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Protistology 18 (3): 177-188 (2024) | doi:10.21685/1680-0826-2024-18-3-1 PPOtÎStOlOây
Review
Types of flagellar apparatus of diatomists: morphological diversity and evolution
Pavel Safonov
Institute of Cytology, Russian Academy of Sciences, Saint Petersburg 194064, Russia
| Submitted March 7, 2024 | Accepted July 6, 2024 |
Summary
Heterokontophyta is a vast group of predominantly photosynthetic stramenopiles. It is divided into two major lineages: Chrysista and Diatomista. Chrysista is further subdivided into clades SI and SII, while Diatomista comprises the SIII clade. The typical flagellar apparatus of stramenopiles consists of two flagella associated with two basal bodies and a system of roots. This morphology is typical of flagellate cells of a large number of heterokontophytes. However, in many cases it underwent profound changes. In certain lineages, one of the two flagella either became vestigial or was completely lost, and other heterokontophytes are aflagellate. While such organisms are found in all three clades of heterokontophytes, the loss of one or both flagella and flagellar roots is especially characteristic of diatomists. This review aims to examine the main types of flagellar apparatus in diatomists and discuss hypotheses regarding the evolution of these structures.
Key words: Heterokontophyta, Diatomista, clade SIII, flagellar apparatus, evolution
Introduction
Heterokontophyta, formerly known as Ochro-phyta (Guiry et al., 2023), constitutes one of the major lineages within the diverse eukaryotic phylum Stramenopiles (Adl et al., 2018). Together with Telonemia, Alveolata and Rhizaria, Stramenopiles form a supergroup TSAR (Strassert et al., 2019). While the vast majority of heterokontophytes are photosynthetic (Guiry et al., 2023), there are also secondarily heterotrophic species, both free-living (Sekiguchi et al., 2003; Kristiansen and Skaloud, 2017; Mann et al., 2017) and parasitic (Heesch et al., 2008).
https://doi.org/10.21685/1680-0826-2024-18-3-1
© 2024 The Author(s)
Protistology © 2024 Protozoological Society Affiliated with RAS
The biodiversity of Heterokontophyta is grouped into three major clades: SI, SII, and SIII (Fig. 1, A). Clade SI consists of Phaeophyceae (brown algae), Xanthophyceae, Raphidophyceae, and a few smaller related taxa. Chrysophyceae (golden algae) and Synchromophyceae belong to the SII clade. Together, clades SI and SII form the taxon Chrysista. Diatomeae, Bolidophyceae, Dictyochophyceae, and Pelagophyceae constitute the Diatomista group, also known as the SIII clade (Yang et al., 2012; Adl et al., 2018). The exact evolutionary affinity of certain classes of heterokontophytes remains a subject of ongoing debate. Depending on the composition of multigene datasets used for phylogenetic analysis,
Corresponding author: Pavel Safonov. Laboratory of Cytology of Unicellular Organisms, Institute ofCytology RAS, Tikhoretsky Ave., 4, 194064 St. Petersburg, Russia; e-mail: safonov.incras@gmail.com
Eustigmatophyceae are positioned within either the SI clade (Thakur et al., 2019; Dorrell et al., 2021; Azuma et al., 2022) or the SII clade (Yang et al., 2012; Stancheva et al., 2019; Barcyte et al., 2022; Di Franco et al., 2022). The position of Pinguiophyceae is relatively well established, with a majority of phylogenies placing this group within the SII clade (Yang et al., 2012; Stancheva et al., 2019; Dorrell et al., 2021; Azuma et al., 2022). However, some phylogenetic reconstructions suggest its potential association with the SIII clade (Di Franco et al., 2022).
