Parasite host specificity related to host susceptibility to be killed?
In a previous post I introduced the Laboulbeniales: microscopic ascomycetous fungi that parasitize invertebrates, mainly beetles. The past few weeks my head was running fast on reading and developing theories on those mysterious organisms.
Parasite having its own troubles
A parasite, any parasite, has one huge problem: an individual must meet all its nutritional demands and avoid all its enemies on that one host or it dies. Especially for ectoparasites, as are Laboulbeniales, living attached to their host can be quite a challenge: physiological changes can occur within the host (e.g. because of the parasite’s feeding behavior), predators can chase the host, and so on.
Say that all these possible problems of attaching to and feeding on a single host are solved. There are left two levels of danger for the parasite. First, a parasite upon a specific host species may be more susceptible to its own enemies than when it occurs on another host. The parasite itself has its potential predators, parasites and/or competitors. It must face all of them without leaving its host. Obligate parasites do not seem to have an easy life. Second, if different hosts are “evenly good” for attachment and feeding, one host may be safest from enemies, possibly narrowing down the range of hosts.
Laboulbeniales exhibiting host specificity
I started thinking, after reading through this, about my Laboulbeniales and their phenomenon of host specificity, which is widespread in the order; one parasite species lives in association with only one single or a few host species. In addition to this host specificity, Laboulbeniales often exhibit remarkably morphological adaptions to exact positions on the host body. This particularity is often referred to as position specificity.
Let’s continue thinking about the host specificity. This feature has mostly been attributed to phylogenetic relatedness of hosts, as - indeed - it will be great and exciting to find congruent phylogenies of hosts and parasites, but the challenging research will be to explain how morphologically similar Laboulbeniales can occur upon phylogenetically unrelated hosts.
For example, Laboulbenia ecitonis G.Blum is a plurivorous species, saying it infects unrelated hosts: it is parasitic to the army ant Eciton quadriglume and it grows quite well on histerid beetles and mites that lodge in the ant colony. Another species of Eciton, E. hamatum, however, has a cohabitant species of staphylinid beetle, but in this case the ant appears to be immune to its guest’s fungal disease. A species of Laboulbenia is quite common on the beetle guest, but it has yet to be found on the ant that acts as host.
Host shifts and habitat
Based on field data, we know that Laboulbeniales with the capacity to infect several host species, will only be found on cohabiting hosts. This plurivory may have three causes: 1) a similar body chemistry of the cohabiting hosts, 2) incidental infections, and 3) adaptation to exploit other host species. Host shifts of species of the genus Laboulbenia (L. davidsonii W.Rossi) have been suggested to occur between Cicindelinae (tiger beetles) and other Carabidae living in the same habitat, resulting in variation in fungal morphology or nutritional requirements.
It seems that the habitat, which is chosen by the host, has a determining effect upon the success of Laboulbeniales. This observation helps to understand why in certain habitats a certain parasite can be found on a certain, atypical host, right?
Differences in host susceptibility
Now I propose the following suggestion: Laboulbeniales are more host-specific in one habitat than in another, because of differences in host susceptibility to being killed.
In a certain habitat a given host (group) can be more susceptible for its own predators, or more predators may be present in that current habitat. Say there's a species of Laboulbenia L growing on hosts A and B. Both hosts A and B are equal in providing nutrients and attachment features for L. Host A is present in habitat A and B in habitat B, with a much lower number of predators that attack the host. This infers that host B will be safer from enemies. My guess would be that natural selection will favor L attacking host B, as L upon host A will more easily fail in completing its complete life cycle and producing fertile ascospores.
In a broader perspective, we might think of morphologically closely related Laboulbeniales and their respective different hosts or host groups, to ask ourselves the question whether a given parasite species was first on the one group, but could also grow on the other group of hosts, and did so because the first group is more susceptible to certain predators in - in this case - even the same habitat (as both host groups share the same habitat here). This is the situation for Laboulbenia davidsonii, as well as for the newly described species Laboulbenia littoralis De Kesel & Haelewaters sp. nov.
Let’s thoroughly think this through. Oh, and comments are most welcome too.
Benjamin RK 1971. Introduction and Supplement to Roland Thaxter’s Contribution towards a Monograph of the Laboulbeniaceae. Bibliotheca Mycologica 80: 1-155.
De Kesel A & D Haelewaters. Laboulbenia slackensis and L. littoralis sp. nov. (Ascomycota, Laboulbeniales), two sibling species as a result of ecological speciation. Mycologia (in press).
Rossi W 2011. New species of Laboulbenia from Ecuador, with evidence for host switch in the Laboulbeniales. Mycologia 103: 184-194.
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