Ultrastructure of Prorodon (Ciliophora, Prostomatida) I. Somatic Cortex and Some Implications Concerning Kinetid Evolution In Prostomatid and Colpodid Ciliates

Abstract

ABSTRACT. This study describes the ultrastructure of the somatic cortex of Prorodon aklitolophon and Prorodon teres. the meridionally arranged somatic kineties of both species can be separated into two parts: a short anterior part, which consists of a few somatic dikinetids (in which both kinetosomes are ciliated), and a longer posterior consisting of monokinetids. the somatic monokinetids are associated with a convergent postciliary microtubular ribbon, a transverse microtubular ribbon flatly inserted in front of the kinetosome, a short and steeply extending kinetodesmal fibre attached to kinetosomal triplet 5 and 7, and a desmose anterior to triplet 3. From this desmose, two to five prekinetosomal microtubules originate and extend anteriorly. the posterior kinetosome of the somatic dikinetids is associated with the same microfibrillar and microtubular structures as the somatic monokinetid, except that no prekinetosomal microtubules originate from the desmose. the anterior kinetosome has a single postciliary microtubule and a tangentially oriented transverse microtubular ribbon. the permanent collecting canals of the unique contractile vacuole system extend parallel and adjacent to the somatic kinetics of Prorodon. the collecting canals are supported by the prekinetosomal microtubules. A similarly organized contractile vacuole system is not yet known from any other ciliate group. One of the most surprising results of this investigation was finding a significant similarity between the somatic dikinetid pattern of Prorodon and the colpodid dikinetid pattern. A hypothesis is presented to illustrate the evolution of the somatic kinetid patterns in colpodid and prostomatid ciliates.

Phylogenetic Relationships between Prostome and Colpodean Ciliates Tested by Small Subunit rRNA Sequences

“…In particular, Aescht et al (1991) provided arguments relating the colpodeans, Bursaria and Colpoda, either to the nassophoreans, Obertrumia, Pseudomicrothorax, and Furgasonia, or to the haptorians, Loxophyllum and Homalozoon. On the other hand, Hiller's (1992Hiller's ( , 1993 hypothesis would predict that the colpodeans, Bursaria and Colpoda, would be most closely related to the prostomes, Coleps and Prorodon.…”

“…hirtus was cultured on Eau Volvic and fed twice a week with Chlorogonium. P. teres was cultured on Eau Volvic and fed with Tubifex, following established procedures (Hiller, 1993). Cells of both species were isolated by centrifugation and washed with sterile springwater prior to DNA extraction.…”

“…For example, competing hypotheses for the phylogenetic relationships of the classes Colpodea, Prostomatea, Nassophorea, and Litostomatea were presented recently, primarily based on ultrastructural data. Hiller (1992Hiller ( , 1993 has argued that prostomes and colpodeans share a more recent common ancestry, basing his argument on stomatogenetic features and a hypothesis for the transformation of kinetids of one class to those of another. Both prostomes and colpodeans exhibit merotelokinetal stomatogenesis (Hiller, 1992), and their kinetids can be transformed from an hypothetical ancestor with a plesiomorphous, ciliate dikinetid pattern (Hiller, 1993).…”

“…Hiller (1992Hiller ( , 1993 has argued that prostomes and colpodeans share a more recent common ancestry, basing his argument on stomatogenetic features and a hypothesis for the transformation of kinetids of one class to those of another. Both prostomes and colpodeans exhibit merotelokinetal stomatogenesis (Hiller, 1992), and their kinetids can be transformed from an hypothetical ancestor with a plesiomorphous, ciliate dikinetid pattern (Hiller, 1993). Earlier, Aescht et al (1991) visualized the transformation of the kinetid of either a haptorian (class Litostomatea) or nassophorean (class Nassophorea) ancestor to give rise to the colpodean dikinetid with its unique posteriorly directed transverse microtubular ribbon or transversodesma.…”

Further insights into the phylogeny of two ciliate classes Nassophorea and Prostomatea (Protista, Ciliophora)

“…Two orders have been recognized based on the position of the mouth and the presence of a brosse and toxicysts, namely Prostomatida and Prorodontida (Dragesco et al, 1974;Lynn, 2008). However, this arrangement is far from certain and the systematics of Prostomatea has been vigorously debated for several decades (Bardele, 1999;Corliss, 1979;Hiller, 1992Hiller, , 1993Lynn and Small, 2002;Stechmann et al, 1998). Availability of SSU rRNA gene sequences of morphologically identified prostomes is limited to a single order, the Prorodontida Kim et al, 2007;Stechmann et al, 1998;Wright and Colorni, 2002;Yi et al, 2012).…”