Troglomyces pusillus

Enghoff, Henrik & Santamaria, Sergi, 2015, Infectious intimacy and contaminated caves-three new species of ectoparasitic fungi (Ascomycota: Laboulbeniales) from blaniulid millipedes (Diplopoda: Julida) and inferences about their transmittal mechanisms, Organisms Diversity & Evolution (New York, N. Y.) 15 (2), pp. 249-263 : 259-262

Troglomyces pusillus

The diffuse distribution of thalli, as well as their occurrence on immature as well as mature hosts, suggests that infection takes place via the substrate rather than during copulation. The higher incidence (infection rate, number of thalli per host specimen) on adult hosts may be a function of size, longevity and activity level of the different stadia of the host: As a rule of thumb, the length of individual post-embryonic stages of arthropods increases through ontogeny ( Minelli and Fusco 2013), and available evidence suggests that this is specifically true in millipedes ( Blower 1974; Voigtländer 1987). Furthermore, immature millipedes are obviously less active than adults, as evidenced by the generally observed predominance of adults in pitfall samples.

Mode of transfer in relation to habitat

Although millipedes sometimes appear in dense masses, such behaviour is not known for the family Blaniulidae ; we therefore focus on direct transfer during courtship/copulation vs. indirect transfer via the substrate.

Of the three Troglomyces species described here, one ( T. pusillus ) has a cave-dwelling host and shows no site specificity on the host, suggesting parasite transfer via the substrate. The two others ( T. triandrus and T. bilabiatus ) have epigean hosts and show very strong site specificity strongly suggesting parasite transfer during host copulation. Considering that Laboulbeniales in general prefer a humid environment and that caves present stable, generally humid conditions, the conclusion lies near that our findings represent a general pattern: Site-specific fungi with sexual transfer on epigean hosts and non-site specific fungi with substrate-borne transfer in caves. Widening the view to other Troglomyces species ( Table 3), the cave-dwelling T. manfrediae shows no strong site specificity, but the two epigean species, T. rossii and T. botryandrus , show a similar lack of site specificity. Although all known Troglomyces species with strong site specificity and inferred transfer during host copulation have epigean hosts, Troglomyces species without strong site specificity and with inferred transfer via the substrate have been found on cave-dwelling as well as epigean millipede species ( Table 4).

Outside the genus Troglomyces , the three described species of Diplopodomyces are all cave-dwelling and parasitize millipedes of the order Callipodida , but whereas two of the species show no site specificity, suggesting substrate-borne transmission, Diplopodomyces veneris shows very pronounced site specificity, strongly suggesting sexual transfer ( Santamaria et al. 2014).

Five millipede-parasitizing species of the large genus Rickia have been described, all from epigean juliformian millipedes (family Harpagophoridae for R. siddharta , family Julidae for the rest). Rickia uncigeri is found all over the body but preferentially on the ventral side of head, as well as the anterior body rings and legs ( Scheloske 1969; Majewski 1974); on newly studied Danish Unciger foetidus (C.L. Koch, 1838) (one male, three females), thalli of R. uncigeri were found only on body rings 2–10, on the legs of these rings and occasionally on legs back to ring 15, as well as on the gnathochilarium. Rickia dendroiuli shows a similar distribution pattern: many thalli on the anterior legs (pairs 1–12), sometimes also on the ventral parts of the corresponding body rings, a few thalli also found on legs further back (to the middle of the millipede’ s body) and on the head, antennae and mouthparts—unpublished observations on four males and three females of Cylindroiulus latzeli (Berlese, 1884) from Italy. Rickia pachyiuli was found on legs and gonopods of male hosts ( Bechet and Bechet 1986); on newly studied Croatian Pachyiulus hungaricus (Karsch, 1881) (five males, seven females, NMW), thalli of R. pachyiuli were found on legs 1–19 and ventrally on the corresponding body rings (2–12)— when only few thalli were present, they were close to the gonopods in males and to the gonopore in females. Rickia laboulbenioides was found mostly on the second pair of legs of its host ( De Kesel et al. 2013). Finally, R. siddharta from Sri Lanka was found on the body rings and (mainly) at the base of the legs of its host ( Rossi and Balazuc 1977). The site specificity seen in R. pachyiuli and R. laboulbenioides is suggestive of sexual transfer, whereas the diffuse distribution of thalli in the other three suggests transfer via the substrate.

Finally, Triainomyces hollowayanus , parasite of an epigean giant pill millipede, order Sphaerotheriida , has been found only on the second pair of legs in female hosts ( Rossi and Weir 1998); although the fungus has so far not been found on male hosts, sexual transfer seems probable.

The overall pattern thus is that epigean millipedes host strongly site-specific as well as not site-specific Laboulbeniales . In contrast, fungi parasitizing cave-dwelling millipedes are generally not site-specific, which is consistent with the idea that substrate-borne transfer is favoured in the generally humid cave climate. (A positive correlation between humidity and substrate-borne infection of beetles with Laboulbeniales is implicit in the discussion by Scheloske [1969: 67–69]). The only exception is D. veneris . This very strongly site-specific species is in the special situation that it coexists with a congeneric species, D. lusitanipodos , on the same host species (sometimes even the same host individual); this may be the reason that a highly focused mode of transfer has evolved.

Role of defensive secretions?

Millipedes are known for their defensive chemicals which in blaniulids and other cylindrical millipedes are at least mainly released from an uninterrupted series of defence glands starting on body ring 6 and continuing almost to the hind end. A great diversity of chemicals has been reported from various millipedes ( Eisner et al. 1978; Shear et al. 2007; Vujisić et al. 2011; Bodner and Raspotnig 2012), and even between closely related species, the chemical profiles may differ significantly ( Vujisić et al. 2011; Bodner and Raspotnig 2012).

Some components of millipede defensive secretions are known to have an antifungal effect (e.g. Kuwahara et al. 2002; Makarov et al. 2014; Roncadori et al. 1985; Wu et al. 2007); thus, interspecific differences in the arsenal of chemicals may entail differences in susceptibility to parasitism by Laboulbeniales .

The defensive secretion of B. guttulatus was studied by Weatherston et al. (1971) and was found to contain lipoid esters (palmityl acetate, a monounsaturated C16-alcohol acetate and a monounsaturated C18-alcohol acetate). Quite different chemical spectra were found in two other blaniulids: Nopoiulus kochii (Gervais, 1847) and Cibiniulus phelpsii (Verhoeff, 1897) by Vujisić et al. (2014). The secretions of several additional blaniulids are currently under study by L. Vujisic, S. Makarov and others; preliminary results indicate that the secretion of A. pallidus does differ from those of B. guttulatus and B. tenuis .

Parthenogenesis—a means of escaping infection?

The limited available evidence suggests that T. triandrus is absent from male-less, presumably parthenogenetic populations of A. pallidus . Laboulbeniales have not been found on the often very abundant, parthenogenetic species Proteroiulus fuscus (Am Stein, 1857) although its bisexual congener P. hispanus is host of an unidentified fungus. There is as yet no numerical basis for claiming a significant correlation between parthenogenesis and freedom from the fungus, but given the very strong circumstantial evidence for transfer during courtship, switching to parthenogenesis is an efficient way to avoid the parasite ( Lehtonen et al. 2012).