7 research outputs found
Overexpression of PaHAP3A stimulates differentiation of ectopic embryos from maturing somatic embryos of Norway spruce
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
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