The motile cells of stramenopiles typically possess two basal bodies: one producing a smooth posterior flagellum and the other producing an anterior flagellum, which bears tripartite mastigone-mes. The flagellar apparatus generally comprises a system ofmicrotubular and sometimes fibrous roots (Yoon et al., 2009). In 2000, Moestrup introduced a unified numbering system for basal bodies and microtubular roots, designating the basal body of the posterior flagellum as B1 and the basal body of the anterior flagellum as B2. Two microtubular roots associated with B1 are denoted as R1 and R2, while two microtubular roots associated with B2 are designated as R3 and R4 (Moestrup, 2000). Although alternative designations can be found in some subsequent works (e.g. Honda and Inouye, 2002; Kai et al., 2008; Elias et al., 2017), Moestrup's notation system is more widely used.
The described morphology characterizes motile cells ofmany present-day heterokontophytes (Yoon et al., 2009). At the same time, significant changes have occurred in some lineages. In many cases, posterior flagellum has become vestigial or was completely lost as observed in chrysophyceans with Chromulina-Mke morphology (Kristiansen and Ska-loud, 2017) or spermatozoids of brown algae from the order Dictyotales (Kawai and Henry, 2016). Heterokontophytes that retain only a posterior fla-gellum are found among pinguiophyceans (Kawachi et al., 2002). Furthermore, many heterokontophytes lack flagellate stages in their life cycles; some are immotile, others have developed alternative locomotion mechanisms such as gliding in pennate diatoms (Mann et al., 2017) or amoeboid movement in synchromophyceans, e.g. Synchroma grande (Horn et al., 2007).
While species lacking biflagellate forms in their life histories are present in all three clades of Heterokontophyta, the evolutionary trend of losing one or both flagella along with the system of roots is particularly prominent within the SIII clade. In
this review, I provide a brief characterization of the flagellar apparatus types found in diatomists and discuss potential evolutionary trends in their morphology.
Types of flagellar apparatus of diatomists
BOLIDOPHYCEAE
Bolidophyceae is a sister group to diatoms (Fig.1, A, B), comprising both biflagellate and aflagellate organisms (Ichinomiya et al., 2011, 2016). In Triparma pacifica, two basal bodies produce two flagella, and both microtubular and fibrous roots are absent (Fig. 2, A). This morphology is also characteristic of T. mediterranea and T. eleuthera (Guillou et al., 1999; Ichinomiya et al., 2016). Other Triparma species are non-motile, with their cells covered by silica plates (Ichinomiya et al., 2016). Interestingly, phylogenetic analysis conducted by Ichinomiya and co-authors (2016) revealed a strain of Triparma possessing flagella group together with several silicified representatives of the genus. This, along with occasional observations of motile cells in cultures of T. laevis, led them to hypothesize that silicified Triparma species may have complex life cycles involving diploid non-motile and haploid motile stages. The subsequent phylogenetic analysis revealed that another silicified species, T. retinervis, is closely related to the flagellate species T. pacifica (Yamada et al., 2020), which provided additional evidence in favor of the hypothesis put forward. Considering the fact that silicified stages have never been observed in flagellate species, Yamada and co-authors (2020) further hypothesized that they may have arisen by secondary loss of the silicified stage and that this process occurred independently in several Triparma lineages.
Diatomeae
Diatoms are divided into two groups: pennate and centric. Pennate diatoms is a late-evolving taxon, while "centric diatoms" is a paraphyletic grouping of all other lineages (Theriot et al., 2010).
A remarkable feature of centric diatoms is the ability to produce uniflagellate spermatozoids (Mann et al., 2017). Despite the fact that there are numerous reports of diatom spermatozoids, the ultrastructure of these cells was studied in a limited number of species: they possess only B2, producing an anterior flagellum with 9 + 0 axoneme
Fig. 1. A — Cladogram illustrating the evolutionary relationships of the three major heterokontophyte clades, with a focus on the structure of the SIII clade (Diatomista); B — ragment of a maximum likelihood phylogeny of stramenopiles, providing a detailed illustration of the evolutionary relationships among diatomists; black dots indicate branches with bootstrap support values = 100 and posterior probabilities = 1.00; adapted from Thakur et al. (2019), with modifications.
configuration (i.e., the central pair of microtubules is absent) (Jensen et al., 2003; Idei et al., 2013). Moreover, in Melosira moniliformis, Coscinodiscus wailesii, Thalassiosira lacustris and Pleurosira laevis the basal body consists of doublets of microtubules rather than triplets (Heath and Darley, 1972; Jensen et al., 2003; Idei et al., 2013).
