The Genome of Hypotrichous Ciliates

Abstract

Publisher Summary This chapter discusses the genome of hypotrichous ciliates. The chapter describes the structure of the macronucleus as well as the cytological and molecular events during macronuclear differentiation. Similar to the majority of ciliated protozoa, hypotrichous ciliates are characterized by their nuclear dimorphism. During their vegetative life cycles the cells contain diploid micronuclei and DNA-rich macronuclei of varying number and shape. Their high-molecular weight DNA is arranged in chromosomes. During mitosis these chromosomes are separated in an organized way with the help of a spindle apparatus. The macronucleus is the major site of RNA transcription and synthesizes all RNA necessary for vegetative growth of the cell. Its DNA is organized in short, gene-sized molecules, which are distributed during cell division by a process called “amitosis.” Vegetative growth usually continues for a limited number of generations that is characteristic for each species. Without sexual reproduction clones of most species die out. Sexual reproduction occurs when two competent cells, which are morphologically identical but of different mating type, meet. In the course of conjugation all micronuclei undergo a series of divisions that finally lead to differentiation into a haploid stationary and migratory micronucleus. In the chapter, this process is described for Euplutes. The chapter illustrates the schematically macronuclear development of hypotrichous ciliates. Intermediate stages in the processing of gene-sized molecules of DNA during macronuclear development have been observed in the marine hypotrich Euplores crassus.