Totipotent Stem Cell – an overview | ScienceDirect Topics

Asexual Reproduction

Although sex evolved early in animal evolution, agametic cloning from somatic tissue by asexual reproductive modes is prevalent in most of the soft-bodied invertebrates such as sponges, cnidarians, flatworms, annelids, and some echinoderms, as well as urochordates, a close relative of vertebrates. In sedentary corals, asexual reproduction is the clonal propagation in which an organism gives rise to the production of genetically identical replicates (ramets) by budding. Conversely, unitary animals are those that have a single body per genet (a genetic individual arising from one zygote by mitosis), which reproduce only sexually. Animals that reproduce asexually are highly regenerative, compared to obligate sexual reproducers.

In invertebrates, adult stem cells play a pivotal role in regeneration and asexual reproduction. Unlike vertebrate systems, invertebrate stem cells are not housed within a regulatory microenvironment (niche). Stem cells involved in agametic cloning in invertebrate taxa are of different kinds: choanocytes in sponges; interstitial cells in cnidarians, neoblast cells in planarians, and haemoblast stem cells in ascidians (Urochordata). Some metazoans like nematodes, rotifers, gastrotrichs and insects lack somatic stem cells and hence the inability to undergo agametic cloning. Different kinds of asexual reproduction occurring in various invertebrate animals are listed in Table2.

Table2. Taxonomic distribution of asexual reproduction in invertebrates

All sponges (phylum Porifera) can regenerate their bodies and reproduce asexually using totipotent stem cells. In addition, many freshwater sponges use choanocytes to develop dormant overwintering gemmules under unfavourable conditions. On return of favourable conditions, such multicellular propagules reactivate and regenerate genetically identical replicas of the original sponge.

In the simplest condition, asexual reproduction can take place by subdivision of an existing body into two or more multicellular parts, followed by regeneration of the missing parts. Placozoans, the basal metazoan animals without distinct tissues or organs, reproduce exclusively by fission, whereby two parts of the animal move away from each other until their connection is ruptured (Fig.1). In cnidarians, asexual reproduction is coupled with the sessile existence of the adult. In the freshwater hydrozoan, Hydra, reproduction occurs almost exclusively by budding, suppressing gamete formation (Fig.2). In the calyx region, the parent tissue cells continuously move towards the bud-forming area and get incorporated into the growing bud. Evidently, reorientation and repolarization of parent tissue play a major role in bud formation. In addition, epithelial stem cells that mediate the morphogenetic plasticity of the tissue in regeneration may also have a similar role in asexual budding.

Fig.1. Budding in Hydra vulgaris. Seen on the right side is a well grown bud and on the left, is early stage bud.

Fig.2. Binary fission in the placozoan, Trichoplax. (a) Before fission, (b)(d) Trichoplax progressing through asexual reproduction by fission.

In Hydra, a new polyp bud eventually becomes a separate individual clone. However, in the coral relatives of Hydra, the clones do not break off, but stay attached and become a branch as in plants. Such branching colonies are also found in the bryozoan, Membranipora. In some anemones, the adults autotomise (self-mutilate) a tentacle, which regenerates in to a small individual. In the scyphozoan cnidarian Aurelia, the polypoid scyphistoma undergoes horizontal fission (strobilation) into a stack of juvenile medusas, called ephyra (Fig.3). The ephyra transforms into male and female medusas, which produce planula larva by sexual reproduction. The planula then settles into a sedentary scyphistoma, thus continuing the life cycle. This is termed as alternation of generation or metagenesis. Whereas asexual reproduction produces genetically identical modules that may prolong the survival of the genotype, sexual reproduction enables genetic recombination and production of new coral genotypes to enhance fitness and survival of the species.

Fig.3. Life cycle of Aurelia to demonstrate alternation of generation.

Scyphozoans also reproduce asexually by podocyst formation, under conditions of limited food supply. Podocysts are cysts produced beneath the pedal discs of polyps of scyphozoans. They excyst small polyps that develop into active scyphistomae, which are capable of producing further podocysts as well as medusas by strobilation.

In the azooxanthellate soft corals, fragmentation of the colony occurs by autotomy. New clonal colonies in the reef corals are also formed by colony fragmentation as a result of storm or wave impacts. In the anemone, longitudinal fission is the common mode of asexual reproduction, although transverse fission occurs in some forms.

Asexual reproduction in sea anemones also occurs by basal laceration, which involves regeneration from a small piece of tissue that typically includes all the three body layers. Different asexual modes of reproduction found in corals reflect the extraordinary ability of cnidarian cell lines to differentiate, dedifferentiate and redifferentiate, providing their tissues with remarkable developmental plasticity.

