85
mid-Cretaceous Myanmar amber (Fig. 3.3k). The author suggests that, although
there are no modern equivalents to the pycnidia, they are most similar to leaf spotproducing members of the coelomyceteous fungi. Because the taxonomy and identification of modern rusts, smuts, and other fungal disease causative agents in plants
is largely dependent on micromorphological characters of the spores, careful palynological preparations and/or examination of permineralized angiosperms are likely
the keys to more accurately resolving the fossil history of these pathogens.
Another approach that has been employed to better understand the geologic history of fungus-plant parasitic interactions is to look for the host plants. For example,
the Erysiphales (Ascomycota), or powdery mildews, produce cleistothecia with
very characteristic appendages and are associated with specific angiosperm hosts
(Braun 1987). Consequently, the presence of certain angiosperm hosts during the
Late Cretaceous has been used as indirect evidence of the initial radiation of this
group of fungi (Takamatsu et al. 2010; Takamatsu 2013). However, we feel that this
approach, although interesting, is also problematic (see De Baets and Littlewood
2015). In the absence of a fossil record, how do we know if the ancestors of presentday (hyper-)host-specific fungal parasites parasitized the ancestors of the presentday hosts and elicited the same disease symptoms? Nevertheless, it should be
possible to identify members of the Erysiphales on the surface of fossil leaf cuticles
since many of the reproductive structures are highly ornamented.
3.3.2 Fungal Parasites of Algae
Algae are critical elements in modern aquatic ecosystems, not only in producing
oxygen for other aquatic life, but also in serving as primary producers of organic
matter at the base of the food chain (Round 1981). Some are pivotal in the biology
of aquatic animals, while others are major structural contributors to the formation of
reefs (Coates and Jackson 1987; Weiss and Martindale 2017). The fossil record of
algae is extensive and dates back to the Late (perhaps even Middle) Proterozoic
(Coniglio and James 1985; Graham and Wilcox 2000; Butterfield 2015; Bengtson
et al. 2017). Fungal parasitism of algae today is common, and some fungi enter into
complex relationships with their algal hosts (e.g., Kohlmeyer 1979; Kohlmeyer and
Kohlmeyer 1979; Gachon et al. 2010). One interesting example consists of endolithic microscopic algae inhabiting coral skeletons as a convenient shelter and endolithic fungi colonizing the corals primarily for food and feeding on both the coral
polyps and the endolithic algae (for details, see Le Campion-Alsumard et al. 1995;
Golubic et al. 2005). Parasitic fungi can have a profound impact on freshwater and/
or marine phytoplankton and algal populations (Ibelings et al. 2004; Kagami et al.
2007; Wang and Johnson 2009; Gleason et al. 2011); however, documented evidence of fungal parasites of fossil algae is very rare. This dearth of evidence is due
probably to the fact that the most common modes of preservation of fossil algae
(e.g., as cysts, calcareous skeletons, or thallus impressions) are not conducive to the
preservation in recognizable form of microbial parasites associated with these
3 Fungi as Parasites: A Conspectus of the Fossil Record
mid-Cretaceous Myanmar amber (Fig. 3.3k). The author suggests that, although
there are no modern equivalents to the pycnidia, they are most similar to leaf spotproducing members of the coelomyceteous fungi. Because the taxonomy and identification of modern rusts, smuts, and other fungal disease causative agents in plants
is largely dependent on micromorphological characters of the spores, careful palynological preparations and/or examination of permineralized angiosperms are likely
the keys to more accurately resolving the fossil history of these pathogens.
Another approach that has been employed to better understand the geologic history of fungus-plant parasitic interactions is to look for the host plants. For example,
the Erysiphales (Ascomycota), or powdery mildews, produce cleistothecia with
very characteristic appendages and are associated with specific angiosperm hosts
(Braun 1987). Consequently, the presence of certain angiosperm hosts during the
Late Cretaceous has been used as indirect evidence of the initial radiation of this
group of fungi (Takamatsu et al. 2010; Takamatsu 2013). However, we feel that this
approach, although interesting, is also problematic (see De Baets and Littlewood
2015). In the absence of a fossil record, how do we know if the ancestors of presentday (hyper-)host-specific fungal parasites parasitized the ancestors of the presentday hosts and elicited the same disease symptoms? Nevertheless, it should be
possible to identify members of the Erysiphales on the surface of fossil leaf cuticles
since many of the reproductive structures are highly ornamented.
3.3.2 Fungal Parasites of Algae
Algae are critical elements in modern aquatic ecosystems, not only in producing
oxygen for other aquatic life, but also in serving as primary producers of organic
matter at the base of the food chain (Round 1981). Some are pivotal in the biology
of aquatic animals, while others are major structural contributors to the formation of
reefs (Coates and Jackson 1987; Weiss and Martindale 2017). The fossil record of
algae is extensive and dates back to the Late (perhaps even Middle) Proterozoic
(Coniglio and James 1985; Graham and Wilcox 2000; Butterfield 2015; Bengtson
et al. 2017). Fungal parasitism of algae today is common, and some fungi enter into
complex relationships with their algal hosts (e.g., Kohlmeyer 1979; Kohlmeyer and
Kohlmeyer 1979; Gachon et al. 2010). One interesting example consists of endolithic microscopic algae inhabiting coral skeletons as a convenient shelter and endolithic fungi colonizing the corals primarily for food and feeding on both the coral
polyps and the endolithic algae (for details, see Le Campion-Alsumard et al. 1995;
Golubic et al. 2005). Parasitic fungi can have a profound impact on freshwater and/
or marine phytoplankton and algal populations (Ibelings et al. 2004; Kagami et al.
2007; Wang and Johnson 2009; Gleason et al. 2011); however, documented evidence of fungal parasites of fossil algae is very rare. This dearth of evidence is due
probably to the fact that the most common modes of preservation of fossil algae
(e.g., as cysts, calcareous skeletons, or thallus impressions) are not conducive to the
preservation in recognizable form of microbial parasites associated with these
3 Fungi as Parasites: A Conspectus of the Fossil Record