In contrast with bolidophyceans, some of the studied diatom spermatozoids also have a peculiar system of microtubular roots. In M. moniliformis, C. wailesii, T. lacustris and P. laevis it is organized as a cone of microtubule bundles connecting the basal body with the surface of the nucleus (Fig. 2, B; Jensen et al., 2003; Idei et al., 2013). At the same time, spermatozoids of Chaetoceros laciniosus have only one microtubular root, and in Lithodesmium undulatum they are absent (Jensen et al., 2003; Idei et al., 2013).
Pelagophyceae
The other two sister groups in the SIII clade are Pelagophyceae and Dictyochophyceae (Fig. 1, A, B). Among pelagophyceans that have motile life cycle stages, the vast majority are biflagellate. However, despite the fact that over the past decade numerous new biflagellate members of the group were described (Han et al., 2018; Wetherbee et al., 2015, 2021, 2023), the information regarding the ultrastructure of their flagellar apparatus is scarce. To date, it has only been characterized in two species that have biflagellate life cycle stages: Sarcinochrysis marina (O'Kelly, 1989) and Ankylochrysis lutea (Honda and Inouye, 1995).
In S. marina zoospores, two basal bodies produce two flagella (Fig. 2, C) (O'Kelly, 1989). Four microtubular roots are present: major anterior root
Fig. 2. Schematic representation of the main morphological variations of flagellar apparatus of diatomists. Structural parts of mastigonemes are drawn schematically, not to scale. Thalassiosira lacustris (B): each blue line designates a microtubular bundle; Sarcinochrysis marina and Ankylochrysis lutea (C, D): each blue line designates an individual microtubule. Microtubular roots in S. marina (C) and A. lutea (D), as well as basal bodies (A-L) are numbered according to the notation system of Moestrup (2000); root R4 of S. marina (C) in the chosen projection is located behind the second basal body; "bypassing" root of A. lutea (D) is not shown; n — nucleus. Each scheme is accompanied by a reference to the original article. Some structures are highlighted in gray, as their morphology was not definitively resolved in the referenced articles. Mastigonemes of Octactis speculum (G, H): Moestrup and Thomsen (1990) reported the presence of 2 out of 3 structural parts — the base and the shaft, but the authors were unable to determine the number of terminal filaments and their morphology. Paraxial rod in the skeleton bearing stage of O. speculum (H): Moestrup and Thomsen (1990) did not observe the structure, as the flagellar wing was poorly preserved; the presence of fibrillar material possibly indicates that the rod, if present, disintegrated during the fixation of the sample. Comments regarding the root R2 in S. marina (C), the bridges between basal bodies in O. speculum (G, H), the bands, connecting B2 with the nucleus in Pteridomonas danica (I) and the second basal body in Ciliophrys infusionum (K) can be found in the corresponding parts of the text.
(MAR) and minor anterior root (mar) are associated with B2, major posterior root (MPR) and minor posterior root (mpr) are associated with B1 (O'Kelly, 1989). As per the notation system of Moestrup (2000), these roots should be designated as follows: "MAR" - R3, "mar" - R4, "MPR" - R2, "mpr" — R1 (Fig. 2, C). R1 and R3 are composed of two microtubules, R4 consists of only one. Either five or six microtubules comprise root R2: based on the drawing of O'Kelly (1989, Fig.13.3 b, p. 260), one can conclude that there are five of them, but in the text, the author stated that "the major posterior rootlet is composed of six strands" (ibid. p. 270). Subsequently, in Table 1 oftheir 1995 article, Honda and Inouye also characterized root R2 (designated as "R3", see below) of S. marina as consisting of 5 microtubules, with the reference to O'Kelly (1989). As I have not found any additional information regarding R2 in this species, I have marked the sixth microtubule of the root in gray on the scheme (Fig. 2, C).