Budding is also common in flatworms (Platyhelminthes), which have excellent regenerative abilities. In the paratomic fission, new individuals differentiate in a chain-like fashion from a parent worm before separating from it, while in architomy, the body simultaneously fragments and only thereafter individuals differentiate from the pieces. The acoelous turbellarians release their progeny by budding from the posterior margin of the body. In the nemertean worm, belonging to genus Lineus, sexually immature adults undergo transverse fragmentation with subsequent regeneration. In the polychaete worm (Annelida), Dodecaceria, the adult worm fragments into several individual segments, each of which reconstitutes a new head and a tail by renewed segment proliferation.

The commonly occurring asexual mode of reproduction in sea stars and brittle stars is the division of the body across the disk, termed fissiparity. Each resulting part regenerates a complete individual which can split again. The population of the fissiparous sea star Coscinasteria stenuispina at Rio de Janeiro, Brazil, appears to be sustained only by fission, although these sea stars are capable of gamete production. Another asexual method is autotomy, by which a whole new animal is regenerated from a single arm or even part of an arm. Remarkably, fragmentation, fission or budding occur regularly in echinoderm larvae. Transverse fission has also been reported in the sea cucumber, Holothuria edulis.

Both exogenous and endogenous factors appear to be involved in regulating asexual and sexual reproduction in echinoderms. Whereas the asexual mode is related to small body size, sexual maturity is attained after an individual reaches a certain size. Regeneration after fission is aided by existing pluripotent stem cells or de-differentiation of tissues into stem cells. Conversely, regeneration after autotomy, which takes place across predefined planes, involves the formation of an extensively proliferating blastema. However, both these regenerative processes are nerve-dependent and require neurotransmitters and neuropeptides, as growth factors.

The ascidian urochordates, sometimes called sea-squirts, are colonial marine invertebrates with remarkable budding capacity to form new entities from existing structures. Two types of stem cells (epithelial and blood-born) are employed in the budding of ascidians. Palleal budding (buds developed from thoracic body region) and stolonial budding (buds originating from distal tip of developing stolon) are derivatives of epithelial stem cells, whereas in vascular budding, the buds are produced by the totipotent haemoblast stem cells inside the vascular system.

Parthenogenesis is the development of a new offspring from an unfertilized egg. Parthenogenetic lineages occur in many insect species, but are widespread among other invertebrate taxa (Table1). It entails modification or absence of meiosis so that the eggs remain diploid and do not have to fuse with sperm to give rise to a diploid zygote. This kind of parthenogenesis is termed as apomictic, which produces genetically identical modules of the same genet (typical asexual reproduction). In contrast, automixis (meiotic parthenogenesis) restores diploidy by the fusion of the egg with the second polar body (e.g., free-living nematode, Rhabditis). Obviously, the resultant modules may not be the exact genetic replicates of the mother.

In insects, parthenogenesis may be thelytokous (female producing), arrhenotokous (male producing) or amphitokous (producing either sex). Haploid parthenogenesis is a special case in which the oocytes undergo regular meiotic division. If the eggs are fertilized, the offspring is a female, and if the eggs remain unfertilized, then parthenogenetic development results in a male offspring, which is haploid in its somatic tissues (e.g., the haplodiploid Hymenopteran insects). In social insects like the subterranean termite, Reticuliter messperatus, the queens produce new queens asexually by thelytokous parthenogenesis, but produce other colony members (workers and soldiers) by sexual reproduction. The parthenogenetic production of these new queens is achieved by the closing of the eggs micropyle (sperm gates) to prevent sperm entry. Yet another type, namely obligate parthenogenesis, occurs in bdelloid rotifers, in which sexual reproduction never takes place due to the lack of males in the population.

In the cladoceran rotifers and aphid insects, parthenogenesis occurs cyclically together with bouts of sexual reproduction. This is called cyclical parthenogenesis. In Daphnia, parthenogenetic reproduction takes place for one to several generations during favourable conditions, followed by sexual reproduction under unfavourable environments. The sexually produced long-lived dormant eggs hatch once favourable conditions return. By this alternation of generations, favourable environmental conditions can be exploited to increase the number of offspring by parthenogenetic reproduction, whereas the periodical appearance of one or more sexual generations will ensure genetic advantages such as increased heterosis, and re-assortment of genetic characters.

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Totipotent Stem Cell - an overview | ScienceDirect Topics