Genomics Spurs Rapid Advances in our Understanding of the Biology of Vascular Wilt Pathogens in the Genus Verticillium

Scientific names are crucial in communicating knowledge about fungi. In plant pathology, they link information regarding the biology, host range, distribution, and potential risk. Our understanding of fungal biodiversity and fungal systematics has undergone an exponential leap, incorporating genomics, web-based systems, and DNA data for rapid identification to link species to metadata. The impact of our ability to recognize hitherto unknown organisms on plant pathology and trade is enormous and continues to grow. Major challenges for phytomycology are intertwined with the Genera of Fungi project, which adds DNA barcodes to known biodiversity and corrects the application of old, established names via epior neotypification. Implementing the one fungus–one name system and linking names to validated type specimens, cultures, and reference sequences will provide the foundation on which the future of plant pathology and the communication of names of plant pathogens will rest. 247 A nn u. R ev . P hy to pa th ol . 2 01 5. 53 :2 47 -2 67 . D ow nl oa de d fr om w w w . a nn ua lr ev ie w s. or g A cc es s pr ov id ed b y N at io na l L ib ra ry o f M ed ic in e on 0 8/ 21 /1 5. F or p er so na l u se o nl y. PY53CH12-Crous ARI 24 July 2015 8:49 Molecular phylogenetics: using DNA data to study evolutionary relationships, usually depicted via phylogenetic trees INTRODUCTION The protection of plants against disease and, more specifically, research on plant pathogens rests primarily on accurate communication regarding the fungi that cause disease. We communicate about species of fungi causing diseases via the application of scientific names. These names provide the key to the accumulated knowledge on the biology, distribution, ecology, host range, and control of, as well as the risks associated with, fungal pathogens. Inaccurate characterization can lead to unnecessary control measures and restrictions being applied or, importantly, in no action being taken to control potentially devastating pathogens. Even when hitherto unknown fungi are encountered, an understanding of their relationships can predict whether they are likely to be a cause for concern. Our understanding of the fungi and their relationships with plants and each other has seen an unprecedented and exponential growth over the past 10–15 years. This has been primarily fueled by advances in molecular phylogenetics, and the impact of this new technology continues to accelerate. Fungal systematics, the field concerned with classification and relationships, is becoming increasingly vibrant in diverse regions of the world and is no longer largely confined to Europe and North America, which was the case into the 1980s. It is also witnessing an unprecedented level of international collaboration and visibility, taking advantage of web-based systems to enhance the procedures for documenting and naming fungi, including those that cause plant disease. Mycologists face a daunting challenge. Just 20 years ago, the notion that we knew only 6% of the fungi on Earth was treated with skepticism by many other biologists. Now, primarily as a consequence of the application of DNA sequence data to the identification of fungi, it has emerged that this was far from an exaggeration and is actually a conservative estimate of the huge problem facing our knowledge of global fungal species richness. There are at least 1.5 million and probably 3 million species to contend with (41, 44), although some estimates are even higher (98). It is thus unfortunate that only around 100,000 species are presently recognized. Fungi are surely the most diverse eukaryotes on Earth. Faced with this knowledge gap, mycologists are beginning to find new approaches to both document this megadiversity and reduce the time traditionally devoted to clarifying the application of old, insufficiently known names (see sidebar, How Many Fungal Species Are There on Earth? ). Plant pathologists and other users of fungal names are understandably often irritated and frustrated by name changes that can appear nonsensical. This frustration is often justified because HOW MANY FUNGAL SPECIES ARE THERE ON EARTH? In the premolecular era, Hawksworth (41) estimated that, conservatively, there were 1.5 million species of fungi on Earth. This estimate was based on three independent data sets, which indicated that the number of fungi in all habitats to be expected in a survey of a particular place was six times that of the vascular plants present, extrapolated to the global scale. Estimates of the number of plant species have risen from 250,000 to 400,000, suggesting that the estimate for fungal species should be increased to 2.4 million, and around 3 million is probable (44). However, some additional data on fungal numbers suggest that the ratio of fungi to plants could be at least 10:1 (8, 66); fungi occurring in unique habitats such as soil and insects or those on lichens have generally not been included in these later estimates. Recent molecular studies have shown that there could be more than a thousand species of fungal endophytes in a single plant host, most being unculturable (49), suggesting that there could be as many as six million species of fungi (86). 248 Crous · Hawksworth ·Wingfield A nn u. R ev . P hy to pa th ol . 2 01 5. 53 :2 47 -2 67 . D ow nl oa de d fr om w w w . a nn ua lr ev ie w s. or g A cc es s pr ov id ed b y N at io na l L ib ra ry o f M ed ic in e on 0 8/ 21 /1 5. F or p er so na l u se o nl y. PY53CH12-Crous ARI 24 July 2015 8:49 Sporophore: spore-bearing organ of a fungus some of these changes are indeed senseless, arising from arcane rules devised in the early twentieth century that are no longer appropriate or necessary. Examples are versions of the International Code of Botanical Nomenclature (ICBN), which allowed different morphs of the same fungal species to be accorded separate scientific names and did not permit fungal species to be based on even permanently preserved cultures. Other changes in names, however, reflect a more logical understanding of relationships, arising from new research with molecular tools and so are more predictive of the biology and ecology of the fungi concerned; something plant pathologists should welcome. In this review, we consider recent developments affecting the identification and naming of plantpathogenic fungi. This is done in the context of past practices, in order that plant pathologists can better appreciate the background of changes now in the pipeline. In addition, we look to new developments and enhancements of the current systems and practices, which promise to provide further precision in the naming and identification of fungi. Through their positive impacts on the knowledge of plant pathology, mycologists are becoming increasingly recognized as world leaders in the organization of naming systems, envied by zoologists and botanists alike, who are now striving to follow suit.