A comprehensive checklist of Estonian myxomycetes

The history of the myxomycetes research in Estonia dates back to the middle of the 19th century, with first data occurring in the H. A. Dietrich’s book published in 1856. The current work summarizes all the published reports of Estonian myxomycetes as well as some unpublished data and herbaria revisions. After the assessment of the taxonomic status of published records and bringing in line with currently accepted taxonomy, we present the updated checklist of the myxomycetes of Estonia, comprising 150 species representing 39 genera. Eleven species were excluded from the list as doubtful.


Introduction
Myxomycetes are the group of mostly terrestrial protists of the kingdom Amoebozoa (Adl et al., 2019) possessing a peculiar life cycle. The morphological structure of their fruiting bodies for a long time served as the first and foremost means for species delimitation before the onset of molecular techniques. The resemblance of myxomycetes fruiting bodies to fungal ones is the reason why, despite the protistean nature of myxomycetes, traditionally they were studied by mycologists.
First data on Estonian myxomycetes were reported in the 19th century by Heinrich August Dietrich, the well-known horticulturist, botanist and mycologist of saxonian origin, who spent most of his life in Estonia. He collected actively in 1850s in the Western and Northern parts of the modern Estonia and his collections resulted in nine volumes of exsiccate Centuria Plantarum Florae Balticae cryptogamarum (I-IX) and the subsequent two-volume monograph "Blicke in die Cryptogamenwelt der Ostseeprovinzen" (Dietrich 1856(Dietrich , 1859. In this book he listed 46 species and varieties of myxomycetes. All but one volume of Centuria exsiccates have been preserved in at least one copy and are kept in Estonian museums. Eleven specimens of myxomycetes, found in Centurias II, IV, V, VII and VIII, were recently revised (Pärtel et al. 2020).
The other historical collection of the 19 th century has been recently rediscovered and attributed to the bryologist and teacher Gustav Carl Girgensohn. Upon revision, the collection appeared to include 11 specimens (two with original identifications) that belonged to 6 species of Myxogastrea (Pärtel et al. 2020).
In the present paper, we aim to update and summarize knowledge about the species diversity of Estonian myxomycetes taking into account new collection data and the taxonomic treatments updated since the publication of the last checklist of Estonian myxomycetes.

Material and methods
The checklist was compiled based on all available published myxomycete data till the present day and also includes unpublished material kept in Estonian herbaria: TAAM -Herbarium of the Department of Mycology in the Institute of Agricultural and Environmental Sciences of the Estonian University of Life Sciences; EAA -herbarium of the Estonian University of Life Sciences; TU -herbarium of the University of Tartu. Published sources that were used are listed in the References. Specimens from historical H.A. Dietrich's exsiccates and G.C. Girgensohn's collection (22 in total) were recently revised using the conventional techniques of stereoscopic dissecting microscopy and light microscopy ( following the methods described in Nannenga-Bremekamp (1991)).
Nomenclature is updated following Lado (2005Lado ( -2020. Names of authors are abbreviated according to Brummit & Powell (1992). In the list currently accepted species name is followed by cited synonyms, if there were any, and a reference where the species was mentioned for the first time. In cases when species are represented only by unpublished herbarium specimen(s), herbarium number(s) is/are provided. Collection data for unpublished specimens are available via the PlutoF platform by the following DOI: https://dx.doi.org/10.15156/BIO/807449 (Abarenkov et al. 2010).

Results
The summarized checklist of myxomycetes of Estonia includes 150 species belonging to 39 genera of Myxomycetes s. l., including 149 species of Myxomycetes G. Winter sensu Leontyev et al., 2019, and one species of Ceratiomyxomycetes D. Hawksw., B. Sutton & Ainsw. Eleven species are reported as doubtful/invalid taxa or were excluded from the checklist after the specimen revision. Some of the noteworthy species are shown in Fig. 1.
Physarum muscicola Pers.-Dietrich (1856). Invalid or doubtful name according to Lado (2005Lado ( -2020. The record was not accompanied by the reference to the herbarium specimen. Didymium physaroides Fr. -Dietrich (1856). If this is Didymium physaroides (Pers.) Fr. & Palmquist and not Didymium physaroides (Link) Link, then according to Lado (2005Lado ( -2020 it is a doubtful name "Cited by Martin & Alexopoulos, Myxomycetes 391. 1969, as a synonym of Didymium melanospermum (Pers.) T. Macbr., but in p. 369 say that may be Diderma spumarioides (Fr.) Fr". H. A. Dietrich did not refer to any specimen and we have not found the species in the revised part of his collections. However, it is to be mentioned that the substrate described by Dietrich ("on decaying wood, especially birch, in autumn, not rare") is untypical for Diderma spumarioides and related species (Buyck 1988).

Discussion
The number of species provided herein, 150, is comparable to, although less than those reported from ge-ographically close regions with an extended history of myxomycetes research. As an example, over 211 species of myxomycetes are known from Lithuania, and no less than 188 species from the Leningrad region of Russia (Adamonytė 2007, Erastova & Novozhilov 2015, Popov et al. 2007). On the other hand, in Latvia, which borders both Estonia and Lithuania, only 108 species of myxomycetes have been discovered so far (Adamonytė, 2020). Such differences in a number of species between the regions similar in terms of the area and environmental conditions are most probably caused by the different research efforts applied.
It is worth mentioning that all species reported herein, are in fact morphospecies. The potential cryptic diversity of myxomycetes has not been yet thoroughly evaluated in Estonia, and further barcoding studies may reveal many more species occurring in this country.
Lastly, although the role of myxomycetes as key microbial predators in detrital food webs can hardly be exaggerated (Fukasawa et al. 2018), their conservation status has not yet gained proper attention in Estonia. As compared to fungi, among which 434 species have already been evaluated according to IUCN criteria and 153 declared as threatened, no myxomycetes have been included into the up-to-date Red List of Estonian fungi (Saar et al. 2019). The assessment of the threat status of myxomycetes in Estonia is an important future task, in which the current checklist might prove useful.

Conclusion
In the current study, all the published reports as well as unpublished data and results of revision of the herbarium collections are summarized to provide the updated checklist of the myxomycetes of Estonia that includes 150 morphological species. Further research is needed to assess the hidden biodiversity of this group, as well as to evaluate the frequency and conservation status of the listed species.