In A. lutea, two basal bodies produce two flagella (Fig. 2, D) (Honda and Inouye, 1995). It also has a pronounced root system: Honda and Inouye (1995) reported the presence of three microtubular roots directly associated with kinetosomes, a bypassing microtubular root (BR), and two striated fibrous roots. In their article, the authors utilized root numbering as per Andersen (1987): the root, associated with the anterior basal body, was designated as R1, while roots, associated with the posterior basal body, were designated as R3 and R4. However, according to the notation system of Moestrup (2000), the "R1" root of A. lutea should be designated as R3, "R3" - as R2, and "R4" - as R1. I used the corrected root designations in the scheme (Fig. 2, D). Roots R1 and R3 comprise two microtubules; root R2 consists of six microtubules, with two notably longer than the others.
Another morphological type, unique among pelagophyceans, is characteristic of Pelagomonas calceolata: it possesses a single basal body that produces an anterior flagellum, and roots are absent (Fig. 2, E; Andersen, 1993). In P. calceolata "the flagellum is anchored in the cell posterior and lies in a long, shallow flagellar groove" (Andersen, 1993, p. 703); "the basal body is abutted to the plasma membrane" (ibid. p. 707). Thus, on TEM images the kinetosome and adjacent nucleus appear to be "sandwiched" between two parts of the plasma membrane.
Dictyochophyceae
Class Dictyochophyceae comprises 4 orders: Florenciellales, Dictyochales, Pedinellales and Rhizochromulinales; the former two and the latter two are sister taxa (Fig. 1, A, B) (Eikrem et al., 2004; Hosoi-Tanabe et al., 2007; Azuma et al., 2022). In general, dictyochophyceans lack microtubular or fibrous roots (besides some pedinellids, see below). However, I find the diversity of the flagellar apparatus morphologies in this group remarkable, as it possibly illustrates different stages of the loss of the posterior flagellum.
Cells offlorenciellids possess two basal bodies of equal size that produce two unequal flagella; a fibrillar bridge connects B1 and B2 (Fig. 2, F; Eikrem et al., 2004; Hosoi-Tanabe et al., 2007). More variations are found in the order Dictyochales. Vicicitus globosus retains both flagella, though the posterior flagellum is "very short (about a tenth the length of the long flagellum)" (Chang et al., 2012, p. 407). In the naked stage of Octactisspeculum, the posterior flagellum lacks the central microtubules and is only distinguishable on electron micrographs (Fig. 2, G). Moestrup and Thomsen (1990) described it as short and stubby; however, it is not certain whether this flagellum is indeed significantly shortened or simply masked by deep flagellar pocket. In the skeleton-bearing stage of O. speculum, the undulipodium of the posterior flagellum is completely absent (Fig. 2, H). Although Moestrup and Thomsen (1990) did not detect distinct bridge interconnecting basal bodies in O. speculum, they noted that structures of some kind - banded fiber or microtubules, might be present. As the nature and exact morphology ofthis bridge remained uncertain, I marked it gray in the Fig. 2 F and G. Finally, another Octactis species, O. octonaria, also retains only one undulipodium (Chang, 2015).
Pedinellids retain both basal bodies while possessing one undulipodium. Noteworthy, B1 can be either equal in size to B2, like in Actinomonas, Apedinella and Pteridomonas (Fig. 2, I; Larsen, 1985; Patterson and Fenchel, 1985; Koutoulis et al., 1988), or approximately two times shorter, like in Pseudopedinella and Pedinella (Fig. 2, J; Thomsen, 1988; Sekiguchi et al., 2003). In Actinomonas, Pteridomonas and Pseudopedinella, B2 is attached to the nucleus by electron-dense bands (Larsen, 1985; Patterson and Fenchel, 1985; Thomsen, 1988); in Apedinella, an amorphous electron-dense material
at the base of B2 near the nuclear membrane was revealed (Koutoulis et al., 1988). Larsen (1985) suggested that the bands observed in Actinomonas may be interpreted as vestigial flagellar roots, but the exact nature of these structures was not resolved. Thus, I highlighted them gray in the scheme (Fig. 2, I).
