Studies in the Lasiosphaeriaceae. Monographs of two key genera and a family-level phylogeny
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Table of Contents:
I. Introduction to the Lasiosphaeriaceae
II. Taxanomic Background of the Lasiosphaeriaceae
III. Known distribution and abundance
IV. Biogeographic questions being addressed
The Study Group and Conceptual Issues
Ascomycetes (Subdivision Ascomycotina) constitute the largest known group
of fungi (Hawksworth et al., 1983, Hawksworth, 1991) and occur in all ecosystems
and geographical areas. Many are small and inconspicuous, parasitic on plants
and evident only from the effects they produce upon their hosts. Others
are saprobic, living on the soil, decaying logs, leaf mold or dung. The
ascomycetes have two distinct reproductive phases, the teleomorph (sexual
or meiotic state) and the anamorph (asexual or mitotic state), which will
most often be found growing and propagating independent of one another in
time and space (Fig. 4). This two-phase life-cycle
poses special problems in understanding the biology, ecology, classification
and taxonomy of these organisms. Ascomycetes have been included in recent
phylogenetic studies (Fig. 1) and have been estimated
to have diverged from the basidiomycetes about 400 million years ago, with
all the major lineages established by the end of the Mesozoic (Berbee and
Taylor, 1994) (Fig. 2).
The order Sordariales is one of several taxonomically diverse and biologically
and ecologically important groups within the Subdivision Ascomycotina, Class
Hymenoascomycetes (superordinal classification follows Barr, 1990) (Table 1). Resolving phylogenetic relationships within
this order will help elucidate relationships within the class as a whole.
Important taxa in this order include Neurospora, a widespread group of molds
and soil inhabitants including N. crassa, the model organism widely used
in molecular and genetic studies (Perkins, 1992; Taylor et al, 1993); Sordaria
fimicola the coprophilous organism, used to demonstrate gene segregation
in spore color; and Chaetomium Kunze, an important cellulolytic organism,
responsible for the destruction of paper and fabrics. Of considerable economic
importance are a number of Ceratocystis Ellis and Halst. species that are
aggressive plant pathogens in both agricultural and natural ecosystems (Kile,
1993).
According to one recent compilation (Eriksson and Hawksworth, 1993) the
Sordariales contains eight families (Table 2).
Barr and Cannon (1994) suggested that two of these families belong in other
orders. The presumed synapomorphies supporting the order Sordariales are
the ascomal ontogenetic characters involved in centrum development (centrum
= structures within the fruitbody such as asci and surrounding sterile elements).
Recent cladistic analyses of molecular data have shown the Sordariales s.
lat. to be non-monophyletic (Fig. 3). The four genera,
representing four families, out of the 99 genera of Sordariales sensu Eriksson
and Hawksworth (1993), that were included in Spatafora and Blackwell's (1994)
cladistic analyses of the ascomycetes segregated into two different clades.
Three of these genera, Neurospora (Sordariaceae), Chaetomium (Chaetomiaceae),
and Cercophora Fuckel (Lasiosphaeriaceae) formed a strongly supported clade,
with a sister group that included the Diaporthales and Ophiostomatales (Fig. 3). These genera represent a core group of typical
Sordariales. The other genus of Sordariales included in their study, Melanospora
Corda (Ceratostomataceae), has been placed variously in different orders
depending on the investigator's emphasis of different morphological characters.
The molecular data are concordant with the notion that the genus is best
removed from the Sordariales.
The Sordariaceae is the taxonomically best studied family in the order.
It primarily contains coprophilous taxa, including Neurospora and Sordaria.
Over the years, as taxa have been removed, the circumscription of the family
has narrowed. Many taxa have been placed in the Lasiosphaeriaceae. The Lasiosphaeriaceae,
as it stands now, is widely circumscribed and needs detailed study to assess
the true affinities of its members. The various genera within the Lasiosphaeriaceae
exhibit a broad range of developmental and morphological characters in both
the teleomorph and anamorph that are currently used as important indicators
of phylogenetic relationships within the ascomycetes.
The family level--Historically, the taxa that make up the present Sordariaceae
and Lasiosphaeriaceae have been combined under one family (Munk, 1957; Dennis,
1968, Carroll and Munk, 1964; Barr and Cannon, 1994), separated into two
families with the addition of subfamily groupings (Lundqvist, 1972), or
split off into a third family, Tripterosporaceae (Barr, 1990). Hawksworth
and Eriksson (1993) place the relevant taxa in two families; that hypothesis
is used here with the understanding that the two families are probably closely
related if not monophyletic. Assessment of the monophyly of these taxa,
and the relationship of the Lasiosphaeriaceae with other families in the
Sordariales is an important aspect of our monographic work.
