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3.1 Introduction
Fungi occur in next to every ecosystem of the world, colonizing numerous substrates and performing multiple functions (Treseder and Lennon 2015; Dighton and
White 2017). As carbon heterotrophs, they have, by necessity, evolved several different nutritional modes and mastered various levels of cooperation with, and
exploitation of, other organisms to acquire carbon (Lewis 1973). Many fungi
degrade complex organic compounds such as lignin and cellulose and, through this
recycling, are important in returning minerals to the soil and CO 2 to the atmosphere
(Hatakka 2005; Baldrian and Valášková 2008). Others partner with certain types of
algae and cyanobacteria to form lichens (Nash 2008), while still others enter into
mycorrhizal associations with non-vascular and vascular land plants (Brundrett and
Tedersoo 2018). Fungi have also evolved mutualistic associations with animals;
some even thrive within the animal, in anaerobic environments (Orpin and Joblin
1997; Dollhofer et al. 2015). On the other hand, what has been termed the “dark
side” of the fungal Kingdom (Taylor et al. 2015) is that, as parasites and pathogens,
fungi negatively affect the performance of other microorganisms, plants, animals,
and even humans and are causative agents of many diseases (e.g., Sharon and
Schlezinger 2013; Köhler et al. 2015; Hall and Noverr 2017; Möller and
Stukenbrock 2017).
Parasitic fungi live and derive the majority of their nutrients at the expense of
other organisms that are alive at the time of infection (Deverall 1969; Zelmer 1998).
Biotrophic parasitic relationships represent physiologically balanced systems, in
which the parasite coexists with its host for an extended period of time, whereas
necrotrophic parasites kill host tissue and then feed saprotrophically on the dead
remains (Glazebrook 2005; Delaye et al. 2013). However, it is known today that,
while this subdivision is generally accurate, the actual situation is more complex
because many fungi behave as both biotrophs and necrotrophs, depending on the
conditions in which they find themselves or the stages of their life cycles
(Glazebrook 2005).
The origin of the true fungi is estimated at between 660 Ma and up to 2.6 Ga ago
based on molecular clock data and some paleontological evidence (for details on
early fungal fossils, see Krings et al. 2017c; Loron et al. 2019; Bonneville et al.
2020), and the divergence of the fungal-animal lineage from the plant lineage at
between 780 Ma and up to 2.5 Ga ago (e.g., Altermann and Schopf 1995; Martin
et al. 2003; Taylor and Berbee 2006; Blair 2009; Lücking et al. 2009; Sharpe et al.
2015; Bengtson et al. 2017; Berbee et al. 2017). The nutritional mode of the common ancestor of the true fungi remains elusive. However, early-diverging branches
of the fungal stem lineage include the Aphelida, which are parasites of planktonic
algae (Letcher et al. 2013, 2017; Karpov et al. 2014, 2017), and the animalendoparasitic Cryptomycota and Microsporidia (Keeling and Fast 2002; James
et al. 2006; Jones et al. 2011a, b; Vávra and Lukeš 2013; Han and Weiss 2017; Bass
et al. 2018), suggesting that the evolutionary arms race of fungi as parasites of other
organisms is of ancient origin (Anderson et al. 2010). Unfortunately, none of these
early-diverging lineages (except possibly Aphelida; see Krings and Kerp 2019) have
C. J. Harper and M. Krings
3.1 Introduction
Fungi occur in next to every ecosystem of the world, colonizing numerous substrates and performing multiple functions (Treseder and Lennon 2015; Dighton and
White 2017). As carbon heterotrophs, they have, by necessity, evolved several different nutritional modes and mastered various levels of cooperation with, and
exploitation of, other organisms to acquire carbon (Lewis 1973). Many fungi
degrade complex organic compounds such as lignin and cellulose and, through this
recycling, are important in returning minerals to the soil and CO 2 to the atmosphere
(Hatakka 2005; Baldrian and Valášková 2008). Others partner with certain types of
algae and cyanobacteria to form lichens (Nash 2008), while still others enter into
mycorrhizal associations with non-vascular and vascular land plants (Brundrett and
Tedersoo 2018). Fungi have also evolved mutualistic associations with animals;
some even thrive within the animal, in anaerobic environments (Orpin and Joblin
1997; Dollhofer et al. 2015). On the other hand, what has been termed the “dark
side” of the fungal Kingdom (Taylor et al. 2015) is that, as parasites and pathogens,
fungi negatively affect the performance of other microorganisms, plants, animals,
and even humans and are causative agents of many diseases (e.g., Sharon and
Schlezinger 2013; Köhler et al. 2015; Hall and Noverr 2017; Möller and
Stukenbrock 2017).
Parasitic fungi live and derive the majority of their nutrients at the expense of
other organisms that are alive at the time of infection (Deverall 1969; Zelmer 1998).
Biotrophic parasitic relationships represent physiologically balanced systems, in
which the parasite coexists with its host for an extended period of time, whereas
necrotrophic parasites kill host tissue and then feed saprotrophically on the dead
remains (Glazebrook 2005; Delaye et al. 2013). However, it is known today that,
while this subdivision is generally accurate, the actual situation is more complex
because many fungi behave as both biotrophs and necrotrophs, depending on the
conditions in which they find themselves or the stages of their life cycles
(Glazebrook 2005).
The origin of the true fungi is estimated at between 660 Ma and up to 2.6 Ga ago
based on molecular clock data and some paleontological evidence (for details on
early fungal fossils, see Krings et al. 2017c; Loron et al. 2019; Bonneville et al.
2020), and the divergence of the fungal-animal lineage from the plant lineage at
between 780 Ma and up to 2.5 Ga ago (e.g., Altermann and Schopf 1995; Martin
et al. 2003; Taylor and Berbee 2006; Blair 2009; Lücking et al. 2009; Sharpe et al.
2015; Bengtson et al. 2017; Berbee et al. 2017). The nutritional mode of the common ancestor of the true fungi remains elusive. However, early-diverging branches
of the fungal stem lineage include the Aphelida, which are parasites of planktonic
algae (Letcher et al. 2013, 2017; Karpov et al. 2014, 2017), and the animalendoparasitic Cryptomycota and Microsporidia (Keeling and Fast 2002; James
et al. 2006; Jones et al. 2011a, b; Vávra and Lukeš 2013; Han and Weiss 2017; Bass
et al. 2018), suggesting that the evolutionary arms race of fungi as parasites of other
organisms is of ancient origin (Anderson et al. 2010). Unfortunately, none of these
early-diverging lineages (except possibly Aphelida; see Krings and Kerp 2019) have
C. J. Harper and M. Krings