Inheritance of Extrachromosomal rDNA in Physarum polycephalum

Abstract

In the acellular slime mold Physarum polycephalum, the several hundred genes coding for rRNA are located on linear extrachromosomal DNA molecules of a discrete size, 60 kilobases. Each molecule contains two genes that are arranged in a palindromic fashion and separated by a central spacer region. We investigated how rDNA is inherited after meiosis. Two Physarum amoebal strains, each with an rDNA recognizable by its restriction endonuclease cleavage pattern, were mated, the resulting diploid plasmodium was induced to sporulate, and haploid progeny clones were isolated from the germinated spores. The type of rDNA in each was analyzed by blotting hybridization, with cloned rDNA sequences used as probes. This analysis showed that rDNA was inherited in an all-or-nothing fashion; that is, progeny clones contained one or the other parental rDNA type, but not both. However, the rDNA did not segregate in a simple Mendelian way; one rDNA type was inherited more frequently than the other. The same rDNA type was also in excess in the diploid plasmodium before meiosis, and the relative proportions of the two rDNAs changed after continued plasmodial growth. The proportion of the two rDNA types in the population of progeny clones reflected the proportion in the parent plasmodium before meoisis. The rDNAs in many of the progeny clones contained specific deletions of some of the inverted repeat sequences at the central palindromic symmetry axis. To explain the pattern of inheritance of Physarum rDNA, we postulate that a single copy of rDNA is inserted into each spore or is selectively replicated after meiosis.

Inheritance of Extrachromosomal Ribosomal DNA During the Asexual Life Cycle of Dictyostelium discoideum: Examination by Use of DNA Polymorphisms

“…A WS38OB-derived band (-5.5 kb), which is present in all haploid segregants, is unrelated to rDNA and reflects the presence of a high-copy-number plasmid to be described elsewhere. exhibits levels of polymorphism between those observed in D. discoideum and T. thermophila (6). This degree of polymorphism correlates with the size of the palindromes: that ofD.…”

“…In Physarum polycephalum the situation is not clear. If there is a chromosomal copy, assumptions need to be made to explain the mode of inheritance of ribosomal DNA (rDNA) (6). In D. discoideum there is no firm evidence supporting the existence of a chromosomal copy of rDNA (13).…”

Mobile group I introns at nuclear rDNA position L2066 harbor sense and antisense homing endonuclease genes intervened by spliceosomal introns

“…9 ). 1) The rDNA in Didymium and other myxomycetes appear as multicopy extrachromosomal mini-chromosomes [ 16 , 42 – 44 ]. Thus, copies of the extrachromosomal rDNA in D. squamulosum may be located both in the nucleolus and nucleoplasm.…”

Genetic structure of the protist Physarum albescens (Amoebozoa) revealed by multiple markers and genotyping by sequencing

“…To produce a new simulated individual, the algorithm randomly samples with replacement two “parent” individuals and randomly chooses one SSU variant (from parent 1 or parent 2, non‐Mendelian inheritance), one COI variant (from parent 1 or parent 2, non‐Mendelian inheritance), and two EF1A variants (one of the two alleles from parent 1 and one of the two alleles from parent 2, Mendelian inheritance). The simulation does not consider that in myxomycetes some variants of SSU and COI are preferentially preserved during the homogenization process (Ferris et al, 1983; Silliker & Collins, 1988; Silliker et al, 2002). To test if this factor would influence the results, an alternative simulation was created: here, a weight value (0, 1, or 2) was randomly assigned to each SSU and each COI variant.…”