Holomastigotoides is a genus of parabasalids found in the hindgut of lower termites. It is characterized by its dense, organized arrangement of flagella on the cell surface and the presence of a mitotic spindle outside its nucleus during the majority of its cell cycle. As a symbiont of termites, Holomastigotoides is able to ingest wood and aid its host in digestion. In return, Holomastigotoides is supplied with a stable habitat and steady supply of food. Holomastigotoides has notably been studied to observe the mechanisms of chromosomal pairing and segregation in haploid and diploid cells.
Taxonomy Holomastigotoides was first described by Max Hartmann in 1910. Hartmann mistakenly identified Holomastigotoides as the female form of the parabasalid Trichonympha hertwigi, which he observed living in a species of termite, Coptotermes sp., in Brazil. After initial discovery, Giovanni Battista Grassi and Anna Foa reclassified Hartmann's “male” form of T. hertwigi to Holomastigotoides in 1911, thus establishing the first use of the genus. The original host species of Holomastigotoides described by Hartmann was later invalidated due to lack of description, and Coptotermes testaceus was subsequently named the type host for Holomastigotoides hertwigi as it is the only species of Coptotermes native to Brazil. The following species are recognized:
H. aureus H. batututi H. bigfooti H. hartmanni H. minor H. mirabile H. oxyrhynchus
Habitat and ecology Holomastigotoides is an obligate symbiont of lower termites. Holomastigotoides lives in hindguts of lower termites, where it feeds on wood and assists the termite in wood digestion. This allows the termite to access and use nutrients found in wood that they would not have been able to digest otherwise. Holomastigotoides can be transferred from termite to termite by way of feeding on anal secretions of other termites during juvenile stages. Since discovery, Holomastigotoides species have been found in multiple termite genera, including Coptotermes, Heterotermes, Prorhinotermes, Psammotermes, and Anacanthotermes. It is possible for multiple species of Holomastigotoides to reside in an individual host termite species. This may be a result of speciation of Holomastigotoides within a single host species or a result of possible co-speciation between Holomastigotoides and its hosts.
Morphology
Cell surface and flagella Holomastigotoides is a cone-shaped cell. One of the most notable features of Holomastigotoides is the high density of flagella on the cell surface, with some reports of up to 10 000 flagella on a single cell. The organization of the flagella in Holomastigotoides is attributed to the arrangement of its flagellar bands in a spiral formation around the cell. The flagellar bands originate from the anterior apex of the cell and spiral posteriorly in progressively larger spirals, wrapping around the circumference of the cell. An individual flagellar band is made up of many basal bodies arranged in a single row, and a single flagellum emerges from each basal body, giving Holomastigotoides its characteristic, highly flagellated appearance. The basal bodies of a flagellar band are linked by a fiber system that consists of three different fiber types. Each flagellar band is associated with an axostyle, endoplasmic reticulum, and Golgi bodies. The high density of external flagella helps prevent pieces of ingested wood in the termite hindgut from contacting and damaging cell surfaces. The number of flagellar bands varies based on the species of Holomastigotoides. The posterior base of Holomastigotoides cells are not flagellated, and contain vesicles that are likely used for phagocytosis of wood.
Basal bodies and fiber system Near the anterior apex of the cell, the basal bodies are arranged tightly together within the flagellar bands, to such an extent that some basal bodies will overlap with each other. The fiber system associated with the basal bodies is also compressed in this apical region, and thus the fiber types are more difficult to distinguish. As basal bodies become more widely spaced further away from the cell apex, the fiber types are also easier to distinguish. Basal bodies transition into flagella distally, and the transition point is indicated by a transition plate. An axosome is found between the transition plate and the central microtubules of an individual flagellum. Holomastigotoides also possesses parabasal bodies, as is characteristic of parabasalids. The parabasal bodies consist of a Golgi body and a parabasal fiber, and are closely associated with the basal bodies of the flagella. Golgi bodies have been observed to overlap with parabasal fibers near the base of the nucleus. The basal bodies of a flagellar band are linked by a fiber system, which consists of the parabasal fiber, fibrous ribbon, and KI fiber. The parabasal fiber provides a surface for microtubule formation, and there is one parabasal fiber for each flagellar band. The parabasal fiber possesses a dark lining that has been suggested to be a microtubule organizing centre for the axostyle. The size of parabasal fibers decreases as they extend further past the apex, to the point where they cannot be observed in the mid-region or base of the cell. Parabasal fibers are densely concentrated in the cell's apex, and axostyles closely associated with the parabasal fibers also accumulate in this location. The fibrous ribbon is a long sheet that looks like an accordion, and connects all the basal bodies in an individual flagellar band. An individual fibrous ribbon is as long as the length of an individual flagellar body. KI fibers are named for their distinctive shape, and specifically link basal bodies in triplets. KI fibers can change shape, which also changes the distance between basal bodies and regulates how close or far they are from each other. The fibrous ribbon and KI fiber are thought to have a role in controlling cell shape by moving the flagellar bands. They also play roles in regulating the direction a Holomastigotoides cell moves in, coordinating the beating of flagella, and assisting in accommodating large pieces of wood during phagocytosis.
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