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A Model on the Evolution of Cryptobiosis

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A Model on the Evolution of Cryptobiosis Ann. Zool. Fennici 40: 331–340 ISSN 0003-455X Helsinki 29 August 2003 © Finnish Zoological and Botanical Publishing Board 2003 A model on the evolution of cryptobiosis K. Ingemar Jönsson* & Johannes Järemo Department of Theoretical Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden (*e-mail: [email protected]) Received 18 Nov. 2002, revised version received 5 Mar. 2003, accepted 6 Mar. 2003 Jönsson, K. I. & Järemo, J. 2003: A model on the evolution of cryptobiosis. — Ann. Zool. Fennici 40: 331–340. Cryptobiosis is an ametabolic state of life entered by some lower organisms (among metazoans mainly rotifers, tardigrades and nematodes) in response to adverse environ- mental conditions. Despite a long recognition of cryptobiotic organisms, the evolution- ary origin and life history consequences of this biological phenomenon have remained unexplored. We present one of the fi rst theoretical models on the evolution of cryptobi- osis, using a hypothetical population of marine tardigrades that migrates between open sea and the tidal zone as the model framework. Our model analyses the conditions under which investments into anhydrobiotic (cryptobiosis induced by desiccation) functions will evolve, and which factors affect the optimal level of such investments. In particular, we evaluate how the probability of being exposed to adverse conditions (getting stranded) and the consequences for survival of such exposure (getting desic- cated) affects the option for cryptobiosis to evolve. The optimal level of investment into anhydrobiotic traits increases with increasing probability of being stranded as well as with increasing negative survival effects of being stranded. However, our analysis shows that the effect on survival of being stranded is a more important parameter than the probability of stranding for the evolution of anhydrobiosis. The existing, although limited, evidence from empirical studies seems to support some of these predictions. Introduction otic abilities are often separated into those in which the cryptobiotic state may be entered Cryptobiosis is the collective name for an only within a specifi c (ontogenetic) life stage, ametabolic state of life utilized by some organ- and those that may enter cryptobiosis over the isms to overcome periods of unfavourable entire life cycle (Crowe 1971, Wright et al. environmental conditions (Keilin 1959, Crowe 1992). The fi rst category includes species from 1975, Wright et al. 1992, Kinchin 1994). Cryp- the taxa Arthropoda, Crustacea, Brachiopoda, tobiotic organisms are known from both the Insecta, spores of various fungi and bacteria, plant and animal kingdom, but in animals only and pollen and seeds of some plants, while the among invertebrates (Wright et al. 1992). The second category mainly includes species from main factors inducing the cryptobiotic state are Protozoa, Rotifera, Nematoda, Tardigrada, and desiccation (anhydrobiosis), freezing (cryo- various species of mosses, lichens and algae, biosis) and oxygen defi ciency (anoxybiosis, as well as some higher plants (Keilin 1959, Wright et al. 1992). Organisms with cryptobi- Wright et al. 1992). 332 Jönsson & Järemo • ANN. ZOOL. FENNICI Vol. 40 Fig. 1. Tardigrade (a) in a hydrated active state (light microscopy photo by K. I. Jöns- son) and (b) in a desiccated anhydrobiotic state (SEM photo by R. M. Kristensen). In the cryptobiotic state, several fundamental ably taken in populations living in seashore and biological characteristics are affected. Since all tidal habitats, where great variation in the water metabolic processes have stopped, reproduction, availability created a strong selection for desic- development, and repair are prevented. There- cation tolerance. Some tardigrade populations fore, as a life history component cryptobiosis of the genus Echiniscoides and Archechiniscus is mainly characterized by survival. However, inhabiting littoral marine environments show a to successfully achieve a high survival in the capacity to survive desiccation (Grøngaard et al. cryptobiotic state, the organism has to make 1990, Wright et al. 1992), whereas other marine biochemical preparations of the body, such as the tardigrades cannot do so. production of carbohydrates to replace the lost In this paper, we present a theoretical model water in anhydrobiotic organisms (e.g., Crowe on the evolution of anhydrobiosis (cryptobiosis 2002). Such processes are energetically costly induced by desiccation). The model describes (Jönsson & Rebecchi 2002), and will withdraw a hypothetical population of an ancient marine resources from potential use in other life history tardigrade inhabiting the littoral zone, where the functions, e.g., reproduction. Thus, most