Timing and characteristics of nuclear events during conjugation and genomic exclusion in Paramecium multimicronucleatum

Ciliated protists are ideal material for studying the origin and evolution of sex,because of their nuclear dimorphism(containing both germline micronucleus and somatic macronucleus in the same cytoplasm),special sexual processes(conjugation and autogamy),and high diversity of mating-type systems.However,the study of sexual process is limited to only a few species,due to the difficulties in inducing or observing conjugation.In the present study,we investigate the conjugation process in Paramecium multimicronucleatum:(1)of the three prezygotic divisions,all micronuclei undergo the first two divisions(meiosis I,II),while a variable number of nuclei undergo the third division(mitosis);(2)the synkaryon divides three times after fertilization,giving rise to eight products that differentiate into four macronuclear anlagen and four micronuclei;(3)cells restore the vegetative stage after two successive cell fissions during which the macronuclear anlagen are distributed into daughter cells without division,while micronuclei divide mitotically;(4)the parental macronucleus begins to fragment following the first meiotic division and finally degenerates completely;(5)the entire process takes about 110 h,of which about 85 h are required for macronuclear development.In addition,we describe for the first time the process of genomic exclusion occurring between amicronucleate and micronucleate cells of P.multimicronucleatum,during which the micronucleate cell contributes a pronucleus to the amicronucleate cell,resulting in both exconjugants being homozygotes.These results provide new insights into the diversity of sexual processes and lay an important cytological basis for future in-depth studies of mating systems in ciliates.

Doublets of the unicellular organism Euplotes vannus(Alveolata,Ciliophora,Euplotida):the morphogenetic patterns of the ciliary and nuclear apparatuses associated with cell division

Ciliated protists are one of the most diverse and highly differentiated group among unicellular organisms.Doublets occur in ciliates when two cells fuse into a single individual.Doublets contain two major cellular components(either cell in a doublet)and have traditionally been considered as developmental anomalies.Nevertheless,doublets can divide or even conjugate effectively,which may represent dispersal forms of the life stages.In addition,morphogenesis,as an important process in the life cycle,will provide important insights into the complex differentiation mechanism and various physiological phenomena.However,morphogenetic studies focusing on doublets of ciliates are very limited,which has become an obstacle to understand their complete life history.Here we isolated a doublet strain from the marine species Euplotes vannus(Müller,1786)Diesing,1850 and investigated its morphogenetic events during asexual reproduction.Our results indicate that:(1)the opisthe’s oral primordium develops de novo beneath the cortex;(2)the frontoventral and transverse cirral anlagen,cirrus I/1,and marginal anlagen in both dividers develop de novo separately;(3)the dorsal kinety anlagen,the three rightmost ones of which produce three caudal cirri for the proter,occur within the parental structures in the mid-body region;(4)the opisthe acquires two caudal cirri,one from the end of each two rightmost kineties;and(5)there are two macronuclei and one micronucleus in the doublet and they divide amitotically and mitotically,respectively.Finally,we speculate that this special differentiation may be an adaptive form to adverse environments.

Nuclear events during conjugation in the poorly studied model ciliate Paramecium jenningsi

