Episome-like behaviour of donor DNA in transformed strains of Neurospora crassa

Abstract

TRANSFORMATION is a well established process for the transfer of genetic information in bacteria^1,2. Although such a system in higher organisms remains to be fully elucidated, certain features of gene transfer in eukaryotes have similarities with bacterial transformation³. The occurrence of inositol-independent (inl⁺) transformants has been reported in Neurospora when an inositol requiring (inl⁻) mutant strain (89601) is treated with a wild-type (RL3-8A) DNA preparation^4,5. Although these transformants are stable for inositol independence (inl⁺) during the somatic cell cycle they differ in the ability to transmit this character to their sexual progeny. Essentially there are two groups of transformants, one capable of Mendelian transmission and another showing only a rare non-Mendelian transmission of the transformed character (inl⁺)5 (Table 1). Such differences in modes of genetic transmission can be explained if the donor DNA exists as an episome in the transformed strains. Thus the physical status of the episomes determines the Mendelian and non-Mendelian transmission of the genetic character controlled by them. When integrated they are transmitted in a Mendelian way like any other chromosomal gene; whereas in the non-integrated (autonomous) form their nucleolytic degradation (or slow replication followed by segregation) during meiosis can lead to non-Mendelian transmission. This hypothesis is supported by demonstration of a continued aberrant transmission of the inositol independence to the sexual progeny in the subsequent generations⁵. Here we describe experiments which establish further the validity of the above hypothesis.