Halteria

The genus Halteria is abundant in many freshwater environments. The ubiquity of this genus is likely why observations date back hundreds of years. The original description of the genus is not clearly established, but it is possible that observations of Halteria date back to Antony van Leewenhoek's observations in 1675 as the fourth animalcule observed in an earthen pot full of rainwater. The organism he observed was small, swift, and seen to stand still before quickly changing direction and travelling straight, which is consistent with the characteristic movement of Halteria. Creating the new genus, Halteria, when the two species were found not to fit the subfamily Vorticellina, under which the genus Trichodina fell. Descriptions of Halteria at this time were still rather vague, focusing on the quick jumping movement that results from the beating of its cirri and the presence of oral cilia. Questions on the classification of Halteria have arisen again in more recent years. Halteria have been most commonly classified as a member of the oligotrich group of ciliates, because they possess the group's characteristic prominent oral cilia arranged in an incomplete circle. However, recent deep sequencing and RNA analysis of Halteria indicate that Halteria may be more closely related to oxytrichids than oligotrichs, suggesting the similarity in oral apparatus with oligotrichs is the result of convergent evolution. == Description ==

Halteria can exist in a trophic, ciliated stage or an encysted stage and the morphology of the cells varies significantly between stages. Within the mitochondria of H. geleiana, microorganisms have been observed within the matrix. The macronucleus is oblong in shape while the micronucleus is more globular. Encystment As Halteria cells transition from the trophic to the encysted stage, initially their globular bodies elongate, primarily at the anterior end, until the length of the cell has nearly doubled. Owing to the uneven elongation, the buccal cavity is flattened, the membranelles of the oral apparatus move closer to the centre of the cell and the rows of cirri move closer to the posterior end of the cell. While the cell stretches, the cytoplasm develops 5 μm long conical structures. After this stage of elongation, the cells become more rounded, and a mucous envelope is extruded. Also during this next stage of encystment, the conical structures formed in the cytoplasm attach to the outer layer of the developing cyst, called the ectocyst. Once attached to the ectocyst, the conical structures are called lepidosomes. After encystment, cysts use the mucous envelope to firmly attach to any available substrate. == Habitat and ecology ==

The genus Halteria consists of freshwater ciliates that typically live a planktonic lifestyle. The species Halteria grandinella is considered cosmopolitan, meaning that it is found in habitats across the world. Other species are less common and so they are less well defined, however frequent descriptions of Halteria grandinella have provided insight into the genus as a whole. Halteria are heterotrophic and unlike many closely related genera like Pelagohalteria, they have no photosynthetic endosymbionts. Halteria do frequently eat green algae which, when observed in food vacuoles, has led to misclassifications in the past when mistaken for endosymbionts. Species of Halteria play a particularly large role in many freshwater habitats as bacteriovores. In a study that used fluorescently labelled bacteria in fishponds to observe protistan bacterivory, ciliate grazing accounted for 56% of total protistan grazing and Halteria, along with two other ciliate genera, Pelagohalteria and Rimostrombidium were responsible approximately 71% of the total ciliate bacterivory. Halteria also act as prey for many metazoan predators. It has been proposed that the characteristic jumping behavior of Halteria was evolved as an escape strategy to avoid such predation. Much of the research related to Halteria is focused on their movement and their ecological roles. Halteria acts as a model organism for the study of their jumping movement through ciliary beating. It can be found in abundance in diverse freshwater habitats interacting with other organisms as both predators and prey. Halteria spend most of the time either stationary or moving smoothly through water propelled by the cilia at their anterior end. The halting jumping movement most associated with Halteria is the result of external stimulus such as currents, which is known because jumping in Halteria has been induced in a laboratory setting. Jumping behavior in Halteria requires 41% of the organism's total metabolic rate, and so employing it too frequently would be an inefficient use of energy. == Reproduction ==

Asexual reproduction Halteria can reproduce asexually by transverse binary fission. During this replication the majority of the ciliature that will be present on the daughter cells is formed de novo. The only exception to this is the oral ciliature of the parent cell which is inherited by the proter daughter cell. The parental cirri are resorbed by the cell during division and the cirri of both daughter cells are produced de novo from cirral anlagen and the oral ciliature of the opisthe daughter cell is generated de novo through the formation of an oral primordium at the posterior end of the cell. Both the macronucleus and micronucleus divide during the process resulting in two daughter cells that are genetically identical to the parent cell. Conjugation Halteria cells can reproduce sexually through a process that has been studied specifically in H. grandinella. During sexual reproduction, the ventral sides of two Halteria cells fuse. Various changes in morphology then occur through maturation divisions including a decrease in the number of cirri in both cells and the loss of buccal membranelles in one of the pair and the entire oral apparatus disappears in the other. The remaining membranelles are shared between the cells at the anterior end. On a nuclear level, during conjugation the original macronuclei fragment and the micronuclei mature and divide three times, with only one derivative of the first two divisions continuing to divide, forming two pronuclei in the third division. A pronuclei from each cell is exchanged and the two that end up in each cell fuse to form the synkaryon. The synkaryon divides twice with one derivative from each of the second divisions degenerating and the remaining derivatives becoming the new micronucleus and the macronucleus analge. After synkaryon division is complete, conjugates separate, now generating two cells with genetics distinct from the parent cells and from each other. == References ==