Ciliophrys infusionum — an organism belonging to the order Rhizochromulinales, possesses one un-dulipodium and two basal bodies (Fig. 2, K). However, the exact morphology of B1 is uncertain, so I marked it gray in the scheme: based on Davidson's drawing (1982), one can conclude that it is shorter than B2, but the author provided neither TEM images nor comments regarding this structure. Among other rhizochromulinids, the ultrastructure of flagellar apparatus has only been studied in two closely related organisms: Rhizochromulina marina and Rhizochromulina sp. B44. In both of them not only the undulipodium of the posterior flagellum is absent, but also the corresponding basal body is significantly shorter than B2 (Fig. 2, L; Hibberd and Chretiennot-Dinet, 1979; Safonov et al., 2024). This morphology likely represents the most profound level of the second flagellum reduction among dic-tyochophyceans.
Evolution of flagellar apparatus in diato-mists: discussion of hypotheses
In this review, I examined the main morphological variations offlagellar apparatus that are found among members of the heterokontophyte clade SIII (Diatomista). Below, I further discuss observations that were mentioned in the text and hypotheses regarding the evolution of structures, comprising flagellar apparatus in diatomists.
The flagellar apparatus of centric diatoms is characterized by three remarkable features: an unusual system of microtubular roots, an axoneme lacking the central pair of microtubules, and a kinetosome composed of microtubule doublets rather than triplets.
A cone of microtubular bundles, found in some diatom spermatozoids, differs dramatically from the typical stramenopile root system and is thought to be a derivate of a spindle apparatus (Andersen, 1991; Jensen et al., 2003; Idei et al., 2013). This hypothesis is supported by some interesting observations. For example, in Melosira, microtubule bundles that comprise the cone nucleate slightly below the basal body, where an electron-dense structure is situated (Idei et al., 2013). This structure is likely
a microtubule center involved in the process of mitotic/meiotic spindle formation in diatoms; in Melosira, microtubule center is known to be very dense and osmiophilic (Pickett-Heaps, 1991). Furthermore, a close association between the pole of meiotic spindle and the basal body was observed in Lithodesmium (Manton et al., 1970), although mature spermatozoids in this species do not possess the cone.
Apart from diatoms, locomotive flagella with 9+0 axonemes were described in spermatozoids of some animals, e.g., eels (Gibbons et al., 1985), horseshoe crabs (Ishijima et al., 1988) and insects (Riparbelli et al., 2009). In eel and horseshoe crab spermatozoids, the absence of the central pair ofmicrotubules (CP) affects the flagellar movement dynamics (Gibbons et al., 1985; Ishijima et al., 1988). However, Idei and co-authors (2013) mentioned that it was not the case with diatom spermatozoids. At the same time, the authors observed vesicles within the axoneme in M. moniliformis and T. lacustris; the similar vesicles were also observed in spermatozoids of other diatoms (Manton and von Stosch, 1966; Heath and Darley, 1972). Taking this into account, and considering the fact that the flagellum is the first part of a diatom spermatozoid to contact the egg, Idei and co-authors (2013) came up with the following hypothesis: the absence ofCP may facilitate transport and release of substances in the process of sperm-egg recognition and subsequent fertilization. Consequently, the question is whether the 9+0 axoneme configuration is common to spermatozoids of all centric diatoms. Although the species where this feature was studied belong to different lineages, Idei and co-authors (2013) called for a study of the flagellar apparatus in some deeply branching genera, such as Corethron and Ellerbeckia (based on phylogenies of Theriot et al., 2009, 2010). Later, Poulickova and Mann (2019) also mentioned Leptocylindrus among basal diatom genera (based on phylogeny of Gargas et al., 2018) that might be of interest to study the axoneme ultrastructure. Unfortunately, to date, the flagellar apparatuses of Leptocylindrus, Corethron and Ellerbeckia have not been studied.