likely risk of exposure to desiccation creates a force there are trade-offs between investment in cryp- of selection for anhydrobiotic ability. The main tobiotic functions and investment in other life purpose of the analysis is to evaluate two general history functions. aspects of anhydrobiotic selection: the prob- Tardigrades (phylum Tardigrada, Fig. 1) are ability of being exposed to a time period of dry well known for their cryptobiotic capacity (e.g., environmental conditions, and the consequences Wright et al. 1992) and occur in a variety of on survival of such exposure. Based on our anal- ecosystems, some of which represent the most ysis, we propose a set of conditions that should extreme natural habitats on earth (e.g., continental favour energy investments in traits that improve Antarctica; Sømme & Meier 1995). They repre- the animalʼs anhydrobiotic capacity. sent an important component of the meiofauna in habitats exposed to desiccation, and may survive for several years in a cryptobiotic state (Jönsson The model & Bertolani 2001, Guidetti & Jönsson 2002). Although the ability of tardigrades to enter General model scenario cryptobiosis has been known for a long time, and has been subject to numerous investigations Our starting point is a non-anhydrobiotic popula- (see Wright et al. 1992, Wright 2001), the evo- tion of ancient marine tardigrades that migrates lutionary background to this ability has not been (actively or passively) between the tidal zone studied much. May (1951) suggested that marine and permanent water. Such migratory behaviour tardigrades were the ancestors of terrestrial and could arise from generally better foraging condi- limnic tardigrades, and that the transition from tions in the tidal zone. Consequently, the organ- marine to terrestrial forms was accompanied by isms forage in the tidal zone during high tide, the acquisition of a tolerance to anoxia and des- and return to the non-tidal zone when the water iccation, and ultimately of cryptobiosis. If this withdraws in order to avoid stranding and sub- scenario is correct, the initial evolutionary steps sequent desiccation. We assume that egg laying towards cryptobiotic tardigrades were prob- in these marine tardigrades takes place in open ANN. ZOOL. FENNICI Vol. 40 • Evolution of cryptobiosis 333 water and that eggs are not able to migrate to the survival parameter, may be distinguished (see tidal zone during high tide (in tardigrades, eggs Jönsson et al. 1995a, 1995b, 1998). A deduction of are either laid freely in the substrate or within the function (Eq. 1) from the Euler–Lotka equation is moulted cuticle (Bertolani 1983)). The reproduc- provided in the Appendix. The point of offspring tive cycle of our model population then begins release, which in the present case is equivalent to with a prebreeding period of foraging in the egg-laying, separates the two phases. tidal zone followed by migration to open water. Now assume the existence of within popula- During the period of foraging and resource accu- tion genetic variation in the ability to survive mulation, eggs are developed. If the population partial or complete desiccation, generating succeeds in migrating back to open water, the continuous variation in traits related to this abil- animals moult and lay their eggs, and in the fol- ity. Such traits may include storage of energy, lowing postbreeding period they migrate back to synthesis of protectant molecules or enzymes, the tidal zone to start a new period of foraging. cuticular structures reducing transpiration, However, the population runs the risk of being aggregation behaviours, or any other trait that exposed to a dry period (low tide) during which increase the stress tolerance of the animal the animals cannot return to open water and (Wright et al. 1992, Jönsson 2001). We call these which exposes them to desiccation stress. The traits “anhydrobiotic traits” and assume that the model assumes no parental care. ability to survive a period of desiccation depends The model scenario corresponds well to a on the amount of resources invested into such marine environment with tidal changes in water traits. The magnitude of these investments is availability, e.g., the supposed environment here denoted by K, which is the phenotypic trait under which the fi rst steps towards anhydro- that we evaluate below. biosis in tardigrades may have been taken (May We assume that anhydrobiotic traits incur 1951). However, the scenario could just as well costs in terms of a reduction in survival under represent the conditions under which anhyd- non-desiccating conditions by withdrawing robiosis evolved in any other metazoan. Also, resources from this life history trait. Survival because the general life history effects should may then be modelled by a function Pi that be similar regardless
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