Ultrastructure Of Nassulid And Hypotrich Ciliates And Their Systematic Significance (Ciliophora)

The cortical organelles are the main executors of the physiological activities of ciliated protozoans,and their structural characteristics and differentiation carry reliable phylogenetic information,which is an important focus of evolutionary biology and systematics research.The ultrastructure of cortical organelles,as an important morphological perspective,is an essential supplement to the traditional classification of infraciliature and morphogenesis,and reconsidering them in the face of the contradiction between "molecular system" and "morphological system" is the main way to solve the problem of tough ciliate systematics.However,due to a variety of reasons,the research on the ultrastructure level of phylum Ciliophora is relatively lagging,and the applicability of the discussion on the relationship between the ultrastructure of cortical organelles in different categories sub-class is not clear.Aiming at the above problems,this paper comprehensively discusses the longterm residual problems by revealing the ultrastructure of represented species and combining SSU r RNA gene phylogenetic analysis for nassulid and hypotrich ciliates with significant systematic problems at the suborder-order level and the order-family level,respectively,and the main results include the following two parts.Nassophorea is not monophyletic of molecular phylogeny,and the relationship between the orders remains questionable.In this study,the ultrastructure of the Apocolpodidium etoschense found that the order Colpodidiida and the order Nassulida have the following common features: having two pairs of alveolocysts in the cortex,the presence of ‘‘B-cartwheel'' in the distal region of kinetosome,the presence of the cytostomal lamellae and the subcytostomal lamellae in the cytopharyngeal basket,and having spindle trichocysts with simple tip.These similarities suggest the presence of a core group of Nassophorea which are morphologically and genetically different from the order Microthoracida,which is more distantly related in terms of morphological characteristics.This further supports the visual view of the Microthoracida as a separate taxon rather than a member of the class Nassophorea.In view of the internal relationships of this core group,Colpodidiida as an independent order is further validated by its delicate cytopharyngeal basket which lacks nematodesmal lamellae;while the non-monophyly of the order Nassulida might be explained by the differentiation of the cartwheels in kinetosomes and the arrangement of kinetosomes with postciliary microtubules in the nassulid organelle 3 within its members.The original definition of the family Oxytrichidae is grouped together with the newly established family in phylogenetic analysis to form the core taxon of the Hypotrichia,and the complex phylogenetic relationships within this taxon cannot be explained by classical morphology.This study revealed the ultrastructure of the oral ciliary organelles and body ciliary organelles of the three species of Oxytricha granulifera,Urosoma emarginata,Aponotohymena australis,further confirming the microstructure of the oral cilia and body cilia of the three species of pinnates,represented by the family Oxytrichidae,and further confirmed that there are some differences between Sporadotrichida and Urostylida in terms of associated microtubules of marginal cirri,the number of longitudinal connections between the kinetosomes of the same longitudinal row in frontal and ventral cirri and the presence or absence of longitudinal connection between the kinetosomes of the same longitudinal row in the paroral membrane.Within the Oxytrichidae,the clade I represented by Aponotohymena australis and Paraurostyla weissei and branch II represented by Urosoma emarginata and Oxytricha granulifera are differentiated in terms of the number of longitudinal connections between the kinetosomes of the same longitudinal row in marginal cirri,which supports Dong Tianyao's(2019)conclusion that not-monophyletic subfamily are divided into pigmented and non-pigmented branch based on the type of extrusomes,and also implies the systematic classification significance of the inter-kinetosomes connections of ciliary organelles to the internal branches of the subfamily.For the internal members of the pigmented(branch ?)and non-pigmented bubbles(branch ?),the ultrastructure of different ciliates are similar,suggesting the stability of these structural features within the genetic clade.This paper provides the ultrastructural information of two ciliates for the first time,and supplements the ultrastructure data of ciliary organelles of two species of hypotrich ciliates,synthesizes the ultrastructure and molecular systematics,discusses the characteristics and changes of cortical structures such as pellicle,ciliary organelles,extrusomes and other cortex structures in multiple levels,and reveals the significance of cortical structure for the classification of class-order,order-family level.This study provides clues for the systematic determination of the subclass Hypotrichia and class Nassophorea,and explains the important value of ultrastructure in solving the systematics problem.