Besides diatoms, basal bodies composed ofmic-rotubule doublets have not been found in other heterokontophytes (Idei et al., 2013). Among non-heterokontophyte stramenopiles, a bicosoecid Si-luania monomastiga is known to possess kinetosomes of such structure (Karpov et al., 1998). Moreover, the reduced kinetosomes have been described in numerous organisms from other eukaryotic phyla. Basal bodies consisting of microtubule doublets
were described in a pelobiontid Mastigamoeba schizophrenia (Archamoebea, Amoebozoa) (Simpson et al. 1997), spermatozoids of some insects (Callaini et al., 1999; Riparbelli et al., 2009), gametes of gregarines from the genus Stylocephalus (Api-complexa, Alveolata) (Schrevel et al., 2013), in zoospores of Gromochytrium mamkaevae (Chytridio-mycota, Nucletmycea) (Karpov et al., 2014) and Aphelidium collabens (Aphelida, Nucletmycea) (Seto et al., 2020). In gametes of a coccidian Eimeria necatrix (Apicomplexa, Alveolata), basal body consists of microtubule singlets and a short axial microtubule (Schrevel et al., 2013). In zoospores of Sanchytrium tribonematis (Sanchytriomycota, Nucletmycea), kinetosome is composed of singlets (Karpov et al., 2019), and in another san-chytriomycete — Amoeboradix gromovi — kinetosome consists of singlets or doublets of microtubules (Karpov et al., 2018).
In summary, several structures comprising the flagellar apparatus have undergone reduction in the Diatomea + Bolidophyceae lineage. The system of microtubular roots was likely lost by a common ancestor of diatoms and bolidophyceans. Subsequently, in some diatom species, a new structure derived from the spindle apparatus emerged. Conversely, the loss of the posterior flagellum and the central pair of microtubules of the anterior flagellum are not characteristic of bolidophyceans (Idei et al., 2013), indicating that the reduction of these structures occurred during diatom evolution. Finally, the available information regarding the ultrastructure ofbasal bodies in bolidophyceans does not allow us to determine whether they are composed of microtubule triplets or doublets. Therefore, it is currently impossible to conclude whether the reduction of kinetosomes is characteristic only of diatoms or also of bolidophyceans.
Interestingly, apart from centric diatoms, there is currently only one known member of the SIII clade that has retained the anterior flagellum, but has completely lost the ancestral system of microtubular roots and the posterior flagellum, including its kinetosome. This organism is Pelago-monas calceolata, which belongs to the class Pelagophyceae. At the same time, two other pelagophy-ceans — Sarcinochrisis marina and Ankylochrysis lutea — possess microtubule root systems that essentially are variations of the ancestral stramenopile root system. Nevertheless, Honda and Inouye (1995) mentioned some remarkable characteristics of A. lutea: root R4 is absent, there are no cytoskeletal microtubules, associated with R3, and there is a BR.
The absence of R4 distinguishes this organism from other heterokontophytes (not considering those that have lost flagellar roots entirely), including S. marina (Honda and Inouye, 1995). R3-associated microtubules are common among heterokontophytes and stramenopiles overall; this feature is thought to be inherited from the last eukaryotic common ancestor, as it is also characteristic of alveolates, apusozoans, amoebozoans, collodictyonids and excavates (Yubuki and Leander, 2013). Besides A. lutea, another example of a heterokontophyte whose flagellate cells lack R3-associated microtubules is Vischeria stellata (Eustigmatophyceae) (Santos and Leedale, 1991). Among heterokontophytes, the BR has been described in brown algae (O'Kelly and Floyd, 1984), Phaeomonas (Pinguiophyceae) (Honda and Inouye, 2002) and Giraudyopsis (O'Kelly and Floyd, 1985); the latter is currently classified as a member of Phaeosacciophyceae (Guiry, 2023) — a class related to xantophyceans (Graf et al., 2020). In zoospores of brown algae, BR consists of five microtubules (O'Kelly and Floyd, 1984), in Giraudyopsis, Ankylochrysis and Phaeomonas it consists of one microtubule (O'Kelly and Floyd, 1985; Honda and Inouye, 1995, 2002). In all cases, the structure is associated with the R3 root. Nevertheless, Honda and Inouye (2002) were not sure whether bypassing roots of these organisms are homologous.
