Biology of Laboulbeniales

Figure 1. Thalli of Laboulbeniales. 

A. Distolomyces, B. Tavaresiella, C. Triceromyces.

Welcome to the webpage that will introduce you to one of the most bizarre groups of fungi that we are aware of! Members of Laboulbeniales constitute an order of ascomycete fungi that are unusual in having obligate associations with living arthropods, and in lacking distinct mycelia, with the entire thallus developing from successive divisions of a two-celled ascospore. This determinate type of development is unusual amongst the Fungi and, in this case, has led to the evolution of a vast array of thallus forms (see Figure 1). These fungi have had a controversial taxonomic history and have been centrally placed in arguments regarding the phylogenetic placement of ascomycete fungi generally.

First discovered in the middle of the 19th century (Robin, 1853) their thalli have been described as abnormal cuticular outgrowths of the arthropod exoskeleton (Mayr, 1853), as acanthocephalan worms (Kolenati, 1857),as zygomycetes, and as basidiomycetes. Clarification of the taxonomic position of the group had to await the detailed studies performed by Roland Thaxter at Harvard University, beginning in 1890.

Although Thaxter made numerous other significant contributions to our knowledge of fungi, algae, and the myxobacteria, it was his exacting study of the Laboulbeniales that persists as one of the great classics in mycology. In his first paper on Laboulbeniales (Thaxter, 1890) Thaxter states that “The Laboulbeniaceae constitute a small group of fungi, without close affinities among other known Ascomycetes….and the total number of forms at present known represent only fifteen described species.” During the more than 40 years that followed Thaxter published a short note in Science (Thaxter, 1903), twenty-one unillustrated papers in the Proceedings of the American Academy of Arts and Sciences, and five beautifully-illustrated treatises entitled “Contribution towards a monograph of the Laboulbeniaceae” published in the Memoirs of the American Academy of Arts and Sciences (Thaxter, 1896, 1908, 1924, 1926, 1931). 

 In total, Thaxter described 103 genera, approximately 1260 species, and 13 varieties of these fungi. To illustrate not only his own species, but many of those described by others, he drafted 3427 line drawings, all done with meticulous attention to detail and arranged in 166 plates (Benjamin, 1971). One plate from volume IV of this work is shown in Figure 2. The magnitude of this work is difficult to comprehend unless one has been engaged in a similar type of study.

 

Figure 2. Thaxter drawings of species of Rickia.

Whilst some of Thaxters’ interpretations are open to debate, and parts of his monograph are particularly difficult to use, his work stands as the baseline for information on the biology of this group. In the almost 75 years that have elapsed since the final part of Thaxters’ monograph an additional 700 or so species have been described with many more awaiting formal description in collections around the world. Despite the continuing addition of new taxa, however, very little has been added to our knowledge regarding aspects of the general biology of these fungi.

General biology of Laboulbeniales

Laboulbeniales are unique due to their occurrence on, and exploitation of, the integument of living arthropods, and the determinate development and reduced hyphal system of their minute thalli. Virtually all of the 2000 known species are considered to be obligate ectoparasites of arthropods, mainly insects. Amongst the Insecta, representatives of ten orders are known as hosts viz.: Blattodea (cockroaches and allies), Coleoptera (beetles), Dermaptera (earwigs), Diptera (true flies), Heteroptera (bugs), Hymenoptera (ants), Isoptera (termites), Mallophaga (bird lice), Orthoptera (crickets and allies), and Thysanoptera (thrips). In addition, both millipedes (Diplopoda) and mites (Arachnida, Acari) are known to carry infections.

One characteristic that suggests that the group as currently recognized is a monophyletic group is the shared character of spindle-shaped bicellular ascospores in all known species. One of the two segments of the ascospore is smaller than the other (see Figure 3), the whole enclosed by a gelatinous sheath which is significantly broader around the longer of the two segments. The longer segment forms the basal portion of the fungal thallus including the organelle of attachment known as the foot, from which an internally penetrating haustorium develops (observed only in a few species but thought to be prevalent throughout the group). We have followed development in one species, Hesperomyces virescens, parasitic on ladybug beetles (Weir and Beakes, 1996). In this species, ascospores appear to elongate on contact with the integument of the host. Initial adhesion appears to rely on the inflated portion of the sheath surrounding the tip of the longer of the two ascospore cells. This tip subsequently swells forming the melanized foot. The longer ascospore cell then elongates, divides, and forms the 3-celled receptacle and perithecium. The smaller cell produces the antheridial appendage. In this, and other species studied, the time required for development from germinating ascospore to fully grown mature thallus seems to vary from 10-21 days, and there is some evidence that ascospores are short-lived, persisting away from their hosts for only a short period (Lindroth, 1948; De Kesel, 1995).

