoloogiat ning esinemist mullas on seni vähe uuritud. Antud doktoritöö käigus uuriti fotosünteesivate orhideeliikide mükoriisa seenekoosluste ajalisi ja ruumilisi mustreid. Töö eesmärgiks oli välja selgitada, 1) kuidas muutuvad ühe vegetatsiooniperioodi vältel täiskasvanud orhideedega seotud seensümbiontide kooslused, 2) kas seensümbiontide kooslused on mõjutatud kasvukohatüübist, 3) kas seensümbiontide rohkus on mõjutatud orhidee lähedusest ning 4) millised on nende seenekoosluste ruumilised mustrid. Lisaks oli eesmärgiks arendada seente globaalsel geenijärjestustel põhineva määraja UNITE referentsandmebaasi. Doktoritöö peamisteks tulemusteks on, et orhideede mükoriisaseente kooslused on esmalt mõjutatud orhideeliigist ning seejärel kasvukeskkonnast ning orhidee arenguetapist. Loopealsetel on orhideede mükoriisaseened mullas levinud juhuslikult ning üldjuhul ei mõjuta nende seente arvukust mullas kaugus peremeestaimest. Andmebaasi täiendamisel leiti, et avalikes andmebaasides esineb mitmeid orhideede mükoriisaseente DNA järjestusi, mis on ebakvaliteetsed ning mille kohta puuduvad metaandmed.

Orchids represent one of the largest plant families and display tight associations with pollinators and mycorrhizal fungi. Orchid mycorrhizal (OrM) symbiosis is unique anatomically, taxonomically as well as functionally. Mycorrhizal fungi play a vital role for the orchids during their germination by providing essential nutrients to the dust-like orchid seeds that are almost devoid of their own food reserves. Even after orchids have developed green leaves and start photosynthesizing, their roots remain colonized by mycorrhizal fungi. However, the abundance of fungal colonization as well as the fungal community composition presumably change in the roots of the mature orchids during the vegetation period. Typically, photosynthetic, autotrophic orchids associate with saprotrophic fungi from the Ceratobasidiaceae, Tulasnellaceae and Sebacinales. So far, the distribution and ecological requirements of these fungi remain poorly explored. The aim of this thesis was therefore to study the temporal and spatial patterns of orchid mycorrhizal fungi in forest and grassland ecosystems. The thesis was particularly focused on 1) how the community composition of the putative OrM fungal taxa changes over the vegetation period in relation to the developmental phases of the host orchid species, 2) how the community composition of the putative OrM fungal taxa varies across different habitats as well as within the same habitat and on 3) how the richness of putative OrM fungi changes with the increasing distance from the orchid patches and 4) whether the putative OrM fungi form spatial patterns along the distance from the orchid patches. In addition, we extended annotations of publicly available OrM fungal ITS sequences. The main results of the thesis can be summarized as follows: 1) the community composition of the OrM fungi in roots was primarily host dependent and only secondarily affected by habitat and even to a lesser degree by the developmental phase and 2) in semi-natural grasslands, OrM fungi were randomly distributed and showed little evidence of a distance-dependent decline away from the adult orchids. We detected a number of insufficiently annotated OrM fungal sequences deposited in public databases, as well as sequences which suffered from low read quality and/or chimeras.