Spo11-Independent Meiosis in Social Amoebae

Sex in social amoebae (or dictyostelids) has a number of striking features. Dictyostelid zygotes do not proliferate but grow to a large size by feeding on other cells of the same species, each zygote ultimately forming a walled structure called a macrocyst. The diploid macrocyst nucleus undergoes meiosis, after which a single meiotic product survives to restart haploid vegetative growth. Meiotic recombination is generally initiated by the Spo11 enzyme, which introduces DNA double-strand breaks. Uniquely, as far as is known among sexual eukaryotes, dictyostelids lack a SPO11 gene. Despite this, recombination occurs at high frequencies during meiosis in dictyostelids, through unknown mechanisms. The molecular processes underlying these events, and the evolutionary drivers that brought them into being, may shed light on the genetic conflicts that occur within and between genomes, and how they can be resolved.

NULLISOMIC TETRAHYMENA. II. A SET OF NULLISOMICS DEFINE THE GERMINAL CHROMOSOMES

ABSTRACT Crosses of a diploid Tetrahymena thermophila to a strain with a haploid germinal nucleus result in chromosome loss during meiosis in the haploid. The resulting monosomics can be made nullisomic by a special cross that induces homozygosis of a meiotic product of the germinal nucleus, but retention of the parental somatic nucleus. The creation and testing of single nullisomics for three of the five chromosome pairs and a triple nullisomic missing another pair is presented. Taken together, these strains make possible a series of crosses in which all but one of the chromosomes is missing in one parent. This set of nullisomics can, therefore, be used to map any mutation in Tetrahymena to a specific chromosome.

CYTOGAMY: AN INDUCIBLE, ALTERNATE PATHWAY OF CONJUGATION IN TETRAHYMENA THERMOPHILA

ABSTRACT We report the occurrence of cytogainy in Tetrahymena thermophila. By analogy to Paramecium, cytogamy generates exconjugant clones that derive their entire genetic information from a single meiotic product of their cytoplasmic parent. Thus, "instant" whole-genome homozygotes are created. Cytogamy has been induced in every strain of T.thermophila tested, and most of the excytogamous progeny have exhibited high fertility. The high frequency with which cytogamy can be induced by hyperosmotic shock, coupled with the foregoing genetic properties, make this process a practical (and already proven) method for the isolation of recessive mutants in T.thermophila. We also report that the cytogamy-inducing treatment induces other rare abnormalities of genetic transmission, which have not yet been characterized.