In a recent compilation, the Lasiosphaeriaceae is considered to include
51 genera with 43 synonyms (Eriksson and Hawksworth, 1993). Relationships
of genera within the family are based on a number of morphological characters
of the teleomorph, for example, ascomal wall structure, wall surface covering,
ascus shape and apical ring structure; and the hyphomycetous development
of the anamorph. However, this information is not known for all of the members
and the number of taxa that really belong in the group is unknown.
Of the 51 genera in the Lasiosphaeriaceae, 19 are predominantly coprophilous
and these are the only recently studied members of the family (Cain, 1962;
Lundqvist, 1972). The important plant pathogenic genus Ceratocystis is currently
included in the family, although this has recently been questioned (Rogers,
1994; Spatafora and Blackwell, 1994). The family also contains members that
have been untouched monographically. Chaetosphaeria and Lasiosphaeria occur
mostly on decaying wood, herbaceous stems and leaves, not on dung, and are
among the most species-rich genera in the family, yet there are no comprehensive
treatments or keys to species.
Characteristics of some genera currently included in the Lasiosphaeriaceae
suggest relationships to other families in the order or even to other orders.
Ascospore germination in Lasiosphaeria and Lasiosphaeriella Sivan. suggests
a relationship with the Nitschkiaceae. Chaetosphaeria has long been allied
with the Trichosphaeriales, while the iodine-induced amyloid, apical, ascus
ring in Lasiobertia Sivan. suggests a connection to the Xylariales. Taxa
with long, narrow ascospores, Acrospermoides Miller and Thomps. and Ophioceras
Sacc., show affinities with the Diaporthales and Mycomedusiospora Carroll
and Munk with light colored ascomata may show a relationship with the Hypocreales.
Phylogenetic analysis of the genera currently maintained within the family
is therefore long overdue and clearly warranted. This rigorous assessment
of relationships needs to incorporate both morphological and molecular data.
As currently circumscribed, the Lasiosphaeriaceae is almost certainly not
monophyletic.
The genus level--The two predominantly lignicolous genera, Chaetosphaeria
and Lasiosphaeria, are chosen for monographic revision because they have
never been treated systematically even though they are key to understanding
phylogenetic relationships in the family and are apparently ecologically
and economically important. Species in the two genera have been described
primarily from north temperate regions. However, preliminary evidence suggests
that these genera are a dominant part of the neotropical mycota involved
with litter decomposition since they were among the most commonly collected
taxa in several recent collecting trips to French Guiana (Table
1 and Table 2).
According to the Index to Saccardo's Sylloge Fungorum (Reed and Farr, 1993),
there are 47 names described in Chaetosphaeria and 171 in its synonym Zignoella.
From 1920 to the present, there have been 52 Chaetosphaeria species and
11 Zignoella species described (Index of Fungi), making a total of 281 described
taxa under Chaetosphaeria. The genus was originally described for a hyaline-spored
species with smooth ascomata (Tulasne and Tulasne, 1863), but when the genus
was taken up by Saccardo (1883) it was conceived as having smooth or tomentose
ascomata and hyaline or brown ascospores. Zignoella was placed into synonymy
with Chaetosphaeria by Booth (1957, 1958). Over the years taxa with many
different character combinations have been included. Ascospore coloration
is still maintained as an important generic character in some groups, and
this character will probably prove important for circumscribing monophyletic
units out of the species currently included in Chaetosphaeria. Species originally
described under Zignoella have hyaline spores, but these spores are multi-septate
while the original circumscription of Chaetosphaeria only included species
with one septum. A number of Chaetosphaeria names have been transferred
to or linked with names in Lasiosphaeria. This is because both share the
general characteristics of superficial, ovoid-shaped ascomata and hyaline
septate ascospores. The two genera can be separated by critical examination
of the teleomorph and anamorph characters.
Chaetosphaeria occurs mainly on decaying twigs or wood, however, a few species
are described from leaves and other substrates. Chaetosphaeria was one of
the first genera in which a teleomorph and anamorph were recognized as belonging
to the same fungus (Tulasne and Tulasne, 1863). Species of Chaetosphaeria
have been linked to several phialidic anamorphs belonging to a various genera
including Chloridium Link and Gonytrichum Nees (Gams and Holubova-Jechova,
1976). Gams and Holubova-Jechova noted that the sections recognized in Chloridium
based on morphology corresponded to sections in Chaetosphaeria based on
ascospore morphology. Some species of the Chloridium anamorph have cellulolytic
activity occurring in advance stages of decay on wood, and a species of
the Gonytrichum anamorph has the ability to decompose lignin (Gams and Holubova-Jechova,
1976).
