The ciliated protozoan Tetrahymena thermophila has evolved remarkable nuclear dualism that involves spatial segregation of the polyploid somatic macro- and canonical diploid germinal micro- nucleus in a single cytoplasm. Programmed nuclear death (PND), also referred to as nuclear apoptosis, is a remarkable catabolic process that occurs during conjugation to finish the lifespan of parental soma, in which only the parental macronucleus is eliminated from the cytoplasm, but other co-existing nuclei are unaffected. We found that PND involves unique aspects of autophagy, which differ from mammalian or yeast macroautophagy. When PND starts, the envelope of the parental macronucleus changes its nature as if it is an autophagic membrane, without the accumulation of other membranous structures from the cytoplasm. The alteration of the parental macronuclear membrane involves exposing certain sugars and phosphatidylserine on the envelope, which are members of a class of “eat-me” signals found on the surface of apoptotic cells, that are not found on other types of nuclei. Subsequently, small autophagic vesicles that contain mitochondria and lysosomes fuse with the nuclear envelope stepwise and release their contents into the nucleus at distinct stages. Mitochondria of Tetrahymena contain apoptosis-inducing factor (AIF) and endonuclease G-like DNase, which are responsible for the nuclear condensation and kb-sized DNA fragmentation, corresponding to the early stage of the nuclear apoptosis. On the other hand, acidic lysosomal enzymes are responsible for final resorption of the nucleus. These elaborate mechanisms, unique to ciliates, ultimately achieve specific macronuclear elimination.
The intronic sequence intervening the small subunit ribosomal DNA (SSU rDNA) of Chlorella sp. T-24-5, an atypical photobiont of Paramecium bursaria was examined. The position of the insertion was found to be 10 nucleotides upstream from that of a major P. bursaria photobiont, Micractinium reisseri (S651), and was found to be a novel insertion site (S641; the numbering reflects the homologous position in the rRNA gene of Escherichia coli: S = SSU rRNA). A secondary structure diagram showed that the intron is classified as a group I intron (subgroup IC), characterized by an extended P5 helix. Phylogenetic analyses could not reveal its evolutionary relationships with other introns, but were suggestive of a monophyletic relationship with introns of some trebouxiophytes. These introns all share the insertion position S641, and their sequences are extremely conserved and are likely to have spread recently. The intron of Chlorella sp. T-24-5 had twelve-nucleotide sequence repeats lying at the head of the intron and after the insertion, which may play a role in intron invasion.
It is well recognized that composition of termite symbiotic protist community generally show host-species specificity at species-to-species level. On the other hand, studies focused on population or colony level variations of the protist composition are few, although such studies are necessary for understanding the termite-protist mutualism. In this study we investigated symbiotic protist fauna of 34 Hodotermopsis sjoestedti nests collected from five islands in the Ryukyu Archipelago, Taiwan Is., and southern part of Chinese continent. Thirteen parabasalid species and six oxymonad species, which were reported from the host termite collected in Yakushima Is., were recognized in this study. Four parabasalids were identified as recently described species from H. sjoestedti collected in Vietnam. The fauna was the most species-rich among the termite hosts studied to date, possibly reflecting large host body size. The prevalence rates of the symbiotic protist species are generally high, 100% in all populations for all protist species, except Gen. A sp. and Hoplonympha sp. All host nests had identical protist species composition except for the presence or absence of Gen. A sp. Two host populations lacked this species. In Yakushima Is., both types of termite nests were found, which exhibited mutually exclusive distribution. The distribution pattern suggests that the host termite is a multiple-site nester and nest-budding occurs in the field.