嗜热四膜虫——具有发展潜力的功能基因组学研究模型

在真核生物的分子生物学和遗传学研究方面,纤毛类原生动物嗜热四膜虫(Tetrahymenathermophila)已经被证明是一种有价值的生物学模型。通过对它的研究,人们发现并且掌握了核酶的分子机制、RNA的自我拼接、端粒的结构和功能、DNA序列重组等机理。这种原生动物的基因组功能分别由两个细胞核执行,即二倍体的小核与生殖过程有关,而多倍体的大核决定细胞的基因转录,并为转化基因的表达提供了强有力的手段。     

Mutation affecting cell separation and macronuclear resorption during conjugation in Tetrahymena thermophila: Early expression of the zygotic genotype

A new recessive conjugation lethal mutation was found in Tetrahymena thermophila which was named mra for macronuclear resorption arrest. Other events affected by the mra mutations are separation of pairs, DNA replication in the macronuclear anlagen, and resorption of one of the two micronuclei. In wild-type crosses 50% of the pairs had separated by 12 hr after mixing two mating types and had completed resorption of the old macronucleus 1–2 hr later. In contrast most mra conjugants did not separate even by 24 hr after mixing and the old relic (condensed) macronucleus was seen in over 90% of them. After addition of 10mM calcium to the conjugation medium, the mra conjugants did separate but they still failed to complete resorption of the old macronucleus and to replicate macronuclear anlagen DNA in the exconjugants. The calcium induced separation of the mra conjugants occurred later than the separation of control pairs. During normal conjugation cell separation occurs before the first expression of known macronuclear genes and prior to processing of the macro-nuclear DNA. Therefore, the mra phenotype infers that separation of conjugants requires a signal which is produced by the macronuclear anlagen at an unusually early time. © 1992 Wiley-Liss, Inc.     

Interdependence of Cell Cycle Events in Tetrahymena thermophila: Formation of Contractile Vacuole Pore and Fission Gap in Ciliary Meridians*

SYNOPSIS The formation of the contractile vacuole pore (CVP) in Tetrahymena thermophila. genotype mol^b/mol^b is temporally and spatially associated with the formation of the fission gap in the CVP meridian. New CVPs arise when fission gaps appear in CVP meridians, the new pores being found anterior to the gaps. When, however, CVP meridians are rotated 180°, the fission gaps develop late. In more than 1/3 of the 180°-rotated CVP meridians, the new CVPs are formed before the appearance of the fission gap. Evidently, the appearance of the gap is not a prerequisite for CVP formation. Nevertheless, mutants exist in which the absence of fission gap and CVP are correlated in some cases and in which the presence of supernumerary fission gap and CVP are correlated in other instances. It is suggested that the 2 developmental events, although not causally related to each other, may be controlled by a common morpho-genetic signal. This commits a certain site (mid-body) along a ciliary meridian to develop the fission gap as well as the CVP; however, after this step of commitment, the appearance of the fission gap is delayed in 180°-rotated CVP meridians.     

Promotion of gas bubble formation by ingested nuclei in the ciliate,Tetrahymena pyriformis

Cells of the ciliateTetrahymena pyriformis were suspended with carmine or graphite particles or with Halobacterium gas vesicles, all of which promote bubble formation in aqueous suspensions when tested with 10 atm and above (0.1−0.5×10⁷ Pa) (carmine and graphite) or 25 atm and above (gas vesicles) of nitrogen supersaturations. All three particles were ingested, but only the gas vesicles promoted intracellular gas bubble formation if the cells containing them were nitrogen or methane saturated in a slow stepwise fashion prior to rapid decompression. Cell rupture did not occur until gas saturation pressures greater than 25 atm were used; this suggests that the ciliate pellicle and cytoplasm cannot resist the mechanical forces of an expanding gas phase induced by decompression from between 25 and 50 atm and thus provides an estimate of the physical strength of these cellular components. The inability of the ingested carmine, graphite, and collapsed gas vesicles to induce intracellular gas bubble formation suggests that the phagocytic process somehow altered them. This procedure may thus provide a tool for the study of early events in the digestive processes of ciliates.     

Altered expression patterns of TCP and MYB genes relating to the floral developmental transition from initial zygomorphy to actinomorphy in Bournea (Gesneriaceae)

The shift from zygomorphy to actinomorphy has been intensively studied in molecular genetic model organisms. However, it is still a key challenge to explain the great morphological diversity of derived actinomorphy in angiosperms, since different underlying mechanisms may be responsible for similar external morphologies. Bournea (Gesneriaceae) is of particular interest in addressing this question, as it is a representative of primarily derived actinomorphy characteristic of a unique developmental transition from zygomorphy to actinomorphic flowers at anthesis. Using RNA in situ hybridization, the expression patterns were investigated of three different Bournea orthologues of TCP and MYB genes that have been shown to control floral symmetry in model species. Here, it is shown that the initial zygomorphic pattern in Bournea is likely a residual zygomorphy resulting from conserved expression of the adaxial (dorsal) identity gene BlCYC1. As a key novel event, the late downregulation of BlCYC1 and BlRAD and the correlative changes in the late specific expression of the abaxial (ventral) identity gene BlDIV should be responsible for the origin of the derived actinomorphy in Bournea. These results further indicate that there might be diverse pathways in the origin and evolution of derived actinomorphy through modifications of pre-existing zygomorphic developmental programs under dynamics of regulatory networks.