15 research outputs found
Variations in Quercus sp. pollen seasons (1996–2011) in Poznań, Poland, in relation to meteorological parameters
Cytology and Cytogenetics
Several cytological aspects have been considered in tardigrades. Firstly,
the cell constancy which is not a true eutely being several mitoses present even after
hatching, even though some organs, such epidermis and nervous ganglia, have the
same cell number in juveniles and adults. The total number of these cells is speciesspecific.
Then the ultrastructure of cuticle, epidermis, feeding and digestive apparatus,
excretory and osmoregulatory organs, muscles, nerve cells, sensory cells and
storage cells has been considered. Instead, the ultrastructure of the germ cells has
been considered in the chapter on reproduction. With regard to chromosome number
and shape, it has been observed that generally there is little difference among the
species (n ¼ 5 or n ¼ 6), but several cases of polyploid populations exist, often very
similar to diploid populations from a morphological point of view. In most cases the
polyploid populations do not have males and reproduce by apomixis. Studies on the
genome size have confirmed the presence of polyploid populations, as well as the
presence of nuclei with multiple amounts of DNA within the same specimen. The
genome size of the tardigrades is always relatively small and does not seem related to
phylogenetic lineages. Studies on tardigrade genomes have placed this phylum at the
centre of discussions on the evolution of Metazoa and have considered the role of
horizontal gene transfer in animal evolution with contrasting results
Dormancy in Freshwater Tardigrades
For more than two centuries, tardigrades have been well known for their ability to undergo dormancy. However, this capability has been well studied mainly in the so-called limnoterrestrial species, i.e., in the species colonizing moist terrestrial habitats, such as mosses, lichens, and leaf litter. In these kinds of substrates, tardigrades are active only when a film of water is available around their body so in this condition they behave like aquatic animals. When the substrate dries or freezes, tardigrades achieve dormancy (quiescence) by entering cryptobiosis, specifically anhydrobiosis or cryobiosis, respectively. In freshwater habitats, both forms of cryptobiosis have been verified only in species able to live both in freshwater and terrestrial habitats. In the truly freshwater (or limnic) species, anhydrobiosis has not been verified, while cryobiosis has been confirmed in a few species. Another dormancy phenomenon bound to diapause is frequent in freshwater species: encystment (sometimes found even in limnoterrestrial species). The cyst state, which involves deep structural and physiological modifications, has been known from the beginning of the past century, but only recently has its morphology and inducing factors been studied in depth. Although data on molecular mechanisms allowing cryptobiosis are available, this information does not exist for encystment
Respiration Measurements of Individual Tardigrades of the Species <i>Richtersius</i> cf <i>coronifer</i> as a Function of Temperature and Salinity and Termination of Anhydrobiosis
Ultrastructure of the trophic chamber and nutritive cord of Aspidiotus hederae (Homoptera, coccoidea)
Reproduction, Development and Life Cycles
In tardigrades reproduction occurs only through eggs, fertilized or
unfertilized, and therefore only through gametes. Tardigrades exploit several reproductive
modes, amphimixis, self-fertilization and thelytokous parthenogenesis (both
apomixis and automixis). These modes are often in close relationship with the
colonized environment. As regards sexuality, tardigrades can be gonochoristic
(bisexual or unisexual) or hermaphroditic. The anatomy of the reproductive apparatus
of males, females and hermaphrodites and the maturative patterns of male and
female germinal elements are presented and discussed, as well as the ultrastructure of
spermatozoa and eggs, including their phylogenetic implications. In addition, mating
and fertilization patterns, embryonic and post-embryonic development, sexual
dimorphism and parental care are considered and discussed. Finally, vegetative
reproduction does not occur in tardigrades, and their capability to regenerate is
limited to a physiological tissue restoration of a few cells
Experimentally induced repeated anhydrobiosis in the Eutardigrade Richtersius coronifer
Tardigrades represent one of the main animal groups with anhydrobiotic capacity at any stage of their life cycle. The ability of tardigrades to survive repeated cycles of anhydrobiosis has rarely been studied but is of interest to understand the factors constraining anhydrobiotic survival. The main objective of this study was to investigate the patterns of survival of the eutardigrade Richtersius coronifer under repeated cycles of desiccation, and the potential effect of repeated desiccation on size, shape and number of storage cells. We also analyzed potential change in body size, gut content and frequency of mitotic storage cells. Specimens were kept under non-cultured conditions and desiccated under controlled relative humidity. After each desiccation cycle 10 specimens were selected for analysis of morphometric characteristics and mitosis. The study demonstrates that tardigrades may survive up to 6 repeated desiccations, with declining survival rates with increased numberof desiccations. We found a significantly higher proportion of animals that were unable to contract properly into a tun stage during the desiccation process at the 5th and 6th desiccations. Also total number of storage cells declined at the 5th and 6th desiccations, while no effect on storage cell size was observed. The frequency of mitotic storage cells tended to decline with higher number of desiccation cycles. Our study shows that the number of consecutive cycles of anhydrobiosis that R. coronifer may undergo is limited, with increased inability for tun formation and energetic constraints as possible causal factors
The Terrestrial and Freshwater Tardigrada of Northeastern North America, with New Records from Maine
Environmental Adaptations:Encystment and Cyclomorphosis
Stressful environmental conditions generally limit animal survival,
growth, and reproduction and may induce dormancy in the form of various resting
stages. Tardigrades represent one of a few animal phyla in which different forms of
dormancy are frequently encountered. One of these forms, cryptobiosis, a quick
response to sudden changes in the environment, has gained a great deal of attention,
whereas much less is known of the slower emerging form of dormancy, diapause. In
this review we present the current knowledge of diapause in tardigrades.
Diapause in tardigrades, represented by encystement and cyclomorphosis, is
likely controlled by exogenous stimuli, such as temperature and oxygen tension,
and perhaps also by endogenous stimuli. These stimuli initiate and direct successive
phases of deep morphological transformations within the individual. Encystment is
characterized by tardigrades that lie dormant\u2014in diapause\u2014within retained cuticular
coats (exuvia). The ability to form cysts is likely widespread but presently only
confirmed for a limited number of species.
In tardigrades, cyclomorphosis was first reported as a characteristic of the marine
eutardigrade genus Halobiotus. This phenomenon is characterized by pronounced
seasonal morphological changes and in Halobiotus involves stages with an extra
protecting cuticle. Cyst formation in moss-dwelling limnic species may also occur as
part of a seasonal cyclic event and can thus be viewed as part of a cyclomorphosis.
Therefore, whereas diapause generally seems to be an optional response to environmental
changes, it may also be an obligate part of the life cycle.
The evolution of encystment and cyclomorphosis finds its starting point in the
molting process. Both phenomena represent an adaptation to environmental constraints.
Notably, the evolution of diapause is not necessarily an alternative to cryptobiosis, and
some tardigrades may enter both forms of dormancy. The simultaneous occurrence of several adaptive strategies within tardigrades has largely increased the resistance of
these enigmatic animals toward extreme environmental stress