Everything about Ciliates totally explained
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The
ciliates are one of the most important groups of
protists, common almost everywhere there's water — lakes, ponds, oceans, rivers, and soils, with many
ectosymbiotic and
endosymbiotic members, as well as some obligate and opportunistic parasites included. Ciliates tend to be large
protozoa, a few reaching 2 mm in length, and are some of the most complex in structure. The name
ciliate comes from the presence of hair-like organelles called
cilia, which are identical in structure to
flagella but typically shorter and present in much larger numbers with a different undulating pattern than flagella. Cilia occur in all members of the group (although the peculiar
suctoria only have them for part of the life-cycle) and are variously used in swimming, crawling, attachment, feeding, and sensation.
Cell structure
Unlike other
eukaryotes, ciliates have two different sorts of
nuclei: a small,
diploid micronucleus (reproduction), and a large,
polyploid macronucleus (general cell regulation). The latter is generated from the micronucleus by amplification of the genome and heavy editing. Division of the macronucleus occurs by amitosis, the segregation of the chromosomes is by a process, whose mechanism is unknown. This process is by no means perfect, and after about 200 generations the cell shows signs of aging. Periodically the macronuclei must be regenerated from the micronuclei. In most, this occurs during
conjugation. Here two cells line up, the micronuclei undergo
meiosis, some of the
haploid daughters are exchanged and then fuse to form new micronuclei and macronuclei.
Most ciliate cell membranes are surrounded by a pellicle - a clear, elastic layer of protein. With a few exceptions, there's a distinct
cytostome or mouth where ingestion takes place. Food
vacuoles are formed through
phagocytosis and typically follow a particular path through the cell as their contents are digested and broken down via
lysosomes so the substances the vacuole contains are then small enough to
diffuse through the membrane of the food vacuole into the cell. Anything left in the food vacuole by the time it reaches the cytoproct (anus) is discharged via
exocytosis. Most ciliates also have one or more prominent contractile vacuoles, which collect water and expel it from the cell to maintain osmotic pressure, or in some function to maintain ionic balance. These often have a distinctive star-shape, with each point being a collecting tube.
Feeding
Most ciliates feed on smaller organisms (heterotrophic), such as
bacteria and
algae, and detritus swept into the oral groove (mouth) by modified oral cilia. This usually include a series of membranelles to the left of the mouth and a paroral membrane to its right, both of which arise from
polykinetids, groups of many cilia together with associated structures. The food is moved by the cilia through the mouth pore into the gullet, which forms food vacuoles.
This varies considerably, however. Some ciliates are mouthless and feed by absorption, while others are predatory and feed on other protozoa and in particular on other ciliates. This includes the suctoria, which feed through several specialized tentacles
Reproduction
Ciliates can undergo both asexual and sexual reproduction. Asexual reproduction occurs by binary fission. The micronucleus undergoes by mitosis and the macronucleus elogates and splits in half. Both new cells each obtain a copy of the micronucleus and macronucleus. Sexual reproduction involves
conjugation, which involve two cells. After conjugation, the two cells divide, forming four new cells.
Specialized structures
In some forms there are also body polykinetids, for instance, among the
spirotrichs where they generally form bristles called
cirri. More often body cilia are arranged in
mono- and
dikinetids, which respectively include one and two kinetosomes (basal bodies), each of which may support a cilium. These are arranged into rows called
kineties, which run from the anterior to posterior of the cell. The body and oral kinetids make up the
infraciliature, an organization unique to the ciliates and important in their classification, and include various fibrils and
microtubules involved in coordinating the cilia.
The infraciliature is one of the main component of the cell cortex. Another are the
alveoli, small vesicles under the cell membrane that are packed against it to form a
pellicle maintaining the cell's shape, which varies from flexible and contractile to rigid. Numerous
mitochondria and
extrusomes are also generally present. The presence of alveoli, the structure of the cilia, the form of mitosis and various other details indicate a close relationship between the ciliates,
Apicomplexa, and
dinoflagellates. These superficially dissimilar groups make up the
alveolates.
Fossil record
Until recently, the oldest ciliate fossils known were
tintinnids from the
Ordovician Period. In 2007, Li
et al. published a description of fossil ciliates from the
Doushantuo Formation, about 580 million years ago, in the
Ediacaran Period. These included two types of tintinnids and a possible ancestral
suctorian.
Classification
Phylum
Ciliophora
Further Information
Get more info on 'Ciliates'.
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