Lasiosphaeria is a species-rich genus which will be crucial to resolve relationships
in the family. A history of the generic name and its relationship to coprophilous
taxa was given by Lundqvist (1972), without further species treatments.
The genus was originally based on 7 species (Cesati and de Notaris, 1863).
As it was revised over the years, with changes in its concepts (Fuckel,
1870; Saccardo, 1883), it gradually became packed with a variety of probably
unrelated elements. The Index to Saccardo's Sylloge Fungorum (Reed and Farr,
1993), lists 131 names in Lasiosphaeria and there are 43 names for species
described after 1920 (Index of Fungi) making a total of 174.
Lasiosphaeria is closely linked to Cercophora and Bombardia (Fr.) Karst.
by similarities in anamorph development, ascomal, ascus and ascospore morphology.
Although Cercophora is considered to be mainly coprophilous, lignicolous
species have been transferred among these three genera without resolving
a clear generic concept in any of them (Hilber and Hilber, 1979; Hilber
et al, 1987). The Nordic coprophilous species of Cercophora have been outlined
with excellent descriptions and illustrations (Lundqvist, 1972), however,
no monograph of either Cercophora or Bombardia exists. While this project
is not intended as a monograph of these two genera, the type species and
other representative species of each must be reassessed and their relationship
to Lasiosphaeria clearly defined. Only after this is done can we understand
the extent of revision of Cercophora and Bombardia that is necessary within
the context of a monograph of Lasiosphaeria.
There has never been a monograph for the genus. For identification we have
had to rely on scattered works by few authors treating limited number of
species (Seaver, 1912; Carroll and Munk, 1964; Hilber and Hilber, 1983,
Hilber et al, 1987; Hilber and Hilber, 1979, make reference to an unpublished
dissertation of Lasiosphaeria which the PI has been unable to locate). In
recent years, a few species have been segregated in other genera e.g. Lasiosphaeriella
based mostly on ascospore characters (Sivanesan, 1975). However, other characters,
such as its anamorph development suggest that these species should be included
in a reassessment of the group.
Lasiosphaeria has been conceived in a broad sense regarding ascospore types
and ascomal wall structure (Carroll and Munk, 1964; Barr, 1990). Variation
includes ascomata with assorted types of vestiture and variation in ascus
shape, apical apparatus and dehiscence. The few reported anamorphs for species
of Lasiosphaeria are phialides that form directly on germinating ascospores
(Hughes, 1951; Gams, 1973; Hilber et al, 1987), a pattern of development
that is not found in many ascomycete genera (Hanlin, 1994; Huhndorf and
Glawe, 1990; Samuels, pers. comm.). However, some species of Cercophora
and Lasiosphaeriella, as well as one species of Bertia de Not. in the Nitschkiaceae,
also have this type of anamorph development (Sivanesan, 1978). A presumed
apomorphy which supports the genus Lasiosphaeria is the precocious development
of the ascospores, where the species have become physiologically mature
in the hyaline stage (Barr and Cannon, 1994). This could also be the case
for Chaetosphaeria.
It is not known at this time how well the number of species described reflects
the actual number of species that will remain in Lasiosphaeria and Chaetosphaeria
after detailed examination. Based on the PI's monographic studies in other
groups (Huhndorf, 1992c), it is estimated that two-thirds of the described
species will belong to other genera, some closely related but others more
distantly so. The type collections of all the species must be examined to
determine the number of actual species known at this time, however the type
collections for the most part will not provide any information about anamorphs.
For this, new collections and pure culture studies will be necessary. Previously
unknown species will undoubtedly be encountered and will be described.
Known distribution and abundance
Genera of Lasiosphaeriaceae have been described from many parts of the world
but many are still known only from single collections. For coprophilous
ascomycetes, Lundqvist (1972) suggests there are two distribution patterns,
north-temperate and tropical-subtropical, with the possibility of a third,
south-temperate group on which information is sparse. Some species have
been found only in certain climatic regions, whereas the genera seem not
to be limited in this way. Cercophora is best known from the temperate zones
but there are a number of records of tropical collections in the literature
(Lundqvist, 1972). Coprophilous genera have been described from Venezuela
(Jeng and Krug, 1976, 1977), and from India (Subramanian and Lodha, 1968)
and one genus appears to be pantropical (Cain and Farrow, 1956).
Data for the lignicolous taxa are too incomplete to discern distribution
patterns, and nothing can be said about the abundance of most. However,
eight genera have been reported mostly from Europe and North America, including
Chaetosphaeria and Lasiosphaeria. Genera have been described from Brazil
(Müller and Samuels, 1982a; Samuels and Müller, 1978), India,
Jamaica, Japan, New Zealand and United Kingdom (Minoura and Muroi, 1978;
Müller and Samuels, 1982b; Samuels et al, 1981; Sivanesan, 1983), Australia
(Hyde, 1992a,b) and Egypt (Schatz, 1985).