Ciliated protists are highly differentiated unicellular eukaryotes that possess special sexual processes(conjugation and autogamy)that rely on their unique nuclear dimorphism,i.e.,the presence of both a germline micronucleus(MIC)and a somatic macronucleus(MAC)in a single cell.The sexual processes show a high diversity in different ciliates.To better understand the differentiation and evolution of sexual processes in closely related species,we investigated the nuclear events during conjugation in Paramecium jenningsi strain GZ,a poorly studied close relative of the well-known P.aurelia complex.The main results include:(1)the conjugation process takes about 48 h,including three prezygotic divisions(meiosis I,II,and mitosis)and three postzygotic divisions;(2)the MICs are dominated by the“parachute”phase at the prophase of meiosis I;(3)after meiosis II,a variable number of nuclei undergo the third prezygotic division,and the two products near the paroral cone become the genetically identical migratory and stationary pronuclei;(4)the synkaryon divides three times to form the MIC and the MAC anlagen,but only one nuclear product of the first postzygotic division completes the next two divisions;and(5)an extra cell division is required to complete the last step of conjugation,during which two MIC anlagen undergo mitotic division while two MAC anlagen are distributed between the daughter cells without division.The comparison of the nuclear events during conjugation in representatives of the class Oligohymenophorea reveals that usually there are three prezygotic divisions and a variable number of postzygotic divisions.However,the number of selected and differentiated nuclei after each division differs among species.This study provided a new model ciliate for further investigations of nuclear selection and differentiation as well as nuclear morphology during meiosis.

Does the Gonostomum-patterned oral apparatus in hypotrichia carry a phylogenetic signal?Evidence from morphological and molecular data based on extended taxon sampling using three nuclear genes(Ciliophora,Spirotrichea)

The classification of hypotrichs based on the gonostomatid oral structure is widely accepted,but the phylogenetic signal of this character is unknown.Here,we infer the species phylogeny of those gonostomatids for which molecular data are available,plus26 new sequences of SSU-rDNA,ITS1-5.8 S-ITS2 and LSU-rDNA genes.The results indicate that:(i)the endoral is more phylogenetically informative than the paroral;(ii)the structure of the endoral and the Gonostomum-pattern adoral zone of membranelles are plesiomorphies for the hypotrichs sensu stricto;(iii)the group of species possessing these features is monophyletic in all our phylogenetic analyses,except that for the SSU-rDNA;(iv)Schmidingerotrichidae is monophyletic in all trees,suggesting that it is a well-defined family;(v)the Gonostomatidae is polyphyletic in the SSU-rDNA and ITS1-5.8 S-ITS2 trees,with Gonostomum,Cladotricha,Cotterillia,Metagonostomum,Paragonostomum and Wallackia distributed among separate clades,but monophyletic in the LSU-rDNA and concatenated trees;(vi)higher hypotrich taxa such as core urostyloids and core sporadotrichids/stichotrichids might have evolved from species that possessed a gonostomatid oral apparatus.

Hypotrichidium tisiae(Gelei,1929)Gelei,1954:a unique hypotrichid ciliate having a highly specialized developmental pattern during binary division

In sharp contrast to their pelagic relatives,the oligotrichs,the overwhelming majority of hypotrich ciliates inhabit the benthos.Only a few species,including those of the genus Hypotrichidium Ilowaisky,1921,have adapted to a planktonic lifestyle.The ontogenetic mode of the highly differentiated ciliate,Hypotrichidium tisiae(Gelei,1929)Gelei,1954,is unknown.In this study,the interphase morphology and the ontogenetic process of this species are investigated.Accordingly,the previously unidentified ciliary pattern of Hypotrichidium is redefined.The main morphogenetic features are as follows:(1)The parental adoral zone of membranelles is inherited completely by the proter and the oral primordium of the opisthe arises in a deep pouch.(2)Five frontoventral cirral anlagen(FVA)are formed:FVA I contributes to the single frontal cirrus,FVA II–IV generate three frontoventral cirral rows,FVA V migrates and forms postoral ventral cirri.(3)All marginal cirral row anlagen develop de novo:each of the two left anlagen forms a single cirral row,while the single right anlage fragments into anterior and posterior parts.(4)Two dorsal kinety anlagen occur de novo,with the right one fragmenting to form kineties 2 and 3.(5)Two long caudal cirral rows are formed at the ends of dorsal kineties 1 and 3.On the basis of the morphogenetic features and phylogenetic analyses,the assignment of Hypotrichidium to the family Spirofilidae Gelei,1929 within Postoralida is supported.The establishment of separate families for the slender“tubicolous”spirofilids and the highly helical spirofilids is also validated.