In this context, it is noteworthy that Cavalier-Smith (2018) viewed BR as a synapomorphy of Chromista; however, the kingdom was later recognized as polyphyletic (Burki et al., 2020; Strassert et al., 2021), so the BRs of its ex-members may not be homologous either. Interestingly, Cavalier-Smith (2018) also argued that BR preadopts chromists for evolving axopodia, and mentioned pedinellids as an example of organisms with a periciliary axopodial feeding mechanism. According to this hypothesis, the multiply duplicated BRs allowed pedinellids to form axopodia, supported by microtubular triads. Although the detailed discussion of this topic is probably beyond the scope of the present article, I find this assumption highly speculative, as no traces of bypassing roots were found in any pedinellid examined.
To date, S. marina and A. lutea remain unique examples of diatomists that retained (although in the case of A. lutea with some modifications) the ancestral stramenopile root system. At the same time, the study of related organisms can possibly reveal that these morphologies are more common among pelagophyceans. Until recently, there were
no known organisms closely related to A. lutea, so it formed a separate branch within the clade of the order Pelagomonadales (Wetherbee et al., 2015, 2021). In their 2023 article, Wetherbee and co-authors have described two biflagellate species
— Chromopallida australis and Wyeophycusjuliehar-rissiae, and it turned out that they form a common clade with A. lutea. As the other members of the order Pelagomonadales are either aflagellate (Au-reococcus, Pelagococcus) or uniflagellate (Pelago-monas), the study of the flagellar apparatus of Chromopallida and Wyeophycus is of great importance. Unfortunately, the authors have not provided any information regarding this issue.
Besides the bands connecting basal bodies with the nuclear membrane that were observed in some pedinellids (Larsen, 1985; Patterson and Fenchel, 1985; Thomsen, 1988), there are no other flagellar roots in dictyochophyceans. However, the other feature — state of the posterior flagellum
— is remarkably diverse among members of the class. Florenciellids and some dictyochids possess the posterior flagellum, and in pedinellids and rhizochromulinids, its undulipodium is absent. Even more interestingly, B1 in Pedinella squamata, Pseudopedinella tricostata and possibly Ciliophrys infusionum is half as long as B2 (Davidson, 1982; Thomsen, 1988; Sekiguchi et al., 2003). Finally, in the studied rhizochromulines, B1 is even shorter, possibly a quarter of the length of B2 (Hibberd and Chretiennot-Dinet, 1979; Safonov et al., 2024). Thus, I assume that such shortening is a type of the basal body reduction. Reduction of the other type is seen in diatoms, where B2 is composed of microtubule doublets instead of triplets. In this context, the question is whether the shortened kinetosomes of the mentioned dictyochophyceans consist oftriplets or not. At least in Ps. tricostata both basal bodies are composed of microtubule triplets (Thomsen, 1988). Alas, this aspect was not studied in P. squamata and C. infusionum, and the quality of the TEM image provided in the article by Hibberd and Chretiennot-Dinet (1979) does not allow for distinguishing the exact composition ofbasal bodies in R. marina.
It is noteworthy that a significant number of strains assigned to the genus Rhizochromulina have never been studied in detail. At the same time, the phylogenetic analysis conducted in our earlier work, has shown that some of these organisms form branches on the phylogenetic tree that are quite distant from the branch of R. marina and
Rhizochromulina sp. B44 (Safonov et al., 2024). Hence, these rhizochromulinids probably should be classified as separate species or even genera, warranting thorough research into their morphology and ultrastructure, including flagellar apparatus.
Acknowledgments
This research was funded by the Budgetary Program # FMFU-2024-0012 at the Institute of Cytology RAS.
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