While working on an ascomycete contribution for a checklist (Courtecuisse
et al., in prep.), the PI examined a large number of French Guiana ascomycete
collections to assess species richness and abundance of the various litter
inhabiting genera and species in this lowland rainforest area (Table
1 and Table 2) The Sordariales, specifically
the Lasiosphaeriaceae, was the first and third most-frequently encountered
group of ascomycetes in French Guiana in these two surveys where it comprised
between 4% and 16% of the ascomycetes encountered. The two other most-frequently
encountered ascomycete families belong to the Hypocreales and Xylariales.
The Lasiosphaeriaceae were not specifically targeted for collection in these
studies. Additionally, no attempt was made at an exhaustive search for coprophilous,
soil-inhabiting or aquatic ascomycetes, so taxa occurring exclusively in
these habitats are noticeably absent from the collections. Nevertheless,
thirteen lignicolous or herbicolous Lasiosphaeriaceae genera were found
of which two are new to the Neotropics. The four genera listed in Table 2 were the most abundant. Most of the genera are
new records for French Guiana.
Most recognized species of Chaetosphaeria and Lasiosphaeria have been described
from temperate Europe and North America but it is unknown how accurately
this reflects the global distribution. Approximately 60% of the species
have been described from Europe, but only a very few of the same names ever
appear in checklists of commonly collected species (Eriksson, 1992). Reports
of Chaetosphaeria and Lasiosphaeria from North America are mostly from scattered
historical collecting sites in the eastern U.S. (Ellis and Everhart, 1892).
Many of these sites no longer exist as natural areas. Most locations in
North America have been uncollected for ascomycetes in general, so it is
difficult to discern distribution patterns. However, interesting finds of
Lasiosphaeria and Cercophora species in recent years (Hilber et al, 1987)
suggest that the Appalachians may be a center of diversity. All collections
of Lasiosphaeria and Chaetosphaeria (and synonymous names) will be requested
from the major North American and European herbaria to supplement the material
collected during this study.
Approximately 15% of the total number of species of Chaetosphaeria and Lasiosphaeria
have been described from tropical-subtropical regions worldwide. Historical
collections are available from Brazil, Ceylon, Cuba, Java and the Philippines.
The actual distribution of these genera in the tropics is unknown. Recent
studies of lignicolous members of the family include four species of Lasiosphaeria
found in Costa Rica (Carroll and Munk, 1964), one species of Cercophora
in Venezuela (Hanlin and Tortolero, 1987) and one species of Chaetosphaeria
found as an endophyte in French Guiana (Petrini and Dreyfuss, 1981). However
our recent data (Table 2) indicate that they are
abundant and diverse in the neotropics. The data indicate that Chaetosphaeria,
and Lasiosphaeria s. lat. (including Lasiosphaeriella and Cercophora) are
the most commonly collected members of the family in French Guiana and together
the four genera make up between 3% and 11% of the ascomycetes encountered.
Biogeographic questions being addressed
No work has been done on the biogeography of the taxa in the Sordariales.
A major precursor to undertaking biogeographic analysis is a good understanding
of phylogenetic relationships within the group. It also requires a more
complete record of geographic distribution than is presently available.
Our collecting sites in North America, the Caribbean, and Central and South
America (see below) were chosen to allow us to contrast the distribution
of ascomycetes in the Americas and begin to think about their biogeographic
distribution in lowland tropical rainforests. Because the vascular plant
flora in these sites is documented, we can test ideas about plant/fungus
ratios used in estimating numbers of fungal species (Hawksworth, 1992) and
about host specificity as it relates to tropical plant diversity. Whereas
Hawksworth (1991) hypothesized a high degree of host specificity worldwide
leading to a high global estimate of fungi, May (1991) suggested that fungal
species in the tropics may be less host specific because the hosts are sparsely
distributed. If this is true then the plant/fungus ratio in the tropics
would be lower and worldwide estimates of numbers of fungal species would
be substantially reduced.
Host specificity also plays a role in ecosystem management and biogeography.
Host-specific fungi may not co-disperse with their host following forest
fragmentation, thus altering the susceptibility of both plant and fungi
to extinction. We will be able to test ideas about fungal diversity as it
relates to habitat and host diversity and to the abundance of resources.
From the taxonomic composition based on our U.S. and tropical collecting
sites we will be able to contrast temperate and tropical mycotas and suggest
distribution patterns and hypotheses on migration versus dispersal that
can be tested with the phylogeny that we will propose.
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