This
material is based upon work supported by the National Science
Foundation
under several grants. Any opinions, findings, and conclusions or
recommendations
expressed in this material are those of the authors and do not
necessarily
reflect the views of the National Science Foundation.
Alternative hypotheses of relationships of the group include a proposal by Cavalier-Smith (1998) that the Laboulbeniales are not ascomycetes, but rather relatives of other invertebrate-associated fungi, including Trichomycetes, Entomophthorales and Zoopagales in his SubKingdom Eomycota, Phylum Archemycota, Class Zoomycetes. This hypothesis has been refuted on the basis of phylogenetic analysis of partial sequences of ssu rDNA including parsimony analysis and maximum likelihood ratio tests applied to trees constructed using different topological constraints (Weir and Blackwell, submitted).
Although their placement within the filamentous ascomycete clade is well-supported a robust phylogeny of Laboulbeniales taxa must await further molecular work and integration of this with a morphologically-based cladistic analysis. The ancestral form in the Laboulbeniales and subsequent evolutionary trends within the order are still matters of some debate. Thaxter (1908) "...found himself unable to arrive at any satisfactory conclusions," and the more recent treatise by Tavares (1985) also concludes with uncertainty regarding these important questions. Once again preliminary molecular data provide possible clues to a number of the questions that have been raised. In particular, members of the Ceratomycetaceae including representatives from the genera Rhynchophoromyces,Autoicomyces, and Ceratomyces appear basal in phylogenetic reconstructions. These taxa share unspecialized exogenous spermatia and are found exclusively on aquatic hosts. Coreomyces, (an unusual genus found only on aquatic water-boatmen [Heteroptera, Corixidae] and characterized by unique perithecial development and production of antheridial cells on the primary axis below the perithecium) appears to represent an intermediate stage between these basal taxa and the more advanced members of Laboulbeniaceae. The latter are typified by the presence of more specialized endogenous antheridia and occurrence largely on terrestrial insects. No firm conclusions can yet be drawn from these datasets however, since many critical genera remain to be sequenced. Nevertheless, the answers to many intriguing questions regarding the evolution of these enigmatic fungi finally appear to be within grasp. It should now be possible to formulate and test higher-level phylogenetic hypotheses regarding Laboulbeniales; to investigate morphological plasticity and relate this to host, position, and sex-of-host specificities; to test morphology-based species concepts within the order- linking these to observed patterns of geographical distribution; and to examine the patterns of coevolution occurring between Laboulbeniales and their arthropod hosts.
SPECIFICITY
As a direct consequence
of their obligate relationship with their hosts Laboulbeniales fungi
exhibit
an often high degree of host specificity. This was recognized early in
the study of these organisms and in varying
degrees is characteristic
of most obligate parasites. Examination of published host-parasite
lists
(Frank, 1982; Hulden, 1983; Weir, 1996) mainly from temperate regions
clearly
shows that levels of host specificity displayed by Laboulbeniales are
generally
high when compared to most other groups of parasites that exploit
arthropods.
A relatively small proportion of the parasite species have been
reported
from more than
one host but host
association
data are too fragmentary for any firm conclusions to be reached. In the
most intensively studied mycobiota for these fungi, that of Poland
(Majewski,
1994), the host range for any given parasite appears, with very few
exceptions,
to be restricted taxonomically, and generally encompasses only species
that belong to the same genus or group of closely related genera. At
odds
with these findings is the realisation that some species of
Laboulbeniales,
for example Laboulbenia vulgaris, have apparently broad
geographical
distributions, encompassing both tropical and cool temperate areas
(Hammond,
1995), something almost unknown in species of the arthropod groups
which
include most of their hosts. The occurrence in nature of
morphologically
similar but genetically and physiologically distinct species of
Laboulbeniales
cannot, at this point, be discounted.
An even more intriguing aspect of Laboulbeniales biology are the numerous reports of so-called 'position specificity'- the often precise occurrence of thalli on very restricted areas of the body of the host. This phenomenon was first observed by Peyritsch (1875) who noted that Stigmatomyces baeri usually grew on the upper surface of the female fly and on the lower surface of the male. Position phenomena of this type are quite frequently encountered and are readily explained by assuming that ascospore transmission from one host individual to another is optimized during copulation. Transmission during mating does not, however, provide an hypothesis to explain many other examples of position specificity found in the Laboulbeniales as, in many instances, both sexes of host are equally heavily infested, and in the same position on the integument. In the most extreme example, found in the genus Chitonomyces on the African whirligig beetle Orectogyrusspecularis (Gyrinidae), 16 parasite species have been described (Thaxter, 1926). Whether these are valid species or represent different physiological growth-forms of the same species remains yet to be determined.
A third type of
specificity
phenomenon called 'sex-of-host specificity' was described by Benjamin
and
Shanor (1952) in their work on the parasites of Bembidion picipes. Here
6 parasite species were distinguished on a
single host, 2 of these
were found only on male beetles, a third species was apparently
restricted
to females. The restricted nature of this specificity phenomenon has
more
recently been questioned (Scheloske, 1976).
Laboratory-based
experiments
on the host specificity of a single species of Laboulbenia
(DeKesel,
1996) indicated that this fungus was potentially plurivorous on a range
of Carabidae but was restricted to a
single host in nature as
a result of reinforced ecological isolation. Further detailed
experimental
and genetical studies are undoubtedly required in order to clarify some
of the issues regarding species-concepts
and specificity raised
here.
REFERENCES
| Acallomyces sp. Autoicomyces falcatus Autoicomyces reflexus* Botryandromyces ornatus* Cantharomyces sp. Ceratomyces mirabilis* Ceratomyces sp. indet.* Chitonomyces melanurus Cochliomyces trinitatis* Coreomyces spp. (4 species) * (one partial sequence) Corethromyces bicolor* Corethromyces sp. nov. * Cucujomyces bilobatus Diaphoromyces kuschelii Dimeromyces anisolabis Dimorphomyces sp. Dioicomyces sp.* Distolomyces forficulae Gloeandromyces sp. Herpomyces sp. |
Hesperomyces coccinelloides* Hesperomyces virescens* Hydraeomyces halipli Hydrophilomyces sp.* Ilytheomyces sp Kyphomyces sp. Laboulbenia cristata* Laboulbenia flagellata Laboulbenia notiophili Laboulbenia pedicellata Laboulbenia philonthi* Laboulbenia vulgaris Monoicomyces invisibilis Monoicomyces sp. Rhachomyces philonthinus* Rhachomyces canariensis Rhadinomyces pallidus* Rhynchophoromyces minor* Rickia passalina* Sphaleromyces lathrobii* Stigmatomyces* (11 species sequenced) |
Stigmatomyces borealis* Stigmatomyces crassicollis Stigmatomyces discocerinae* Stigmatomyces entomophilus Stigmatomyces ephydrae* Stigmatomyces hydrelliae* Stigmatomyces limnophorae* Stigmatomyces limosinae* Stigmatomyces ochtherae Stigmatomyces papuanus Stigmatomyces paralimnae* Stigmatomyces parydrae Stigmatomyces protrudens* Stigmatomyces purpureus* Stigmatomyces rugosus* Stigmatomyces scaptomyzae* Teratomyces mirabilis Tettigomyces sp. Zeugandromyces sp. Zodiomyces vorticellarius* |