As a rule, transmission of the adhesive ascospores appears to take place during direct contact, often that involved in sexual encounters between two host individuals. Opinions differ (Kaur and Mukerji, 1994; De Kesel, 1995) as to the likelihood and prevalence of indirect transmission, i.e. infection by ascospores shed onto a hosts substrate, although this is known to occur in at least some instances. Hulden (1983) and Weir and Hammond (1997) have identified factors that are essential or favorable for the existence of these parasites within host populations. These factors include overwintering (at least partially) in the imaginal (adult) stage, overlapping generations of host adults, mating between members of different host generations, large and/or dense host populations, warmth, and moisture. Host cuticle characteristics and host defences may also be of prime importance in determining the likelihood and patterns of infection. It is important to point out here that these factors are based largely on samples and observations from north temperate locales and may differ in other parts of the world. It seems quite likely, as pointed out by Weir and Hammond (1997), that the ‘suitability’ of any given arthropod  as a host is likely to involve an interplay between a range of factors, many of which remain to be clearly identified. These will include both phonological and demographic attributes of populations of the potential host, some of which are listed in crude form above. However, host habitat choice and various aspects of host behavior are also likely to be involved.

Compared with most other groups of parasites that exploit arthropods, the level of host specificity displayed by Laboulbeniales is clearly generally high, with the host range for any given parasite, with very few exceptions, being restricted to species within a host genus, or a group of closely related genera. In addition to host specificity, these fungi are also reported to be specific to particular sexes of host (so called “sex –of-host” specificity), or to particular body parts of hosts (so called “position specificity”). The validity of these interpretations continues to attract debate and further information on the levels of specificity displayed by these fungi and our attempts to clarify or resolve these issues can be found by following this link.

A glance through the literature from the time of the discovery of the Laboulbeniales to almost the present day reveals a story of confusion with regard to the morphology of the thallus. Mush of this confusion arises from different interpretations of thallus development and its organization that have been prevalent (compare, for example, the definitions of the word receptacle used by Thaxter [1896, 1908] with those of Sugiyama (1973). This confusion has been detrimental to the study of the Laboulbeniales but has been recently largely resolved by the adoption of a standardized terminology as presented by Tavares (1985).

For further information on the principal components of the thallus click on portions of interest in the thallus of Aporomyces  to the right.

Collection and preparation of Laboulbeniales

Mycologists generally have great difficulty in finding Laboulbeniales due, in large measure, to their unfamiliarity with the hosts of these fungi. This single fact has severely limited the number of people taking a detailed interest in the Laboulbeniales and has meant that they are usually passed over, or only mentioned by name in undergraduate courses in mycology, and in general textbooks on the fungi (eg Webster, 1980). The few workers who have contributed much to our knowledge of the biology of these organisms have usually started out as entomologists, and later turned their attention to these curious parasites (Balazuc, pers. comm., Rossi, pers. comm.., Thaxter). As mentioned above, members of the Laboulbeniales are known on a wide range of insects, as well as mites and millipedes. These fungi can be found on suitable hosts in a variety of habitat types including open water, marsh, bog and seashore, soil, mud, decomposing plant and animal remains, foliage and flowers, the fruit bodies of larger fungi, and even the bodies of living animals such as birds and bats! Levels of infection vary considerably with host, habitat and time of collection, although the most consistent results are usually obtained when collecting in damp environments. Predacious beetles belonging to the families Carabidae and Staphylinidae are particularly prone to infection and are relatively easy to collect in and around wet, or otherwise moist environments. Infection levels appear to be further enhanced when host individuals are in some way forced to co-habit in a given habitat such as the strand-line formed by drifts of seaweed on the coast or similar linear habitats around lakes or other water bodies (P. M. Hammond and A. Weir, unpublished observation). In north temperate regions in early autumn rich assemblages of flies around ponds, lakes, and other water courses may also provide abundant material as can the beetles found in the fruit bodies of larger fungi. In the cold winter months in these climes, hibernating aggregations or individuals in and around buildings, or under the bark of trees can also be a good source of infection.

One of the easiest ways to secure these fungi is to scan through large numbers of dried, preserved insects in museum collections. Much of the extensive material studied by Thaxter and later workers has been obtained in this way. In terms of molecular work and material for DNA extraction we have found it better to collect fresh insects into 95% or 100% ethanol (see Weir and Blackwell, 2001).