Polar filament discharge of Myxobolus cerebralis actinospores is triggered by combined non-specific mechanical and chemical cues

Abstract

S U M M A R YThis study presents initial evidence for the requirement of both chemical and mechanical stimuli to discharge polar capsules of Myxobolus cerebralis actinospores, the causative agent of salmonid whirling disease. The obligate need for combined discharge triggers was concluded from data obtained in a before/after experimental set-up carried out with individual locally immobilized actinospores. Homogenized rainbow trout mucus as chemostimulus and tangency of the apical region of the spores to achieve mechanical stimulation were applied subsequently. The actinospores showed discharged polar filaments exclusively when mucus substrate application was followed by touching the polar capsule-bearing region, but not when either stimulus was offered solely to the same individuals. We measured filament discharge rates to mucus preparations in a microscopic assay using supplementary vibration stimuli to ensure mechanical excitation. The actinospores responded similarly to different frequencies, which suggested a touch-sensitive recognition mechanism. Discharge specificity for salmonid mucus could not be confirmed, as mucus of common carp and bream could trigger similar filament expulsion rates. To a lesser extent homogenized frog epidermis and bovine submaxillary mucin could also stimulate the attachment reaction. In contrast, mucus of a pulmonate freshwater snail elicited no response.

Myxozoan transmission via actinospores: new insights into mechanisms and adaptations for host invasion

“…In M. cerebralis actinospores, there is a putative chemically triggered thigmoperceptive structure (Kallert et al 2005 a), suggested by the observation that the actinospores only discharged their filament when a chemical (mucusderived) signal was present prior to physical contact. An intrinsic retraction mechanism was always observed immediately after discharge of the filament in several species.…”

Tetracapsuloides bryosalmonae (Myxozoa: Malacosporea) portal of entry into the fish host

“…Rainbow trout mucus was obtained and purified according to Kallert et al (2005). Briefly, trout mucus was scraped off, homogenized, and centrifuged, and the supernatant was then used for activation.…”

“…Briefly, trout mucus was scraped off, homogenized, and centrifuged, and the supernatant was then used for activation. Spores were activated using the modified procedure of Kallert et al (2005) by pipetting 14 µl of spore suspension and 6 µl of trout mucus homogenate up and down 3×. Subsequently, a cover slip was carefully placed on the slide without applying any pressure and the spores were investigated with a phase contrast microscope (Zeiss).…”

“…Spores discharge polar filaments that attach to the host. Subsequently, the sporoplasm leaves the spore shell and penetrates the epithelium of the host (Kallert et al 2005(Kallert et al , 2007. As the spores of T. bryosalmonae that are released from the bryozoan host (malacospores) have the same cellular organization as the actinospores of myxosporeans, a similar mode of infection can be postulated.…”

“…Before this time, only intact spores adjacent to the gills were detected in histological sections, with sporoplasms still being enclosed by valve cells. Mechanisms and stimuli that induce actinospore activation, firing of polar filaments and release of the sporoplasm have been investigated in previous studies (Kallert et al 2005, Yokoyama et al 2006, Kallert et al 2007). Differences in reaction time after chemical or a combination of chemical and mechanical stimulation were noted between actinospores of various myxozoan species.…”