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HETEROGONY | ||
Heterogony is a form of life cycle in which parthenogenetic and sexual reproduction alternate. Heterogony is found in groups as diverse as the Cladocera (Crustacea), Rotatoria (syn. Rotifera; Aschelminthes), Malacobothrii (Plathelminthes) and the Aphidina (Insecta). | ||
Why heterogony? One might ask why animals may evolve such a complex form of reproduction type. As species of very diverse taxonomic groups have evolved the same type, it seems only reasonable to look for environmental clues which are analogous to the different forms exhibiting heterogony and which could therefore stand as an evolutive factor enfavouring heterogony to take place. In this context it is important to note that the parthenogentic part of the life cycle always means hyperexploration and mass population of a so far bare environment whereas sexual reproduction correlates more or less to the preparation for the next explorative parthenogenetic stage. Keeping this in mind, we will, in the following, have a closer look on the heterogony cyles and environmental clues in the four groups known to exhibit heterogony. |
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1. Cladocera Most species of both, cladocera and rotatoria, live as cosmopolitans in the freshwater. In both groups heterogony can mainly be observed in populations living in regions with a clear summer-winter seasonal pattern, thus where environmental conditions change relatively reproducively and drastically. In tropical waters heterogony is less dominant. The cladoceran life cycle begins in the early spring, when the first planctonic blooms (e.g. of chrysophyt algae) take place. Then, the youngsters (all of them are so called "partheno" females) exit the latency eggs. Utilizing the upcoming algal blooms, those partheno females rapidly reproduce themselves via parthenogensis. That means that they produce diploid eggs, also named "subitane eggs". From these, again parthenofemales are hatched. The parthenogenetic cycle can continue over zero up to several generations, depending on the environmental conditions. In a laboratory experiment, for example, Moina macrocopa could, under good conditions (much food and oxygen, low population density, constant temperature), be kept for 200 generations in the parthenogenetic mode. Under less good conditions, the animals instantaneously started to undergo sexual reproduction. This laboratory experiment finds it´s natural correlate in the different reproduction behaviour of cladoceran populations living in differetn types of waters. Thus, in big lakes with relatively constant seasons the populations normally exhibit a monocyclic pattern (many partheno- generations, but only one reproductive generation), whereas in little lakes and pools, which frequently run low in oxygen or which are easily overpopulated, usually a di- or even a polycyclic behaviour can be found. There, between a number of amicitic generations, are two or more phases of sexual reproduction are inserted. Sexual reproduction is enabled by the rapid switch of the partheno-stage to the sexual stage. Sexual females, then, produce diploid male eggs. The sex of an egg is determined only one hour before spawning. Therefore, cladocera show an extremely fast adaptation competence to rapidly upcoming deteriorating environmental conditions in a habitate. The sexual reproduction between males and sexualfemales results in the production of the formerly mentioned latency eggs. Interestingly, these are are not only capable of enduring a winter season and other bad conditions (therefore they are the bridge to the next year´s season) but often they can even survive being taken up, digested and excremented by birds (therefore they serve as distribution entity). To this end, in some species the latency eggs are enclosed in skinned carapax, which is called the ephippium (which is due to the saddle-like shape of the carapax in daphniids). BACK TO TOP |
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2. Rotatoria The rotatoria´s life cycle resembles very much that of cladocera This might be due to the identical reasons for it´s existence like in the crustacean group. Heterogony is found in all freshwater digonont rotatoria species insofar those inhabited waters exhibiting seasons with changing environmental conditions. As with the cladocerans, also in rotatoria, the start up of a season is characterized by explorative parthenogenetic reproduction. For Brachionus it has been determined that the individuals lay their first eggs already 24 hours after their own birth. They do so for the next two or three weeks and produce 2-3 eggs each day. Thus, a population of Brachionus can grow by a factor of 100 within only 10 days. The parthenogenic females of rotatoria are also called Ñamicticâ (not mixing sexually). In contrast to the situation in cladocera, where sexual females produce diploid male eggs, the sexual period in rotatoria is beginning with so called Ñmicticâ females producing facultatively parthenogenetic haploid eggs, which had went through meiosis. Production of these eggs is stimulated by changing temperatures, rich food and elongated light exposure. The first, naturally unfertilized, eggs become males which themselves reproduce with the first mictic female generation. This results in fertilized latency eggs with dure shells. Latency eggs give rise to a new start with amictic females after a resting period of only 4 days or first after wekks, months or even years. As in cladocera, the heterogony is again the means for duration, exploration and distribution of inhabitants of and extreme environment. BACK TO TOP |
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3. Malacobothrii The group of the Malacobothrii is known to contain some of the most prominent parasite species (e.g. Fasciola, Schistosoma [bilharziosis], Leucochloridium, Paragonimus). In addition they are a group with outstandingly complex life cycles as they have refined the mechanisms of heterogony within their phylogeny presumably from a primitive two-generation model (one in a mollusc host, the other free-living, e.g. Fasciola hepatica) to a complex 4-host cycle (e.g. Strigea falconispalumbi). Common to the first two groups of the cladocera and the rotatoria is that the parthenogenetic exploration and the sexual refinement and remixing of genes to start a new cycle. In short, a malacobothriid life cycle contains at least an exploring, parthenogenetic miracidium, which is frequently living in the freshwater. This can then invade a first host or it can directly convert to an invasion larva, the so called cercaria. This invades the final host, where it maturates to the so called hermaphrodite distomum which again releases the free-living miracidia. Additionally, a number of 1 to 3 intermediate hosts can be inserted into the life cycle of a malacobothriid worm. BACK TO TOP |
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4. Aphidina Everyone knows them from their deteriorating effect on the beloved roseplant or any other plant held in or at the house: the greenfly of the group aphidina. This example is extremely good in explaining the extremely high potency of the heterogony strategy: explore (in this case your plants) by mass parthenogenetic production until a critical stage is reached. Then switch to sexual reproduction to prepare for harder times and for a new round of exploration (maybe in your neighbourhood). The aphidine life cycle again starts from a latency egg which carry so called fundatrices (founders, wingless females) which parthenogenetically reproduce at a very high rate. After a specific number of parthenogenetic generations, the production of males and females can be observed. These produce by sexual reproduction the latency eggs. Within the parthenogenetic generations also individuals with wings can occur. BACK TO TOP |