Diapause in tardigrades: a study of factors involved in encystment

Abstract

SUMMARYStressful environmental conditions limit survival, growth and reproduction, or these conditions induce resting stages indicated as dormancy. Tardigrades represent one of the few animal phyla able to perform both forms of dormancy: quiescence and diapause. Different forms of cryptobiosis (quiescence) are widespread and well studied, while little attention has been devoted to the adaptive meaning of encystment (diapause). Our goal was to determine the environmental factors and token stimuli involved in the encystment process of tardigrades. The eutardigrade Amphibolus volubilis, a species able to produce two types of cyst (type 1 and type 2), was considered. Laboratory experiments and long-term studies on cyst dynamics of a natural population were conducted. Laboratory experiments demonstrated that active tardigrades collected in April produced mainly type 2 cysts, whereas animals collected in November produced mainly type 1 cysts, indicating that the different responses are functions of the physiological state at the time they were collected. The dynamics of the two types of cyst show opposite seasonal trends: type 2 cysts are present only during the warm season and type 1 cysts are present during the cold season. Temperature represents the environmental factor involved in induction, maintenance and termination of the cyst. We also obtained evidence that A. volubilis is able to perform both diapause and cryptobiosis, even overlapping the two phenomena. The induction phase of tardigrade encystment can be compared to the induction phase of insect diapause, also indicating an involvement of endogenous factors in tardigrade encystment. As in insect diapause, tardigrade encystment can be considered a diapausing state controlled by exogenous and endogenous stimuli.

Identification of the trehalose-6-phosphate synthase (tps) gene in desiccation tolerant and intolerant tardigrades

“…A peculiar finding was the amplification of the tps gene in the limnic species Dactylobiotus parthenogeneticus, which was not able to survive desiccation (Table III) (Powell et al 1986a,b;Wienkem 1990;Becker et al 1996), as a defence against reactive oxygen species (Banaroudj et al 2001;França et al 2007), and in the regulation process of different biological functions, such as moulting (Chung 2008). In tardigrades moulting is strictly correlated with encystment, a diapause stage involving a reduction in metabolism, the loss of a small amount of body water, and the synthesis of several new cuticular layers without ecdysis (Guidetti et al 2006(Guidetti et al , 2008. Considering that D. parthenogeneticus is a limnic species able to encyst (Guidetti et al 2006, the most probable scenario could be that trehalose is involved in the regulation of the encystment process.…”

Heat shock proteins in encysted and anhydrobiotic eutardigrades

“…Encystment represents one of the enigmatic dormancy states of tardigrades and the causes of its induction and evolution remain still unclear (Guidetti et al 2008). Even though several morphological changes occur during the encystment process ) almost nothing is known about its physiology.…”

“…For the first time, our data revealed that both Hsp70 and Hsp90 are not up-regulated in encysted specimens of B. volubilis with respect to desiccated or hydrated specimens. This absence could be due to the fact that tardigrades undergo encystment in advance to environmental adversities (Guidetti et al 2008) so they probably are not in a stressed state facing stressful conditions that need high level of Hsps. To protect themselves from the coming environmental stresses (related to summer period), and therefore to maintain their homeostasis, the tardigrade cysts minimize exchange with their environment by remaining within their old cuticles (exuvia) and producing further cuticular layers .…”

“…Two types of cysts are produced, which show opposite seasonal trends. The type 2 cysts are the most abundant and are produced only during the warm season while the type 1 cysts are very scarce and are present only during the cold season (Guidetti et al , 2008. Consequently for B. volubilis three different physiological conditions, namely hydrated (active) and desiccated (anhydrobiotic) animals and type 2 cysts (diapause) were used.…”

“…Their persistence in these habitats is related to a wide variety of life histories and dormancy strategies . Encystment, cyclomorphosis and resting eggs are considered types of diapauses and are well documented in terrestrial and freshwater tardigrades (Hansen, and Katholm 2002;Guidetti et al 2006Guidetti et al , 2008Guidetti et al , 2011Møbjerg et al 2007;Halberg et al 2009;Altiero et al 2010). Encystment involves the synthesis of several cuticular layers, a reduction in metabolism, and the loss of a small amount of body water (Hansen, and Katholm 2002;Guidetti et al 2006Guidetti et al , 2008Guidetti et al , 2011.…”